Developer Guide

Coding conventions

The source code follows the following structure and conventions.

Code layout

• auto — Build scripts

• src

  • core — Basic types and functions — string, array, log, pool, etc.

  • event — Event core

    • modules — Event notification modules: epoll, kqueue, select etc.

  • http — Core HTTP module and common code

    • modules — Other HTTP modules

    • v2 — HTTP/2

  • mail — Mail modules

  • os — Platform-specific code

    • unix

    • win32

  • stream — Stream modules

Include files

The following two #include statements must appear at the beginning of every Angie file:

#include <ngx_config.h>
#include <ngx_core.h>

In addition to that, HTTP code should include

#include <ngx_http.h>

Mail code should include

#include <ngx_mail.h>

Stream code should include

#include <ngx_stream.h>

Integers

For general purposes, Angie code uses two integer types, ngx_int_t and ngx_uint_t, which are typedefs for intptr_t and uintptr_t respectively.

Common return codes

Most functions in Angie return the following codes:

• NGX_OK — Operation succeeded.

• NGX_ERROR — Operation failed.

• NGX_AGAIN — Operation incomplete; call the function again.

• NGX_DECLINED — Operation rejected, for example, because it is disabled in the configuration. This is never an error.

• NGX_BUSY — Resource is not available.

• NGX_DONE — Operation complete or continued elsewhere. Also used as an alternative success code.

• NGX_ABORT — Function was aborted. Also used as an alternative error code.

Error handling

The ngx_errno macro returns the last system error code. It's mapped to errno on POSIX platforms and to a GetLastError() call in Windows. The ngx_socket_errno macro returns the last socket error number. Like the ngx_errno macro, it's mapped to errno on POSIX platforms. It's mapped to a WSAGetLastError() call in Windows. Accessing the values of ngx_errno or ngx_socket_errno more than once in a row can cause performance issues. If the error value might be used multiple times, store it in a local variable of type ngx_err_t. To set errors, use the ngx_set_errno(errno) and ngx_set_socket_errno(errno) macros.

The values of ngx_errno and ngx_socket_errno can be passed to the logging functions ngx_log_error() and ngx_log_debugX(), in which case system error text is added to the log message.

Example using ngx_errno:

ngx_int_t
ngx_my_kill(ngx_pid_t pid, ngx_log_t *log, int signo)
{
    ngx_err_t  err;

    if (kill(pid, signo) == -1) {
        err = ngx_errno;

        ngx_log_error(NGX_LOG_ALERT, log, err, "kill(%P, %d) failed", pid, signo);

        if (err == NGX_ESRCH) {
            return 2;
        }

        return 1;
    }

    return 0;
}

Strings

Overview

For C strings, Angie uses the unsigned character type pointer u_char *.

The Angie string type ngx_str_t is defined as follows:

typedef struct {
    size_t      len;
    u_char     *data;
} ngx_str_t;

The len field holds the string length and data holds the string data. The string, held in ngx_str_t, may or may not be null-terminated after the len bytes. In most cases it's not. However, in certain parts of the code (for example, when parsing configuration), ngx_str_t objects are known to be null-terminated, which simplifies string comparison and makes it easier to pass the strings to syscalls.

The string operations in Angie are declared in src/core/ngx_string.h. Some of them are wrappers around standard C functions:

• ngx_strcmp()

• ngx_strncmp()

• ngx_strstr()

• ngx_strlen()

• ngx_strchr()

• ngx_memcmp()

• ngx_memset()

• ngx_memcpy()

• ngx_memmove()

Other string functions are Angie-specific:

• ngx_memzero() — Fills memory with zeroes.

• ngx_explicit_memzero() — Does the same as ngx_memzero(), but this call is never removed by the compiler's dead store elimination optimization. This function can be used to clear sensitive data such as passwords and keys.

• ngx_cpymem() — Does the same as ngx_memcpy(), but returns the final destination address. This one is handy for appending multiple strings in a row.

• ngx_movemem() — Does the same as ngx_memmove(), but returns the final destination address.

• ngx_strlchr() — Searches for a character in a string, delimited by two pointers.

The following functions perform case conversion and comparison:

• ngx_tolower()

• ngx_toupper()

• ngx_strlow()

• ngx_strcasecmp()

• ngx_strncasecmp()

The following macros simplify string initialization:

• ngx_string(text) — static initializer for the ngx_str_t type from the C string literal text

• ngx_null_string — static empty string initializer for the ngx_str_t type

• ngx_str_set(str, text) — initializes string str of ngx_str_t * type with the C string literal text

• ngx_str_null(str) — initializes string str of ngx_str_t * type with the empty string

Formatting

The following formatting functions support Angie-specific types:

• ngx_sprintf(buf, fmt, ...)

• ngx_snprintf(buf, max, fmt, ...)

• ngx_slprintf(buf, last, fmt, ...)

• ngx_vslprintf(buf, last, fmt, args)

• ngx_vsnprintf(buf, max, fmt, args)

The full list of formatting options supported by these functions is in src/core/ngx_string.c. Some of them are:

• %O — off_t

• %T — time_t

• %z — ssize_t

• %i — ngx_int_t

• %p — void *

• %V — ngx_str_t *

• %s — u_char * (null-terminated)

• %*s — size_t + u_char *

You can prepend u on most types to make them unsigned. To convert output to hex, use X or x.

Numeric conversion

Several functions for numeric conversion are implemented in Angie. The first four each convert a string of given length to a positive integer of the indicated type. They return NGX_ERROR on error.

• ngx_atoi(line, n) — ngx_int_t

• ngx_atosz(line, n) — ssize_t

• ngx_atoof(line, n) — off_t

• ngx_atotm(line, n) — time_t

There are two additional numeric conversion functions. Like the first four, they return NGX_ERROR on error.

• ngx_atofp(line, n, point) — Converts a fixed point number of given length to a positive integer of type ngx_int_t. The result is shifted left by point decimal positions. The string representation of the number is expected to have no more than point fractional digits. For example, ngx_atofp("10.5", 4, 2) returns 1050.

• ngx_hextoi(line, n) — Converts a hexadecimal representation of a positive integer to ngx_int_t.

Regular expressions

The regular expressions interface in Angie is a wrapper around the PCRE library. The corresponding header file is src/core/ngx_regex.h.

To use a regular expression for string matching, it first needs to be compiled, which is usually done at the configuration phase. Note that since PCRE support is optional, all code using the interface must be protected by the surrounding NGX_PCRE macro:

#if (NGX_PCRE)
ngx_regex_t          *re;
ngx_regex_compile_t   rc;

u_char                errstr[NGX_MAX_CONF_ERRSTR];

ngx_str_t  value = ngx_string("message (\\d\\d\\d).*Codeword is '(?<cw>\\w+)'");

ngx_memzero(&rc, sizeof(ngx_regex_compile_t));

rc.pattern = value;
rc.pool = cf->pool;
rc.err.len = NGX_MAX_CONF_ERRSTR;
rc.err.data = errstr;
/* rc.options can be set to NGX_REGEX_CASELESS */

if (ngx_regex_compile(&rc) != NGX_OK) {
    ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "%V", &rc.err);
    return NGX_CONF_ERROR;
}

re = rc.regex;
#endif

After successful compilation, the captures and named_captures fields in the ngx_regex_compile_t structure contain the count of all captures and named captures, respectively, found in the regular expression.

The compiled regular expression can then be used for matching against strings:

ngx_int_t  n;
int        captures[(1 + rc.captures) * 3];

ngx_str_t input = ngx_string("This is message 123. Codeword is 'foobar'.");

n = ngx_regex_exec(re, &input, captures, (1 + rc.captures) * 3);
if (n >= 0) {
    /* string matches expression */

} else if (n == NGX_REGEX_NO_MATCHED) {
    /* no match was found */

} else {
    /* some error */
    ngx_log_error(NGX_LOG_ALERT, log, 0, ngx_regex_exec_n " failed: %i", n);
}

The arguments to ngx_regex_exec() are the compiled regular expression re, the string to match input, an optional array of integers to hold any captures that are found, and the array's size. The size of the captures array must be a multiple of three, as required by the PCRE API. In the example, the size is calculated from the total number of captures plus one for the matched string itself.

If there are matches, captures can be accessed as follows:

u_char     *p;
size_t      size;
ngx_str_t   name, value;

/* all captures */
for (i = 0; i < n * 2; i += 2) {
    value.data = input.data + captures[i];
    value.len = captures[i + 1] - captures[i];
}

/* accessing named captures */

size = rc.name_size;
p = rc.names;

for (i = 0; i < rc.named_captures; i++, p += size) {

    /* capture name */
    name.data = &p[2];
    name.len = ngx_strlen(name.data);

    n = 2 * ((p[0] << 8) + p[1]);

    /* captured value */
    value.data = &input.data[captures[n]];
    value.len = captures[n + 1] - captures[n];
}

The ngx_regex_exec_array() function accepts an array of ngx_regex_elt_t elements (which are just compiled regular expressions with associated names), a string to match, and a log. The function applies expressions from the array to the string until either a match is found or no more expressions are left. The return value is NGX_OK when there is a match and NGX_DECLINED otherwise, or NGX_ERROR in case of error.

Time

The ngx_time_t structure represents time with three separate types for seconds, milliseconds, and the GMT offset:

typedef struct {
    time_t      sec;
    ngx_uint_t  msec;
    ngx_int_t   gmtoff;
} ngx_time_t;

The ngx_tm_t structure is an alias for struct tm on UNIX platforms and SYSTEMTIME on Windows.

To obtain the current time, it is usually sufficient to access one of the available global variables, representing the cached time value in the desired format.

The available string representations are:

• ngx_cached_err_log_time — Used in error log entries: "1970/09/28 12:00:00"

• ngx_cached_http_log_time — Used in HTTP access log entries: "28/Sep/1970:12:00:00 +0600"

• ngx_cached_syslog_time — Used in syslog entries: "Sep 28 12:00:00"

• ngx_cached_http_time — Used in HTTP headers: "Mon, 28 Sep 1970 06:00:00 GMT"

• ngx_cached_http_log_iso8601 — The ISO 8601 standard format: "1970-09-28T12:00:00+06:00"

The ngx_time() and ngx_timeofday() macros return the current time value in seconds and are the preferred way to access the cached time value.

To obtain the time explicitly, use ngx_gettimeofday(), which updates its argument (pointer to struct timeval). The time is always updated when Angie returns to the event loop from system calls. To update the time immediately, call ngx_time_update(), or ngx_time_sigsafe_update() if updating the time in the signal handler context.

The following functions convert time_t into the indicated broken-down time representation. The first function in each pair converts time_t to ngx_tm_t and the second (with the _libc_ infix) to struct tm:

• ngx_gmtime(), ngx_libc_gmtime() — Time expressed as UTC

• ngx_localtime(), ngx_libc_localtime() — Time expressed relative to the local time zone

The ngx_http_time(buf, time) function returns a string representation suitable for use in HTTP headers (for example, "Mon, 28 Sep 1970 06:00:00 GMT"). The ngx_http_cookie_time(buf, time) function returns a string representation suitable for HTTP cookies ("Thu, 31-Dec-37 23:55:55 GMT").

Containers

Array

The Angie array type ngx_array_t is defined as follows

typedef struct {
    void        *elts;
    ngx_uint_t   nelts;
    size_t       size;
    ngx_uint_t   nalloc;
    ngx_pool_t  *pool;
} ngx_array_t;

The elements of the array are available in the elts field. The nelts field holds the number of elements. The size field holds the size of a single element and is set when the array is initialized.

Use the ngx_array_create(pool, n, size) call to create an array in a pool, and the ngx_array_init(array, pool, n, size) call to initialize an array object that has already been allocated.

ngx_array_t  *a, b;

/* create an array of strings with preallocated memory for 10 elements */
a = ngx_array_create(pool, 10, sizeof(ngx_str_t));

/* initialize string array for 10 elements */
ngx_array_init(&b, pool, 10, sizeof(ngx_str_t));

Use the following functions to add elements to an array:

• ngx_array_push(a) adds one tail element and returns a pointer to it

• ngx_array_push_n(a, n) adds n tail elements and returns a pointer to the first one

If the currently allocated amount of memory is not large enough to accommodate the new elements, a new block of memory is allocated and the existing elements are copied to it. The new memory block is normally twice as large as the existing one.

s = ngx_array_push(a);
ss = ngx_array_push_n(&b, 3);

List

In Angie a list is a sequence of arrays, optimized for inserting a potentially large number of items. The ngx_list_t list type is defined as follows:

typedef struct {
    ngx_list_part_t  *last;
    ngx_list_part_t   part;
    size_t            size;
    ngx_uint_t        nalloc;
    ngx_pool_t       *pool;
} ngx_list_t;

The actual items are stored in list parts, which are defined as follows:

typedef struct ngx_list_part_s  ngx_list_part_t;

struct ngx_list_part_s {
    void             *elts;
    ngx_uint_t        nelts;
    ngx_list_part_t  *next;
};

Before use, a list must be initialized by calling ngx_list_init(list, pool, n, size) or created by calling ngx_list_create(pool, n, size). Both functions take as arguments the size of a single item and a number of items per list part. To add an item to a list, use the ngx_list_push(list) function. To iterate over the items, directly access the list fields as shown in the example:

ngx_str_t        *v;
ngx_uint_t        i;
ngx_list_t       *list;
ngx_list_part_t  *part;

list = ngx_list_create(pool, 100, sizeof(ngx_str_t));
if (list == NULL) { /* error */ }

/* add items to the list */

v = ngx_list_push(list);
if (v == NULL) { /* error */ }
ngx_str_set(v, "foo");

v = ngx_list_push(list);
if (v == NULL) { /* error */ }
ngx_str_set(v, "bar");

/* iterate over the list */

part = &list->part;
v = part->elts;

for (i = 0; /* void */; i++) {

    if (i >= part->nelts) {
        if (part->next == NULL) {
            break;
        }

        part = part->next;
        v = part->elts;
        i = 0;
    }

    ngx_do_smth(&v[i]);
}

Lists are primarily used for HTTP input and output headers.

Lists do not support item removal. However, when needed, items can internally be marked as missing without actually being removed from the list. For example, to mark HTTP output headers (which are stored as ngx_table_elt_t objects) as missing, set the hash field in ngx_table_elt_t to zero. Items marked in this way are explicitly skipped when the headers are iterated over.

Queue

In Angie a queue is an intrusive doubly linked list, with each node defined as follows:

typedef struct ngx_queue_s  ngx_queue_t;

struct ngx_queue_s {
    ngx_queue_t  *prev;
    ngx_queue_t  *next;
};

The head queue node is not linked with any data. Use the ngx_queue_init(q) call to initialize the list head before use. Queues support the following operations:

• ngx_queue_insert_head(h, x), ngx_queue_insert_tail(h, x) — Insert a new node

• ngx_queue_remove(x) — Remove a queue node

• ngx_queue_split(h, q, n) — Split a queue at a node, returning the queue tail in a separate queue

• ngx_queue_add(h, n) — Add a second queue to the first queue

• ngx_queue_head(h), ngx_queue_last(h) — Get first or last queue node

• ngx_queue_sentinel(h) — Get a queue sentinel object to end iteration at

• ngx_queue_data(q, type, link) — Get a reference to the beginning of a queue node data structure, considering the queue field offset in it

An example:

typedef struct {
    ngx_str_t    value;
    ngx_queue_t  queue;
} ngx_foo_t;

ngx_foo_t    *f;
ngx_queue_t   values, *q;

ngx_queue_init(&values);

f = ngx_palloc(pool, sizeof(ngx_foo_t));
if (f == NULL) { /* error */ }
ngx_str_set(&f->value, "foo");

ngx_queue_insert_tail(&values, &f->queue);

/* insert more nodes here */

for (q = ngx_queue_head(&values);
     q != ngx_queue_sentinel(&values);
     q = ngx_queue_next(q))
{
    f = ngx_queue_data(q, ngx_foo_t, queue);

    ngx_do_smth(&f->value);
}

Red-Black tree

The src/core/ngx_rbtree.h header file provides access to the effective implementation of red-black trees.

typedef struct {
    ngx_rbtree_t       rbtree;
    ngx_rbtree_node_t  sentinel;

    /* custom per-tree data here */
} my_tree_t;

typedef struct {
    ngx_rbtree_node_t  rbnode;

    /* custom per-node data */
    foo_t              val;
} my_node_t;

To deal with a tree as a whole, you need two nodes: root and sentinel. Typically, they are added to a custom structure, allowing you to organize your data into a tree in which the leaves contain a link to or embed your data.

To initialize a tree:

my_tree_t  root;

ngx_rbtree_init(&root.rbtree, &root.sentinel, insert_value_function);

To traverse a tree and insert new values, use the "insert_value" functions. For example, the ngx_str_rbtree_insert_value function deals with the ngx_str_t type. Its arguments are pointers to a root node of an insertion, the newly created node to be added, and a tree sentinel.

void ngx_str_rbtree_insert_value(ngx_rbtree_node_t *temp,
                                 ngx_rbtree_node_t *node,
                                 ngx_rbtree_node_t *sentinel)

The traversal is pretty straightforward and can be demonstrated with the following lookup function pattern:

my_node_t *
my_rbtree_lookup(ngx_rbtree_t *rbtree, foo_t *val, uint32_t hash)
{
    ngx_int_t           rc;
    my_node_t          *n;
    ngx_rbtree_node_t  *node, *sentinel;

    node = rbtree->root;
    sentinel = rbtree->sentinel;

    while (node != sentinel) {

        n = (my_node_t *) node;

        if (hash != node->key) {
            node = (hash < node->key) ? node->left : node->right;
            continue;
        }

        rc = compare(val, node->val);

        if (rc < 0) {
            node = node->left;
            continue;
        }

        if (rc > 0) {
            node = node->right;
            continue;
        }

        return n;
    }

    return NULL;
}

The compare() function is a classic comparator function that returns a value less than, equal to, or greater than zero. To speed up lookups and avoid comparing user objects that can be big, an integer hash field is used.

To add a node to a tree, allocate a new node, initialize it and call ngx_rbtree_insert():

my_node_t          *my_node;
ngx_rbtree_node_t  *node;

my_node = ngx_palloc(...);
init_custom_data(&my_node->val);

node = &my_node->rbnode;
node->key = create_key(my_node->val);

ngx_rbtree_insert(&root->rbtree, node);

To remove a node, call the ngx_rbtree_delete() function:

ngx_rbtree_delete(&root->rbtree, node);

Hash

Hash table functions are declared in src/core/ngx_hash.h. Both exact and wildcard matching are supported. The latter requires extra setup and is described in a separate section below.

Before initializing a hash, you need to know the number of elements it will hold so that Angie can build it optimally. Two parameters that need to be configured are max_size and bucket_size, as detailed in a separate document. They are usually configurable by the user. Hash initialization settings are stored with the ngx_hash_init_t type, and the hash itself is ngx_hash_t:

ngx_hash_t       foo_hash;
ngx_hash_init_t  hash;

hash.hash = &foo_hash;
hash.key = ngx_hash_key;
hash.max_size = 512;
hash.bucket_size = ngx_align(64, ngx_cacheline_size);
hash.name = "foo_hash";
hash.pool = cf->pool;
hash.temp_pool = cf->temp_pool;

The key is a pointer to a function that creates the hash integer key from a string. There are two generic key-creation functions: ngx_hash_key(data, len) and ngx_hash_key_lc(data, len). The latter converts a string to all lowercase characters, so the passed string must be writable. If that is not true, pass the NGX_HASH_READONLY_KEY flag to the function, initializing the key array (see below).

The hash keys are stored in ngx_hash_keys_arrays_t and are initialized with ngx_hash_keys_array_init(arr, type): The second parameter (type) controls the amount of resources preallocated for the hash and can be either NGX_HASH_SMALL or NGX_HASH_LARGE. The latter is appropriate if you expect the hash to contain thousands of elements.

ngx_hash_keys_arrays_t  foo_keys;

foo_keys.pool = cf->pool;
foo_keys.temp_pool = cf->temp_pool;

ngx_hash_keys_array_init(&foo_keys, NGX_HASH_SMALL);

To insert keys into a hash keys array, use the ngx_hash_add_key(keys_array, key, value, flags) function:

ngx_str_t k1 = ngx_string("key1");
ngx_str_t k2 = ngx_string("key2");

ngx_hash_add_key(&foo_keys, &k1, &my_data_ptr_1, NGX_HASH_READONLY_KEY);
ngx_hash_add_key(&foo_keys, &k2, &my_data_ptr_2, NGX_HASH_READONLY_KEY);

To build the hash table, call the ngx_hash_init(hinit, key_names, nelts) function:

ngx_hash_init(&hash, foo_keys.keys.elts, foo_keys.keys.nelts);

The function fails if max_size or bucket_size parameters are not big enough.

When the hash is built, use the ngx_hash_find(hash, key, name, len) function to look up elements:

my_data_t   *data;
ngx_uint_t   key;

key = ngx_hash_key(k1.data, k1.len);

data = ngx_hash_find(&foo_hash, key, k1.data, k1.len);
if (data == NULL) {
    /* key not found */
}

Wildcard matching

To create a hash that works with wildcards, use the ngx_hash_combined_t type. It includes the hash type described above and has two additional keys arrays: dns_wc_head and dns_wc_tail. The initialization of basic properties is similar to a regular hash:

ngx_hash_init_t      hash
ngx_hash_combined_t  foo_hash;

hash.hash = &foo_hash.hash;
hash.key = ...;

It is possible to add wildcard keys using the NGX_HASH_WILDCARD_KEY flag:

/* k1 = ".example.org"; */
/* k2 = "foo.*";        */
ngx_hash_add_key(&foo_keys, &k1, &data1, NGX_HASH_WILDCARD_KEY);
ngx_hash_add_key(&foo_keys, &k2, &data2, NGX_HASH_WILDCARD_KEY);

The function recognizes wildcards and adds keys into the corresponding arrays. Please refer to the Map module documentation for the description of the wildcard syntax and the matching algorithm.

Depending on the contents of added keys, you may need to initialize up to three key arrays: one for exact matching (described above), and two more to enable matching starting from the head or tail of a string:

if (foo_keys.dns_wc_head.nelts) {

    ngx_qsort(foo_keys.dns_wc_head.elts,
              (size_t) foo_keys.dns_wc_head.nelts,
              sizeof(ngx_hash_key_t),
              cmp_dns_wildcards);

    hash.hash = NULL;
    hash.temp_pool = pool;

    if (ngx_hash_wildcard_init(&hash, foo_keys.dns_wc_head.elts,
                               foo_keys.dns_wc_head.nelts)
        != NGX_OK)
    {
        return NGX_ERROR;
    }

    foo_hash.wc_head = (ngx_hash_wildcard_t *) hash.hash;
}

The keys array needs to be sorted, and initialization results must be added to the combined hash. The initialization of dns_wc_tail array is done similarly.

The lookup in a combined hash is handled by the ngx_hash_find_combined(chash, key, name, len):

/* key = "bar.example.org"; - will match ".example.org" */
/* key = "foo.example.com"; - will match "foo.*"        */

hkey = ngx_hash_key(key.data, key.len);
res = ngx_hash_find_combined(&foo_hash, hkey, key.data, key.len);

Memory management

Heap

To allocate memory from system heap, use the following functions:

• ngx_alloc(size, log) — Allocate memory from system heap. This is a wrapper around malloc() with logging support. Allocation error and debugging information is logged to log.

• ngx_calloc(size, log) — Allocate memory from system heap like ngx_alloc(), but fill memory with zeros after allocation.

• ngx_memalign(alignment, size, log) — Allocate aligned memory from system heap. This is a wrapper around posix_memalign() on those platforms that provide that function. Otherwise implementation falls back to ngx_alloc() which provides maximum alignment.

• ngx_free(p) — Free allocated memory. This is a wrapper around free().

Pooling

Most Angie allocations are done in pools. Memory allocated in an Angie pool is freed automatically when the pool is destroyed. This provides good allocation performance and makes memory control easy.

A pool internally allocates objects in continuous blocks of memory. Once a block is full, a new one is allocated and added to the pool memory block list. When the requested allocation is too large to fit into a block, the request is forwarded to the system allocator and the returned pointer is stored in the pool for further deallocation.

The type for Angie pools is ngx_pool_t. The following operations are supported:

• ngx_create_pool(size, log) — Create a pool with specified block size. The pool object returned is allocated in the pool as well. The size should be at least NGX_MIN_POOL_SIZE and a multiple of NGX_POOL_ALIGNMENT.

• ngx_destroy_pool(pool) — Free all pool memory, including the pool object itself.

• ngx_palloc(pool, size) — Allocate aligned memory from the specified pool.

• ngx_pcalloc(pool, size) — Allocate aligned memory from the specified pool and fill it with zeroes.

• ngx_pnalloc(pool, size) — Allocate unaligned memory from the specified pool. Mostly used for allocating strings.

• ngx_pfree(pool, p) — Free memory that was previously allocated in the specified pool. Only allocations that result from requests forwarded to the system allocator can be freed.

u_char      *p;
ngx_str_t   *s;
ngx_pool_t  *pool;

pool = ngx_create_pool(1024, log);
if (pool == NULL) { /* error */ }

s = ngx_palloc(pool, sizeof(ngx_str_t));
if (s == NULL) { /* error */ }
ngx_str_set(s, "foo");

p = ngx_pnalloc(pool, 3);
if (p == NULL) { /* error */ }
ngx_memcpy(p, "foo", 3);

Chain links (ngx_chain_t) are actively used in Angie, so the Angie pool implementation provides a way to reuse them. The chain field of ngx_pool_t keeps a list of previously allocated links ready for reuse. For efficient allocation of a chain link in a pool, use the ngx_alloc_chain_link(pool) function. This function looks up a free chain link in the pool list and allocates a new chain link if the pool list is empty. To free a link, call the ngx_free_chain(pool, cl) function.

Cleanup handlers can be registered in a pool. A cleanup handler is a callback with an argument which is called when the pool is destroyed. A pool is usually tied to a specific Angie object (like an HTTP request) and is destroyed when the object reaches the end of its lifetime. Registering a pool cleanup is a convenient way to release resources, close file descriptors or make final adjustments to the shared data associated with the main object.

To register a pool cleanup, call ngx_pool_cleanup_add(pool, size), which returns a ngx_pool_cleanup_t pointer to be filled in by the caller. Use the size argument to allocate context for the cleanup handler.

ngx_pool_cleanup_t  *cln;

cln = ngx_pool_cleanup_add(pool, 0);
if (cln == NULL) { /* error */ }

cln->handler = ngx_my_cleanup;
cln->data = "foo";

...

static void
ngx_my_cleanup(void *data)
{
    u_char  *msg = data;

    ngx_do_smth(msg);
}

Shared memory

Shared memory is used by Angie to share common data between processes. The ngx_shared_memory_add(cf, name, size, tag) function adds a new shared memory entry ngx_shm_zone_t to a cycle. The function receives the name and size of the zone. Each shared zone must have a unique name. If a shared zone entry with the provided name and tag already exists, the existing zone entry is reused. The function fails with an error if an existing entry with the same name has a different tag. Usually, the address of the module structure is passed as tag, making it possible to reuse shared zones by name within one Angie module.

The shared memory entry structure ngx_shm_zone_t has the following fields:

• init — Initialization callback, called after the shared zone is mapped to actual memory

• data — Data context, used to pass arbitrary data to the init callback

• noreuse — Flag that disables reuse of a shared zone from the old cycle

• tag — Shared zone tag

• shm — Platform-specific object of type ngx_shm_t, having at least the following fields:

  • addr — Mapped shared memory address, initially NULL

  • size — Shared memory size

  • name — Shared memory name

  • log — Shared memory log

  • exists — Flag that indicates shared memory was inherited from the master process (Windows-specific)

Shared zone entries are mapped to actual memory in ngx_init_cycle() after the configuration is parsed. On POSIX systems, the mmap() syscall is used to create the shared anonymous mapping. On Windows, the CreateFileMapping()/ MapViewOfFileEx() pair is used.

For allocating in shared memory, Angie provides the slab pool ngx_slab_pool_t type. A slab pool for allocating memory is automatically created in each Angie shared zone. The pool is located in the beginning of the shared zone and can be accessed by the expression (ngx_slab_pool_t *) shm_zone->shm.addr. To allocate memory in a shared zone, call either ngx_slab_alloc(pool, size) or ngx_slab_calloc(pool, size). To free memory, call ngx_slab_free(pool, p).

The slab pool divides all shared zone into pages. Each page is used for allocating objects of the same size. The specified size must be a power of 2, and greater than the minimum size of 8 bytes. Nonconforming values are rounded up. A bitmask for each page tracks which blocks are in use and which are free for allocation. For sizes greater than a half page (which is usually 2048 bytes), allocation is done an entire page at a time.

To protect data in shared memory from concurrent access, use the mutex available in the mutex field of ngx_slab_pool_t. A mutex is most commonly used by the slab pool while allocating and freeing memory, but it can be used to protect any other user data structures allocated in the shared zone. To lock or unlock a mutex, call ngx_shmtx_lock(&shpool->mutex) or ngx_shmtx_unlock(&shpool->mutex) respectively.

ngx_str_t        name;
ngx_foo_ctx_t   *ctx;
ngx_shm_zone_t  *shm_zone;

ngx_str_set(&name, "foo");

/* allocate shared zone context */
ctx = ngx_pcalloc(cf->pool, sizeof(ngx_foo_ctx_t));
if (ctx == NULL) {
    /* error */
}

/* add an entry for 64k shared zone */
shm_zone = ngx_shared_memory_add(cf, &name, 65536, &ngx_foo_module);
if (shm_zone == NULL) {
    /* error */
}

/* register init callback and context */
shm_zone->init = ngx_foo_init_zone;
shm_zone->data = ctx;


...


static ngx_int_t
ngx_foo_init_zone(ngx_shm_zone_t *shm_zone, void *data)
{
    ngx_foo_ctx_t  *octx = data;

    size_t            len;
    ngx_foo_ctx_t    *ctx;
    ngx_slab_pool_t  *shpool;

    value = shm_zone->data;

    if (octx) {
        /* reusing a shared zone from old cycle */
        ctx->value = octx->value;
        return NGX_OK;
    }

    shpool = (ngx_slab_pool_t *) shm_zone->shm.addr;

    if (shm_zone->shm.exists) {
        /* initialize shared zone context in Windows Angie worker */
        ctx->value = shpool->data;
        return NGX_OK;
    }

    /* initialize shared zone */

    ctx->value = ngx_slab_alloc(shpool, sizeof(ngx_uint_t));
    if (ctx->value == NULL) {
        return NGX_ERROR;
    }

    shpool->data = ctx->value;

    return NGX_OK;
}

Logging

For logging, Angie uses ngx_log_t objects. The Angie logger supports several types of output:

• stderr — logging to standard error (stderr)

• file — logging to a file

• syslog — logging to syslog

• memory — logging to internal memory storage for development purposes; the memory can be accessed later with a debugger

A logger instance can be a chain of loggers, linked to each other with the next field. In this case, each message is written to all loggers in the chain.

For each logger, a severity level controls which messages are written to the log (only events assigned that level or higher are logged). The following severity levels are supported:

• NGX_LOG_EMERG

• NGX_LOG_ALERT

• NGX_LOG_CRIT

• NGX_LOG_ERR

• NGX_LOG_WARN

• NGX_LOG_NOTICE

• NGX_LOG_INFO

• NGX_LOG_DEBUG

For debug logging, the debug mask is checked as well. The debug masks are:

• NGX_LOG_DEBUG_CORE

• NGX_LOG_DEBUG_ALLOC

• NGX_LOG_DEBUG_MUTEX

• NGX_LOG_DEBUG_EVENT

• NGX_LOG_DEBUG_HTTP

• NGX_LOG_DEBUG_MAIL

• NGX_LOG_DEBUG_STREAM

Normally, loggers are created by existing Angie code from error_log directives and are available at nearly every stage of processing in cycle, configuration, client connection and other objects.

Angie provides the following logging macros:

• ngx_log_error(level, log, err, fmt, ...) — error logging

• ngx_log_debug0(level, log, err, fmt), ngx_log_debug1(level, log, err, fmt, arg1) etc. — debug logging with up to eight supported formatting arguments

A log message is formatted in a buffer of size NGX_MAX_ERROR_STR (currently, 2048 bytes) on the stack. The message is prepended with the severity level, process ID (PID), connection ID (stored in log->connection), and the system error text. For non-debug messages, log->handler is called as well to prepend more specific information to the log message. The HTTP module sets the ngx_http_log_error() function as log handler to log client and server addresses, current action (stored in log->action), client request line, server name, etc.

/* specify what is currently done */
log->action = "sending mp4 to client";

/* error and debug log */
ngx_log_error(NGX_LOG_INFO, c->log, 0, "client prematurely
              closed connection");

ngx_log_debug2(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
               "mp4 start:%ui, length:%ui", mp4->start, mp4->length);

The example above results in log entries like these:

2016/09/16 22:08:52 [info] 17445#0: *1 client prematurely closed connection while
sending mp4 to client, client: 127.0.0.1, server: , request: "GET /file.mp4 HTTP/1.1"
2016/09/16 23:28:33 [debug] 22140#0: *1 mp4 start:0, length:10000

Cycles

A cycle object stores the Angie runtime context created from a specific configuration. Its type is ngx_cycle_t. The current cycle is referenced by the ngx_cycle global variable and inherited by Angie workers as they start. Each time the Angie configuration is reloaded, a new cycle is created from the new Angie configuration; the old cycle is usually deleted after the new one is successfully created.

A cycle is created by the ngx_init_cycle() function, which takes the previous cycle as its argument. The function locates the previous cycle's configuration file and inherits as many resources as possible from the previous cycle. A placeholder cycle called "init cycle" is created at Angie start, then is replaced by an actual cycle built from configuration.

Members of the cycle include:

• pool — Cycle pool. Created for each new cycle.

• log — Cycle log. Initially inherited from the old cycle, it is set to point to new_log after the configuration is read.

• new_log — Cycle log, created by the configuration. It's affected by the root-scope error_log directive.

• connections, connection_n — Array of connections of type ngx_connection_t, created by the event module while initializing each Angie worker. The worker_connections directive in the Angie configuration sets the number of connections connection_n.

• free_connections, free_connection_n — List and number of currently available connections. If no connections are available, an Angie worker refuses to accept new clients or connect to upstream servers.

• files, files_n — Array for mapping file descriptors to Angie connections. This mapping is used by the event modules having the NGX_USE_FD_EVENT flag (currently, it's poll and devpoll).

• conf_ctx — Array of core module configurations. The configurations are created and filled while reading Angie configuration files.

• modules, modules_n — Array of modules of type ngx_module_t, both static and dynamic, loaded by the current configuration.

• listening — Array of listening objects of type ngx_listening_t. Listening objects are normally added by the listen directive of different modules which call the ngx_create_listening() function. Listen sockets are created based on the listening objects.

• paths — Array of paths of type ngx_path_t. Paths are added by calling the function ngx_add_path() from modules which are going to operate on certain directories. These directories are created by Angie after reading configuration, if missing. Moreover, two handlers can be added for each path:

  • path loader — Executes only once in 60 seconds after starting or reloading Angie. Normally, the loader reads the directory and stores data in Angie shared memory. The handler is called from the dedicated Angie process "cache loader".

  • path manager — Executes periodically. Normally, the manager removes old files from the directory and updates Angie memory to reflect the changes. The handler is called from the dedicated "cache manager" process.

• open_files — List of open file objects of type ngx_open_file_t, which are created by calling the function ngx_conf_open_file(). Currently, Angie uses this kind of open files for logging. After reading the configuration, Angie opens all files in the open_files list and stores each file descriptor in the object's fd field. The files are opened in append mode and are created if missing. The files in the list are reopened by Angie workers upon receiving the reopen signal (most often USR1). In this case the descriptor in the fd field is changed to a new value.

• shared_memory — List of shared memory zones, each added by calling the ngx_shared_memory_add() function. Shared zones are mapped to the same address range in all Angie processes and are used to share common data, for example the HTTP cache in-memory tree.

Buffer

For input/output operations, Angie provides the buffer type ngx_buf_t. Normally, it's used to hold data to be written to a destination or read from a source. A buffer can reference data in memory or in a file and it's technically possible for a buffer to reference both at the same time. Memory for the buffer is allocated separately and is not related to the buffer structure ngx_buf_t.

The ngx_buf_t structure has the following fields:

• start, end — The boundaries of the memory block allocated for the buffer.

• pos, last — The boundaries of the memory buffer; normally a subrange of start .. end.

• file_pos, file_last — The boundaries of a file buffer, expressed as offsets from the beginning of the file.

• tag — Unique value used to distinguish buffers; created by different Angie modules, usually for the purpose of buffer reuse.

• file — File object.

• temporary — Flag indicating that the buffer references writable memory.

• memory — Flag indicating that the buffer references read-only memory.

• in_file — Flag indicating that the buffer references data in a file.

• flush — Flag indicating that all data prior to the buffer need to be flushed.

• recycled — Flag indicating that the buffer can be reused and needs to be consumed as soon as possible.

• sync — Flag indicating that the buffer carries no data or special signal like flush or last_buf. By default Angie considers such buffers an error condition, but this flag tells Angie to skip the error check.

• last_buf — Flag indicating that the buffer is the last in output.

• last_in_chain — Flag indicating that there are no more data buffers in a request or subrequest.

• shadow — Reference to another ("shadow") buffer related to the current buffer, usually in the sense that the buffer uses data from the shadow. When the buffer is consumed, the shadow buffer is normally also marked as consumed.

• last_shadow — Flag indicating that the buffer is the last one that references a particular shadow buffer.

• temp_file — Flag indicating that the buffer is in a temporary file.

For input and output operations buffers are linked in chains. A chain is a sequence of chain links of type ngx_chain_t, defined as follows:

typedef struct ngx_chain_s  ngx_chain_t;

struct ngx_chain_s {
    ngx_buf_t    *buf;
    ngx_chain_t  *next;
};

Each chain link keeps a reference to its buffer and a reference to the next chain link.

An example of using buffers and chains:

ngx_chain_t *
ngx_get_my_chain(ngx_pool_t *pool)
{
    ngx_buf_t    *b;
    ngx_chain_t  *out, *cl, **ll;

    /* first buf */
    cl = ngx_alloc_chain_link(pool);
    if (cl == NULL) { /* error */ }

    b = ngx_calloc_buf(pool);
    if (b == NULL) { /* error */ }

    b->start = (u_char *) "foo";
    b->pos = b->start;
    b->end = b->start + 3;
    b->last = b->end;
    b->memory = 1; /* read-only memory */

    cl->buf = b;
    out = cl;
    ll = &cl->next;

    /* second buf */
    cl = ngx_alloc_chain_link(pool);
    if (cl == NULL) { /* error */ }

    b = ngx_create_temp_buf(pool, 3);
    if (b == NULL) { /* error */ }

    b->last = ngx_cpymem(b->last, "foo", 3);

    cl->buf = b;
    cl->next = NULL;
    *ll = cl;

    return out;
}

Networking

Connections

The connection type ngx_connection_t is a wrapper around a socket descriptor. It includes the following fields:

• fd — Socket descriptor

• data — Arbitrary connection context. Normally, it is a pointer to a higher-level object built on top of the connection, such as an HTTP request or a Stream session.

• read, write — Read and write events for the connection.

• recv, send, recv_chain, send_chain — I/O operations for the connection.

• pool — Connection pool.

• log — Connection log.

• sockaddr, socklen, addr_text — Remote socket address in binary and text forms.

• local_sockaddr, local_socklen — Local socket address in binary form. Initially, these fields are empty. Use the ngx_connection_local_sockaddr() function to get the local socket address.

• proxy_protocol_addr, proxy_protocol_port — PROXY protocol client address and port, if the PROXY protocol is enabled for the connection.

• ssl — SSL context for the connection.

• reusable — Flag indicating the connection is in a state that makes it eligible for reuse.

• close — Flag indicating that the connection is being reused and needs to be closed.

An Angie connection can transparently encapsulate the SSL layer. In this case the connection's ssl field holds a pointer to an ngx_ssl_connection_t structure, keeping all SSL-related data for the connection, including SSL_CTX and SSL. The recv, send, recv_chain, and send_chain handlers are set to SSL-enabled functions as well.

The worker_connections directive in the Angie configuration limits the number of connections per Angie worker. All connection structures are precreated when a worker starts and stored in the connections field of the cycle object. To retrieve a connection structure, use the ngx_get_connection(s, log) function. It takes as its s argument a socket descriptor, which needs to be wrapped in a connection structure.

Because the number of connections per worker is limited, Angie provides a way to grab connections that are currently in use. To enable or disable reuse of a connection, call the ngx_reusable_connection(c, reusable) function. Calling ngx_reusable_connection(c, 1) sets the reuse flag in the connection structure and inserts the connection into the reusable_connections_queue of the cycle. Whenever ngx_get_connection() finds out there are no available connections in the cycle's free_connections list, it calls ngx_drain_connections() to release a specific number of reusable connections. For each such connection, the close flag is set and its read handler is called which is supposed to free the connection by calling ngx_close_connection(c) and make it available for reuse. To exit the state when a connection can be reused, ngx_reusable_connection(c, 0) is called. HTTP client connections are an example of reusable connections in Angie; they are marked as reusable until the first request byte is received from the client.

Events

Event

Event object ngx_event_t in Angie provides a mechanism for notification that a specific event has occurred.

Fields in ngx_event_t include:

• data — Arbitrary event context used in event handlers, usually as a pointer to a connection related to the event.

• handler — Callback function to be invoked when the event happens.

• write — Flag indicating a write event. Absence of the flag indicates a read event.

• active — Flag indicating that the event is registered for receiving I/O notifications, normally from notification mechanisms like epoll, kqueue, poll.

• ready — Flag indicating that the event has received an I/O notification.

• delayed — Flag indicating that I/O is delayed due to rate limiting.

• timer — Red-black tree node for inserting the event into the timer tree.

• timer_set — Flag indicating that the event timer is set and not yet expired.

• timedout — Flag indicating that the event timer has expired.

• eof — Flag indicating that EOF occurred while reading data.

• pending_eof — Flag indicating that EOF is pending on the socket, even though there may be some data available before it. The flag is delivered via the EPOLLRDHUP epoll event or EV_EOF kqueue flag.

• error — Flag indicating that an error occurred during reading (for a read event) or writing (for a write event).

• cancelable — Timer event flag indicating that the event should be ignored while shutting down the worker. Graceful worker shutdown is delayed until there are no non-cancelable timer events scheduled.

• posted — Flag indicating that the event is posted to a queue.

• queue — Queue node for posting the event to a queue.

I/O events

Each connection obtained by calling the ngx_get_connection() function has two attached events, c->read and c->write, which are used for receiving notification that the socket is ready for reading or writing. All such events operate in Edge-Triggered mode, meaning that they only trigger notifications when the state of the socket changes. For example, doing a partial read on a socket does not make Angie deliver a repeated read notification until more data arrives on the socket. Even when the underlying I/O notification mechanism is essentially Level-Triggered (poll, select etc), Angie converts the notifications to Edge-Triggered. To make Angie event notifications consistent across all notifications systems on different platforms, the functions ngx_handle_read_event(rev, flags) and ngx_handle_write_event(wev, lowat) must be called after handling an I/O socket notification or calling any I/O functions on that socket. Normally, the functions are called once at the end of each read or write event handler.

Timer events

An event can be set to send a notification when a timeout expires. The timer used by events counts milliseconds since some unspecified point in the past truncated to ngx_msec_t type. Its current value can be obtained from the ngx_current_msec variable.

The function ngx_add_timer(ev, timer) sets a timeout for an event, ngx_del_timer(ev) deletes a previously set timeout. The global timeout red-black tree ngx_event_timer_rbtree stores all timeouts currently set. The key in the tree is of type ngx_msec_t and is the time when the event occurs. The tree structure enables fast insertion and deletion operations, as well as access to the nearest timeouts, which Angie uses to find out how long to wait for I/O events and for expiring timeout events.

Posted events

An event can be posted which means that its handler will be called at some point later within the current event loop iteration. Posting events is a good practice for simplifying code and escaping stack overflows. Posted events are held in a post queue. The ngx_post_event(ev, q) macro posts the event ev to the post queue q. The ngx_delete_posted_event(ev) macro deletes the event ev from the queue it's currently posted in. Normally, events are posted to the ngx_posted_events queue, which is processed late in the event loop — after all I/O and timer events are already handled. The function ngx_event_process_posted() is called to process an event queue. It calls event handlers until the queue is empty. This means that a posted event handler can post more events to be processed within the current event loop iteration.

An example:

void
ngx_my_connection_read(ngx_connection_t *c)
{
    ngx_event_t  *rev;

    rev = c->read;

    ngx_add_timer(rev, 1000);

    rev->handler = ngx_my_read_handler;

    ngx_my_read(rev);
}


void
ngx_my_read_handler(ngx_event_t *rev)
{
    ssize_t            n;
    ngx_connection_t  *c;
    u_char             buf[256];

    if (rev->timedout) { /* timeout expired */ }

    c = rev->data;

    while (rev->ready) {
        n = c->recv(c, buf, sizeof(buf));

        if (n == NGX_AGAIN) {
            break;
        }

        if (n == NGX_ERROR) { /* error */ }

        /* process buf */
    }

    if (ngx_handle_read_event(rev, 0) != NGX_OK) { /* error */ }
}

Event loop

Except for the Angie master process, all Angie processes do I/O and so have an event loop. (The Angie master process instead spends most of its time in the sigsuspend() call waiting for signals to arrive.) The Angie event loop is implemented in the ngx_process_events_and_timers() function, which is called repeatedly until the process exits.

The event loop has the following stages:

• Find the timeout that is closest to expiring, by calling ngx_event_find_timer(). This function finds the leftmost node in the timer tree and returns the number of milliseconds until the node expires.

• Process I/O events by calling a handler, specific to the event notification mechanism, chosen by Angie configuration. This handler waits for at least one I/O event to happen, but only until the next timeout expires. When a read or write event occurs, the ready flag is set and the event's handler is called. For Linux, the ngx_epoll_process_events() handler is normally used, which calls epoll_wait() to wait for I/O events.

• Expire timers by calling ngx_event_expire_timers(). The timer tree is iterated from the leftmost element to the right until an unexpired timeout is found. For each expired node the timedout event flag is set, the timer_set flag is reset, and the event handler is called.

• Process posted events by calling ngx_event_process_posted(). The function repeatedly removes the first element from the posted events queue and calls the element's handler, until the queue is empty.

All Angie processes handle signals as well. Signal handlers only set global variables which are checked after the ngx_process_events_and_timers() call.

Processes

There are several types of processes in Angie. The type of a process is kept in the ngx_process global variable, and is one of the following:

• NGX_PROCESS_MASTER — The master process, which reads the NGINX configuration, creates cycles, and starts and controls child processes. It does not perform any I/O and responds only to signals. Its cycle function is ngx_master_process_cycle().

• NGX_PROCESS_WORKER — The worker process, which handles client connections. It is started by the master process and responds to its signals and channel commands as well. Its cycle function is ngx_worker_process_cycle(). There can be multiple worker processes, as configured by the worker_processes directive.

• NGX_PROCESS_SINGLE — The single process, which exists only in master_process off mode, and is the only process running in that mode. It creates cycles (like the master process does) and handles client connections (like the worker process does). Its cycle function is ngx_single_process_cycle().

• NGX_PROCESS_HELPER — The helper process, of which currently there are two types: cache manager and cache loader. The cycle function for both is ngx_cache_manager_process_cycle().

The Angie processes handle the following signals:

• NGX_SHUTDOWN_SIGNAL (SIGQUIT on most systems) — Gracefully shutdown. Upon receiving this signal, the master process sends a shutdown signal to all child processes. When no child processes are left, the master destroys the cycle pool and exits. When a worker process receives this signal, it closes all listening sockets and waits until there are no non-cancelable events scheduled, then destroys the cycle pool and exits. When the cache manager or the cache loader process receives this signal, it exits immediately. The ngx_quit variable is set to 1 when a process receives this signal, and is immediately reset after being processed. The ngx_exiting variable is set to 1 while a worker process is in the shutdown state.

• NGX_TERMINATE_SIGNAL (SIGTERM on most systems) — Terminate. Upon receiving this signal, the master process sends a terminate signal to all child processes. If a child process does not exit within 1 second, the master process sends the SIGKILL signal to kill it. When no child processes are left, the master process destroys the cycle pool and exits. When a worker process, the cache manager process or the cache loader process receives this signal, it destroys the cycle pool and exits. The variable ngx_terminate is set to 1 when this signal is received.

• NGX_NOACCEPT_SIGNAL (SIGWINCH on most systems) — Shut down all worker and helper processes. Upon receiving this signal, the master process shuts down its child processes. If a previously started new Angie binary exits, the child processes of the old master are started again. When a worker process receives this signal, it shuts down in debug mode set by the debug_points directive.

• NGX_RECONFIGURE_SIGNAL (SIGHUP on most systems) — Reconfigure. Upon receiving this signal, the master process re-reads the configuration and creates a new cycle based on it. If the new cycle is created successfully, the old cycle is deleted and new child processes are started. Meanwhile, the old child processes receive the NGX_SHUTDOWN_SIGNAL signal. In single-process mode, Angie creates a new cycle, but keeps the old one until there are no longer clients with active connections tied to it. The worker and helper processes ignore this signal.

• NGX_REOPEN_SIGNAL (SIGUSR1 on most systems) — Reopen files. The master process sends this signal to workers, which reopen all open_files related to the cycle.

• NGX_CHANGEBIN_SIGNAL (SIGUSR2 on most systems) — Change the Angie binary. The master process starts a new Angie binary and passes in a list of all listen sockets. The text-format list, passed in the "NGINX" environment variable, consists of descriptor numbers separated with semicolons. The new Angie binary reads the "NGINX" variable and adds the sockets to its init cycle. Other processes ignore this signal.

While all Angie worker processes are able to receive and properly handle POSIX signals, the master process does not use the standard kill() syscall to pass signals to workers and helpers. Instead, Angie uses inter-process socket pairs which allow sending messages between all Angie processes. Currently, however, messages are only sent from the master to its children. The messages carry the standard signals.

Threading

It is possible to offload into a separate thread tasks that would otherwise block the Angie worker process. For example, Angie can be configured to use threads to perform file I/O. Another use case is a library that doesn't have asynchronous interface and thus cannot be normally used with Angie. Keep in mind that the threads interface is a helper for the existing asynchronous approach to processing client connections, and by no means intended as a replacement.

To deal with synchronization, the following wrappers over pthreads primitives are available:

• typedef pthread_mutex_t  ngx_thread_mutex_t;

  • ngx_int_t ngx_thread_mutex_create(ngx_thread_mutex_t *mtx, ngx_log_t *log);

  • ngx_int_t ngx_thread_mutex_destroy(ngx_thread_mutex_t *mtx, ngx_log_t *log);

  • ngx_int_t ngx_thread_mutex_lock(ngx_thread_mutex_t *mtx, ngx_log_t *log);

  • ngx_int_t ngx_thread_mutex_unlock(ngx_thread_mutex_t *mtx, ngx_log_t *log);

• typedef pthread_cond_t  ngx_thread_cond_t;

  • ngx_int_t ngx_thread_cond_create(ngx_thread_cond_t *cond, ngx_log_t *log);

  • ngx_int_t ngx_thread_cond_destroy(ngx_thread_cond_t *cond, ngx_log_t *log);

  • ngx_int_t ngx_thread_cond_signal(ngx_thread_cond_t *cond, ngx_log_t *log);

  • ngx_int_t ngx_thread_cond_wait(ngx_thread_cond_t *cond, ngx_thread_mutex_t *mtx, ngx_log_t *log);

Instead of creating a new thread for each task, Angie implements a thread_pool strategy. Multiple thread pools may be configured for different purposes (for example, performing I/O on different sets of disks). Each thread pool is created at startup and contains a limited number of threads that process a queue of tasks. When a task is completed, a predefined completion handler is called.

The src/core/ngx_thread_pool.h header file contains relevant definitions:

struct ngx_thread_task_s {
    ngx_thread_task_t   *next;
    ngx_uint_t           id;
    void                *ctx;
    void               (*handler)(void *data, ngx_log_t *log);
    ngx_event_t          event;
};

typedef struct ngx_thread_pool_s  ngx_thread_pool_t;

ngx_thread_pool_t *ngx_thread_pool_add(ngx_conf_t *cf, ngx_str_t *name);
ngx_thread_pool_t *ngx_thread_pool_get(ngx_cycle_t *cycle, ngx_str_t *name);

ngx_thread_task_t *ngx_thread_task_alloc(ngx_pool_t *pool, size_t size);
ngx_int_t ngx_thread_task_post(ngx_thread_pool_t *tp, ngx_thread_task_t *task);

At configuration time, a module willing to use threads has to obtain a reference to a thread pool by calling ngx_thread_pool_add(cf, name), which either creates a new thread pool with the given name or returns a reference to the pool with that name if it already exists.

To add a task into a queue of a specified thread pool tp at runtime, use the ngx_thread_task_post(tp, task) function.

To execute a function in a thread, pass parameters and set up a completion handler using the ngx_thread_task_t structure:

typedef struct {
    int    foo;
} my_thread_ctx_t;


static void
my_thread_func(void *data, ngx_log_t *log)
{
    my_thread_ctx_t *ctx = data;

    /* this function is executed in a separate thread */
}


static void
my_thread_completion(ngx_event_t *ev)
{
    my_thread_ctx_t *ctx = ev->data;

    /* executed in Angie event loop */
}


ngx_int_t
my_task_offload(my_conf_t *conf)
{
    my_thread_ctx_t    *ctx;
    ngx_thread_task_t  *task;

    task = ngx_thread_task_alloc(conf->pool, sizeof(my_thread_ctx_t));
    if (task == NULL) {
        return NGX_ERROR;
    }

    ctx = task->ctx;

    ctx->foo = 42;

    task->handler = my_thread_func;
    task->event.handler = my_thread_completion;
    task->event.data = ctx;

    if (ngx_thread_task_post(conf->thread_pool, task) != NGX_OK) {
        return NGX_ERROR;
    }

    return NGX_OK;
}

Modules

Adding new modules

Each standalone Angie module resides in a separate directory that contains at least two files: config and a file with the module source code. The config file contains all information needed for Angie to integrate the module, for example:

ngx_module_type=CORE
ngx_module_name=ngx_foo_module
ngx_module_srcs="$ngx_addon_dir/ngx_foo_module.c"

. auto/module

ngx_addon_name=$ngx_module_name

The config file is a POSIX shell script that can set and access the following variables:

• ngx_module_type — Type of module to build. Possible values are CORE, HTTP, HTTP_FILTER, HTTP_INIT_FILTER, HTTP_AUX_FILTER, MAIL, STREAM, or MISC.

• ngx_module_name — Module names. To build multiple modules from a set of source files, specify a whitespace-separated list of names. The first name indicates the name of the output binary for the dynamic module. The names in the list must match the names used in the source code.

• ngx_addon_name — Name of the module as it appears in output on the console from the configure script.

• ngx_module_srcs — Whitespace-separated list of source files used to compile the module. The $ngx_addon_dir variable can be used to represent the path to the module directory.

• ngx_module_incs — Include paths required to build the module

• ngx_module_deps — Whitespace-separated list of the module's dependencies. Usually, it is the list of header files.

• ngx_module_libs — Whitespace-separated list of libraries to link with the module. For example, use ngx_module_libs=-lpthread to link libpthread library. The following macros can be used to link against the same libraries as Angie: LIBXSLT, LIBGD, GEOIP, PCRE, OPENSSL, MD5, SHA1, ZLIB, and PERL.

• ngx_module_link — Variable set by the build system to DYNAMIC for a dynamic module or ADDON for a static module and used to determine different actions to perform depending on linking type.

• ngx_module_order — Load order for the module; useful for the HTTP_FILTER and HTTP_AUX_FILTER module types. The format for this option is a whitespace-separated list of modules. All modules in the list following the current module's name end up after it in the global list of modules, which sets up the order for modules initialization. For filter modules later initialization means earlier execution.

  The following modules are typically used as references. The ngx_http_copy_filter_module reads the data for other filter modules and is placed near the bottom of the list so that it is one of the first to be executed. The ngx_http_write_filter_module writes the data to the client socket and is placed near the top of the list, and is the last to be executed.

  By default, filter modules are placed before the ngx_http_copy_filter in the module list so that the filter handler is executed after the copy filter handler. For other module types the default is the empty string.

To compile a module into Angie statically, use the --add-module=/path/to/module argument to the configure script. To compile a module for later dynamic loading into Angie, use the --add-dynamic-module=/path/to/module argument.

Core modules

Modules are the building blocks of Angie, and most of its functionality is implemented as modules. The module source file must contain a global variable of type ngx_module_t, which is defined as follows:

struct ngx_module_s {

    /* private part is omitted */

    void                 *ctx;
    ngx_command_t        *commands;
    ngx_uint_t            type;

    ngx_int_t           (*init_master)(ngx_log_t *log);

    ngx_int_t           (*init_module)(ngx_cycle_t *cycle);

    ngx_int_t           (*init_process)(ngx_cycle_t *cycle);
    ngx_int_t           (*init_thread)(ngx_cycle_t *cycle);
    void                (*exit_thread)(ngx_cycle_t *cycle);
    void                (*exit_process)(ngx_cycle_t *cycle);

    void                (*exit_master)(ngx_cycle_t *cycle);

    /* stubs for future extensions are omitted */
};

The omitted private part includes the module version and a signature and is filled using the predefined macro NGX_MODULE_V1.

Each module keeps its private data in the ctx field, recognizes the configuration directives, specified in the commands array, and can be invoked at certain stages of Angie lifecycle. The module lifecycle consists of the following events:

• Configuration directive handlers are called as they appear in configuration files in the context of the master process.

• After the configuration is parsed successfully, init_module handler is called in the context of the master process. The init_module handler is called in the master process each time a configuration is loaded.

• The master process creates one or more worker processes and the init_process handler is called in each of them.

• When a worker process receives the shutdown or terminate command from the master, it invokes the exit_process handler.

• The master process calls the exit_master handler before exiting.

Because threads are used in Angie only as a supplementary I/O facility with its own API, init_thread and exit_thread handlers are not currently called. There is also no init_master handler, because it would be unnecessary overhead.

The module type defines exactly what is stored in the ctx field. Its value is one of the following types:

• NGX_CORE_MODULE

• NGX_EVENT_MODULE

• NGX_HTTP_MODULE

• NGX_MAIL_MODULE

• NGX_STREAM_MODULE

The NGX_CORE_MODULE is the most basic and thus the most generic and most low-level type of module. The other module types are implemented on top of it and provide a more convenient way to deal with corresponding domains, like handling events or HTTP requests.

The set of core modules includes ngx_core_module, ngx_errlog_module, ngx_regex_module, ngx_thread_pool_module, and ngx_openssl_module modules. The HTTP module, the stream module, the mail module, and event modules are core modules too. The context of a core module is defined as:

typedef struct {
    ngx_str_t             name;
    void               *(*create_conf)(ngx_cycle_t *cycle);
    char               *(*init_conf)(ngx_cycle_t *cycle, void *conf);
} ngx_core_module_t;

where the name is a module name string, create_conf and init_conf are pointers to functions that create and initialize module configuration respectively. For core modules, Angie calls create_conf before parsing a new configuration and init_conf after all configuration is parsed successfully. The typical create_conf function allocates memory for the configuration and sets default values.

For example, a simplistic module called ngx_foo_module might look like this:

/*
 * Copyright (C) Author.
 */


#include <ngx_config.h>
#include <ngx_core.h>


typedef struct {
    ngx_flag_t  enable;
} ngx_foo_conf_t;


static void *ngx_foo_create_conf(ngx_cycle_t *cycle);
static char *ngx_foo_init_conf(ngx_cycle_t *cycle, void *conf);

static char *ngx_foo_enable(ngx_conf_t *cf, void *post, void *data);
static ngx_conf_post_t  ngx_foo_enable_post = { ngx_foo_enable };


static ngx_command_t  ngx_foo_commands[] = {

    { ngx_string("foo_enabled"),
      NGX_MAIN_CONF|NGX_DIRECT_CONF|NGX_CONF_FLAG,
      ngx_conf_set_flag_slot,
      0,
      offsetof(ngx_foo_conf_t, enable),
      &ngx_foo_enable_post },

      ngx_null_command
};


static ngx_core_module_t  ngx_foo_module_ctx = {
    ngx_string("foo"),
    ngx_foo_create_conf,
    ngx_foo_init_conf
};


ngx_module_t  ngx_foo_module = {
    NGX_MODULE_V1,
    &ngx_foo_module_ctx,                   /* module context */
    ngx_foo_commands,                      /* module directives */
    NGX_CORE_MODULE,                       /* module type */
    NULL,                                  /* init master */
    NULL,                                  /* init module */
    NULL,                                  /* init process */
    NULL,                                  /* init thread */
    NULL,                                  /* exit thread */
    NULL,                                  /* exit process */
    NULL,                                  /* exit master */
    NGX_MODULE_V1_PADDING
};


static void *
ngx_foo_create_conf(ngx_cycle_t *cycle)
{
    ngx_foo_conf_t  *fcf;

    fcf = ngx_pcalloc(cycle->pool, sizeof(ngx_foo_conf_t));
    if (fcf == NULL) {
        return NULL;
    }

    fcf->enable = NGX_CONF_UNSET;

    return fcf;
}


static char *
ngx_foo_init_conf(ngx_cycle_t *cycle, void *conf)
{
    ngx_foo_conf_t *fcf = conf;

    ngx_conf_init_value(fcf->enable, 0);

    return NGX_CONF_OK;
}


static char *
ngx_foo_enable(ngx_conf_t *cf, void *post, void *data)
{
    ngx_flag_t  *fp = data;

    if (*fp == 0) {
        return NGX_CONF_OK;
    }

    ngx_log_error(NGX_LOG_NOTICE, cf->log, 0, "Foo Module is enabled");

    return NGX_CONF_OK;
}

Configuration Directives

The ngx_command_t type defines a single configuration directive. Each module that supports configuration provides an array of such structures that describe how to process arguments and what handlers to call:

typedef struct ngx_command_s  ngx_command_t;

struct ngx_command_s {
    ngx_str_t             name;
    ngx_uint_t            type;
    char               *(*set)(ngx_conf_t *cf, ngx_command_t *cmd, void *conf);
    ngx_uint_t            conf;
    ngx_uint_t            offset;
    void                 *post;
};

Terminate the array with the special value ngx_null_command. The name is the name of the directive as it appears in the configuration file, for example, "worker_processes" or "listen". The type is a bitfield of flags that specify the number of arguments the directive takes, its type, and the context in which it appears. The flags are:

• NGX_CONF_NOARGS — The directive takes no arguments.

• NGX_CONF_1MORE — The directive takes one or more arguments.

• NGX_CONF_2MORE — The directive takes two or more arguments.

• NGX_CONF_TAKE1 .. NGX_CONF_TAKE7 — The directive takes exactly the indicated number of arguments.

• NGX_CONF_TAKE12, NGX_CONF_TAKE13, NGX_CONF_TAKE23, NGX_CONF_TAKE123, NGX_CONF_TAKE1234 — The directive can take a different number of arguments. Options are limited to the specified numbers. For example, NGX_CONF_TAKE12 means it takes one or two arguments.

Flags for directive types:

• NGX_CONF_BLOCK — The directive is a block, that is, it can contain other directives within its opening and closing braces, or even implement its own parser to handle content inside.

• NGX_CONF_FLAG — The directive takes a boolean value, either on or off.

The directive context defines where it can appear in the configuration:

• NGX_MAIN_CONF — In the top-level context.

• NGX_HTTP_MAIN_CONF — In the http block.

• NGX_HTTP_SRV_CONF — In a server block within the http block.

• NGX_HTTP_LOC_CONF — In a location block within the http block.

• NGX_HTTP_UPS_CONF — In an upstream block within the http block.

• NGX_HTTP_SIF_CONF — In an if block within a server block in the http block.

• NGX_HTTP_LIF_CONF — In an if block within a location block in the http block.

• NGX_HTTP_LMT_CONF — In a limit_except block within the http block.

• NGX_STREAM_MAIN_CONF — In the stream block.

• NGX_STREAM_SRV_CONF — In a server block within the stream block.

• NGX_STREAM_UPS_CONF — In an upstream block within the stream block.

• NGX_MAIL_MAIN_CONF — In the mail block.

• NGX_MAIL_SRV_CONF — In a server block within the mail block.

• NGX_EVENT_CONF — In the events block.

• NGX_DIRECT_CONF — Used by modules that don't create a hierarchy of contexts and have only a single global configuration. This configuration is passed to the handler as the conf argument.

The configuration parser uses these flags to throw an error for a misplaced directive and calls directive handlers supplied with the appropriate configuration pointer, so that the same directives in different locations can store their values in distinct locations.

The set field defines a handler that processes the directive and stores parsed values into the corresponding configuration. There are a number of functions that perform common conversions:

• ngx_conf_set_flag_slot — Converts the literal strings on and off into an ngx_flag_t value with values 1 or 0, respectively.

• ngx_conf_set_str_slot — Stores a string as a value of the ngx_str_t type.

• ngx_conf_set_str_array_slot — Appends a value to an array ngx_array_t of strings ngx_str_t. The array is created if it does not already exist.

• ngx_conf_set_keyval_slot — Appends a key-value pair to an array ngx_array_t of key-value pairs ngx_keyval_t. The first string becomes the key and the second the value. The array is created if it does not already exist.

• ngx_conf_set_num_slot — Converts a directive's argument to an ngx_int_t value.

• ngx_conf_set_size_slot — Converts a size to a size_t value expressed in bytes.

• ngx_conf_set_off_slot — Converts an offset to an off_t value expressed in bytes.

• ngx_conf_set_msec_slot — Converts a time to an ngx_msec_t value expressed in milliseconds.

• ngx_conf_set_sec_slot — Converts a time to a time_t value expressed in seconds.

• ngx_conf_set_bufs_slot — Converts the two supplied arguments into an ngx_bufs_t object that holds the number and size of buffers.

• ngx_conf_set_enum_slot — Converts the supplied argument to an ngx_uint_t value. The null-terminated array of ngx_conf_enum_t passed in the post field defines the acceptable strings and corresponding integer values.

• ngx_conf_set_bitmask_slot — Converts the supplied arguments to an ngx_uint_t value. The mask values for each argument are ORed producing the result. The null-terminated array of ngx_conf_bitmask_t passed in the post field defines the acceptable strings and corresponding mask values.

• ngx_conf_set_path_slot — Converts the supplied arguments to an ngx_path_t value and performs all necessary initializations. For details, see the documentation for the proxy_temp_path directive.

• ngx_conf_set_access_slot — Converts the supplied arguments to a file permissions mask. For details, see the documentation for the proxy_store_access directive.

The conf field defines which configuration structure is passed to the directive handler. Core modules only have the global configuration and set the NGX_DIRECT_CONF flag to access it. Modules such as HTTP, Stream, or Mail create hierarchies of configurations. For example, a module's configuration is created for the server, location, and if scopes.

• NGX_HTTP_MAIN_CONF_OFFSET — Configuration for the http block.

• NGX_HTTP_SRV_CONF_OFFSET — Configuration for a server block within the http block.

• NGX_HTTP_LOC_CONF_OFFSET — Configuration for a location block within the http block.

• NGX_STREAM_MAIN_CONF_OFFSET — Configuration for the stream block.

• NGX_STREAM_SRV_CONF_OFFSET — Configuration for a server block within the stream block.

• NGX_MAIL_MAIN_CONF_OFFSET — Configuration for the mail block.

• NGX_MAIL_SRV_CONF_OFFSET — Configuration for a server block within the mail block.

The offset field defines the offset of a field in a module's configuration structure that holds values for this particular directive. The typical use is to employ the offsetof() macro.

The post field has two purposes: it can be used to define a handler to be called after the main handler has completed, or to pass additional data to the main handler. In the first case, the ngx_conf_post_t structure needs to be initialized with a pointer to the handler, for example:

static char *ngx_do_foo(ngx_conf_t *cf, void *post, void *data);
static ngx_conf_post_t  ngx_foo_post = { ngx_do_foo };

The post argument is the ngx_conf_post_t object itself, and the data is a pointer to the value, converted from arguments by the main handler with the appropriate type.

HTTP

Connection

Each HTTP client connection runs through the following stages:

• ngx_event_accept() accepts a client TCP connection. This handler is called in response to a read notification on a listen socket. A new ngx_connection_t object is created at this stage to wrap the newly accepted client socket. Each Angie listener provides a handler to pass the new connection object to. For HTTP connections it's ngx_http_init_connection(c).

• ngx_http_init_connection() performs early initialization of the HTTP connection. At this stage an ngx_http_connection_t object is created for the connection and its reference is stored in the connection's data field. Later it will be replaced by an HTTP request object. A PROXY protocol parser and the SSL handshake are started at this stage as well.

• ngx_http_wait_request_handler() read event handler is called when data is available on the client socket. At this stage an HTTP request object ngx_http_request_t is created and set to the connection's data field.

• ngx_http_process_request_line() read event handler reads client request line. The handler is set by ngx_http_wait_request_handler(). The data is read into connection's buffer. The size of the buffer is initially set by the directive client_header_buffer_size. The entire client header is supposed to fit in the buffer. If the initial size is not sufficient, a bigger buffer is allocated, with the capacity set by the large_client_header_buffers directive.

• ngx_http_process_request_headers() read event handler, is set after ngx_http_process_request_line() to read the client request header.

• ngx_http_core_run_phases() is called when the request header is completely read and parsed. This function runs request phases from NGX_HTTP_POST_READ_PHASE to NGX_HTTP_CONTENT_PHASE. The last phase is intended to generate a response and pass it along the filter chain. The response is not necessarily sent to the client at this phase. It might remain buffered and be sent at the finalization stage.

• ngx_http_finalize_request() is usually called when the request has generated all the output or produced an error. In the latter case an appropriate error page is looked up and used as the response. If the response is not completely sent to the client by this point, an HTTP writer ngx_http_writer() is activated to finish sending outstanding data.

• ngx_http_finalize_connection() is called when the complete response has been sent to the client and the request can be destroyed. If the client connection keepalive feature is enabled, ngx_http_set_keepalive() is called, which destroys the current request and waits for the next request on the connection. Otherwise, ngx_http_close_request() destroys both the request and the connection.

Request

For each client HTTP request the ngx_http_request_t object is created. Some of the fields of this object are:

• connection — Pointer to a ngx_connection_t client connection object. Several requests can reference the same connection object at the same time - one main request and its subrequests. After a request is deleted, a new request can be created on the same connection.

  Note that for HTTP connections ngx_connection_t's data field points back to the request. Such requests are called active, as opposed to the other requests tied to the connection. An active request is used to handle client connection events and is allowed to output its response to the client. Normally, each request becomes active at some point so that it can send its output.

• ctx — Array of HTTP module contexts. Each module of type NGX_HTTP_MODULE can store any value (normally, a pointer to a structure) in the request. The value is stored in the ctx array at the module's ctx_index position. The following macros provide a convenient way to get and set request contexts:

  • ngx_http_get_module_ctx(r, module) — Returns the module's context

  • ngx_http_set_ctx(r, c, module) — Sets c as the module's context

• main_conf, srv_conf, loc_conf — Arrays of current request configurations. Configurations are stored at the module's ctx_index positions.

• read_event_handler, write_event_handler - Read and write event handlers for the request. Normally, both the read and write event handlers for an HTTP connection are set to ngx_http_request_handler(). This function calls the read_event_handler and write_event_handler handlers for the currently active request.

• cache — Request cache object for caching the upstream response.

• upstream — Request upstream object for proxying.

• pool — Request pool. The request object itself is allocated in this pool, which is destroyed when the request is deleted. For allocations that need to be available throughout the client connection's lifetime, use ngx_connection_t's pool instead.

• header_in — Buffer into which the client HTTP request header is read.

• headers_in, headers_out — Input and output HTTP headers objects. Both objects contain the headers field of type ngx_list_t for keeping the raw list of headers. In addition to that, specific headers are available for getting and setting as separate fields, for example content_length_n, status etc.

• request_body — Client request body object.

• start_sec, start_msec — Time point when the request was created, used for tracking request duration.

• method, method_name — Numeric and text representation of the client HTTP request method. Numeric values for methods are defined in src/http/ngx_http_request.h with the macros NGX_HTTP_GET, NGX_HTTP_HEAD, NGX_HTTP_POST, etc.

• http_protocol — Client HTTP protocol version in its original text form ("HTTP/1.0", "HTTP/1.1" etc).

• http_version — Client HTTP protocol version in numeric form (NGX_HTTP_VERSION_10, NGX_HTTP_VERSION_11, etc.).

• http_major, http_minor — Client HTTP protocol version in numeric form split into major and minor parts.

• request_line, unparsed_uri — Request line and URI in the original client request.

• uri, args, exten — URI, arguments and file extension for the current request. The URI value here might differ from the original URI sent by the client due to normalization. Throughout request processing, these values can change as internal redirects are performed.

• main — Pointer to a main request object. This object is created to process a client HTTP request, as opposed to subrequests, which are created to perform a specific subtask within the main request.

• parent — Pointer to the parent request of a subrequest.

• postponed — List of output buffers and subrequests, in the order in which they are sent and created. The list is used by the postpone filter to provide consistent request output when parts of it are created by subrequests.

• post_subrequest — Pointer to a handler with the context to be called when a subrequest gets finalized. Unused for main requests.

• posted_requests — List of requests to be started or resumed, which is done by calling the request's write_event_handler. Normally, this handler holds the request main function, which at first runs request phases and then produces the output.

  A request is usually posted by the ngx_http_post_request(r, NULL) call. It is always posted to the main request posted_requests list. The function ngx_http_run_posted_requests(c) runs all requests that are posted in the main request of the passed connection's active request. All event handlers call ngx_http_run_posted_requests, which can lead to new posted requests. Normally, it is called after invoking a request's read or write handler.

• phase_handler — Index of current request phase.

• ncaptures, captures, captures_data — Regex captures produced by the last regex match of the request. A regex match can occur at a number of places during request processing: map lookup, server lookup by SNI or HTTP Host, rewrite, proxy_redirect, etc. Captures produced by a lookup are stored in the above mentioned fields. The field ncaptures holds the number of captures, captures holds captures boundaries and captures_data holds the string against which the regex was matched and which is used to extract captures. After each new regex match, request captures are reset to hold new values.

• count — Request reference counter. The field only makes sense for the main request. Increasing the counter is done by simple r->main->count++. To decrease the counter, call ngx_http_finalize_request(r, rc). Creation of a subrequest and running the request body read process both increment the counter.

• subrequests — Current subrequest nesting level. Each subrequest inherits its parent's nesting level, decreased by one. An error is generated if the value reaches zero. The value for the main request is defined by the NGX_HTTP_MAX_SUBREQUESTS constant.

• uri_changes — Number of URI changes remaining for the request. The total number of times a request can change its URI is limited by the NGX_HTTP_MAX_URI_CHANGES constant. With each change the value is decremented until it reaches zero, at which point an error is generated. Rewrites and internal redirects to normal or named locations are considered URI changes.

• blocked — Counter of blocks held on the request. While this value is non-zero, the request cannot be finalized. Currently, this value is increased by pending AIO operations (POSIX AIO and thread operations) and active cache locks.

• buffered — Bitmask showing which modules have buffered output produced by the request. A number of filters can buffer output; for example, sub_filter can buffer data because of a partial string match, copy filter can buffer data because of a lack of free output buffers, etc. As long as this value is non-zero, the request is not finalized, pending a flush.

• header_only — Flag indicating that output does not require a body. For example, this flag is used by HTTP HEAD requests.

• keepalive — Flag indicating whether client connection keepalive is supported. The value is inferred from the HTTP version and the value of the "Connection" header.

• header_sent — Flag indicating that the output header has already been sent by the request.

• internal — Flag indicating that the current request is internal. To enter the internal state, a request must pass through an internal redirect or be a subrequest. Internal requests are allowed to enter internal locations.

• allow_ranges — Flag indicating that a partial response can be sent to the client, as requested by the HTTP Range header.

• subrequest_ranges — Flag indicating that a partial response can be sent while processing a subrequest.

• single_range — Flag indicating that only a single continuous range of output data can be sent to the client. This flag is usually set when sending a stream of data, for example, from a proxy server, and the entire response is not available in a single buffer.

• main_filter_need_in_memory, filter_need_in_memory — Flags requesting that output be produced in memory buffers but not in files. This is a signal to the copy filter to read data from file buffers even if sendfile is enabled. The difference between the two flags is the location of the filter modules that set them. Filters called before the postpone filter in the filter chain set filter_need_in_memory, requesting that only the current request's output come into memory buffers. Filters called later in the filter chain set main_filter_need_in_memory, requesting that both the main request and all subrequests read files into memory when sending output.

• filter_need_temporary — Flag requesting that the request output be produced in temporary buffers, but not in read-only memory buffers or file buffers. This is used by filters that may change the output directly in the buffers where it is sent.

HTTP Module Configuration

Each HTTP module can have three types of configuration:

• Main configuration — Applies to the entire http block. Serves as global settings for the module.

• Server configuration — Applies to a single server block. Serves as server-specific settings for the module.

• Location configuration — Applies to a single location, if, or limit_except block. Serves as location-specific settings for the module.

Configuration structures are created at the Angie configuration stage by calling functions that allocate the structures, initialize them, and merge them. The following example shows how to create a simple location configuration for a module. The configuration has one setting, foo, of type unsigned integer.

typedef struct {
    ngx_uint_t  foo;
} ngx_http_foo_loc_conf_t;


static ngx_http_module_t  ngx_http_foo_module_ctx = {
    NULL,                                  /* preconfiguration */
    NULL,                                  /* postconfiguration */

    NULL,                                  /* create main configuration */
    NULL,                                  /* init main configuration */

    NULL,                                  /* create server configuration */
    NULL,                                  /* merge server configuration */

    ngx_http_foo_create_loc_conf,          /* create location configuration */
    ngx_http_foo_merge_loc_conf            /* merge location configuration */
};


static void *
ngx_http_foo_create_loc_conf(ngx_conf_t *cf)
{
    ngx_http_foo_loc_conf_t  *conf;

    conf = ngx_pcalloc(cf->pool, sizeof(ngx_http_foo_loc_conf_t));
    if (conf == NULL) {
        return NULL;
    }

    conf->foo = NGX_CONF_UNSET_UINT;

    return conf;
}


static char *
ngx_http_foo_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child)
{
    ngx_http_foo_loc_conf_t *prev = parent;
    ngx_http_foo_loc_conf_t *conf = child;

    ngx_conf_merge_uint_value(conf->foo, prev->foo, 1);
}

As seen in the example, the ngx_http_foo_create_loc_conf() function creates a new configuration structure, and ngx_http_foo_merge_loc_conf() merges a configuration with configuration from a higher level. In fact, server and location configurations exist not only at the server and location levels, but are also created for all levels above them. Specifically, a server configuration is also created at the main level, and location configurations are created at the main, server, and location levels. These configurations make it possible to specify server- and location-specific settings at any level of an Angie configuration file. Eventually configurations are merged down. A number of macros, such as NGX_CONF_UNSET and NGX_CONF_UNSET_UINT, are provided for indicating a missing setting and ignoring it during the merge. Standard Angie merge macros, such as ngx_conf_merge_value() and ngx_conf_merge_uint_value(), provide a convenient way to merge a setting and set the default value if none of the configurations provided an explicit value. For a complete list of macros for different types, see src/core/ngx_conf_file.h.

The following macros are available for accessing configuration of HTTP modules at configuration time. They all take ngx_conf_t reference as the first argument.

• ngx_http_conf_get_module_main_conf(cf, module)

• ngx_http_conf_get_module_srv_conf(cf, module)

• ngx_http_conf_get_module_loc_conf(cf, module)

The following example obtains a pointer to a location configuration of the standard core HTTP module and replaces the location content handler stored in the handler field of the structure.

static ngx_int_t ngx_http_foo_handler(ngx_http_request_t *r);


static ngx_command_t  ngx_http_foo_commands[] = {

    { ngx_string("foo"),
      NGX_HTTP_LOC_CONF|NGX_CONF_NOARGS,
      ngx_http_foo,
      0,
      0,
      NULL },

      ngx_null_command
};


static char *
ngx_http_foo(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    ngx_http_core_loc_conf_t  *clcf;

    clcf = ngx_http_conf_get_module_loc_conf(cf, ngx_http_core_module);
    clcf->handler = ngx_http_bar_handler;

    return NGX_CONF_OK;
}

The following macros are available for accessing configuration of HTTP modules at runtime.

• ngx_http_get_module_main_conf(r, module)

• ngx_http_get_module_srv_conf(r, module)

• ngx_http_get_module_loc_conf(r, module)

These macros receive a reference to an HTTP request ngx_http_request_t. The main configuration of a request never changes. Server configuration can change from the default after choosing a virtual server for a request. The location configuration selected for processing a request can change multiple times as a result of a rewrite operation or internal redirect. The following example shows how to access HTTP configuration of a module at runtime.

static ngx_int_t
ngx_http_foo_handler(ngx_http_request_t *r)
{
    ngx_http_foo_loc_conf_t  *flcf;

    flcf = ngx_http_get_module_loc_conf(r, ngx_http_foo_module);

    ...
}

Phases

Each HTTP request passes through a sequence of phases. In each phase a distinct type of processing is performed on the request. Module-specific handlers can be registered in most phases, and many standard Angie modules register their phase handlers as a way of being invoked at a specific stage of request processing. Phases are processed successively and the phase handlers are called once the request reaches the phase. Following is the list of Angie HTTP phases.

• NGX_HTTP_POST_READ_PHASE — First phase. The RealIP module registers its handler at this phase to enable substitution of client addresses before any other module is invoked.

• NGX_HTTP_SERVER_REWRITE_PHASE — Phase where rewrite directives defined in a server block (but outside a location block) are processed. The Rewrite module installs its handler at this phase.

• NGX_HTTP_FIND_CONFIG_PHASE — Special phase where a location is chosen based on the request URI. Before this phase, the default location for the relevant virtual server is assigned to the request, and any module requesting a location configuration receives the configuration for the default server location. This phase assigns a new location to the request. No additional handlers can be registered at this phase.

• NGX_HTTP_REWRITE_PHASE — Same as NGX_HTTP_SERVER_REWRITE_PHASE, but for rewrite rules defined in the location chosen in the previous phase.

• NGX_HTTP_POST_REWRITE_PHASE — Special phase where the request is redirected to a new location if its URI changed during a rewrite. This is implemented by the request going through the NGX_HTTP_FIND_CONFIG_PHASE again. No additional handlers can be registered at this phase.

• NGX_HTTP_PREACCESS_PHASE — A common phase for different types of handlers, not associated with access control. The standard Angie modules Limit Conn and Limit Req register their handlers at this phase.

• NGX_HTTP_ACCESS_PHASE — Phase where it is verified that the client is authorized to make the request. Standard Angie modules such as Access and Auth Basic register their handlers at this phase. By default the client must pass the authorization check of all handlers registered at this phase for the request to continue to the next phase. The satisfy directive can be used to permit processing to continue if any of the phase handlers authorizes the client.

• NGX_HTTP_POST_ACCESS_PHASE — Special phase where the satisfy directive is processed. If some access phase handlers denied access and none explicitly allowed it, the request is finalized. No additional handlers can be registered at this phase.

• NGX_HTTP_PRECONTENT_PHASE — Phase for handlers to be called prior to generating content. Standard modules such as try_files and Mirror register their handlers at this phase.

• NGX_HTTP_CONTENT_PHASE — Phase where the response is normally generated. Multiple standard Angie modules register their handlers at this phase, including Index. They are called sequentially until one of them produces the output. It's also possible to set content handlers on a per-location basis. If the HTTP Module module's location configuration has handler set, it is called as the content handler and the handlers installed at this phase are ignored.

• NGX_HTTP_LOG_PHASE — Phase where request logging is performed. Currently, only the Log module registers its handler at this stage for access logging. Log phase handlers are called at the very end of request processing, right before freeing the request.

Following is an example of a preaccess phase handler.

static ngx_http_module_t  ngx_http_foo_module_ctx = {
    NULL,                                  /* preconfiguration */
    ngx_http_foo_init,                     /* postconfiguration */

    NULL,                                  /* create main configuration */
    NULL,                                  /* init main configuration */

    NULL,                                  /* create server configuration */
    NULL,                                  /* merge server configuration */

    NULL,                                  /* create location configuration */
    NULL                                   /* merge location configuration */
};


static ngx_int_t
ngx_http_foo_handler(ngx_http_request_t *r)
{
    ngx_table_elt_t  *ua;

    ua = r->headers_in.user_agent;

    if (ua == NULL) {
        return NGX_DECLINED;
    }

    /* reject requests with "User-Agent: foo" */
    if (ua->value.len == 3 && ngx_strncmp(ua->value.data, "foo", 3) == 0) {
        return NGX_HTTP_FORBIDDEN;
    }

    return NGX_DECLINED;
}


static ngx_int_t
ngx_http_foo_init(ngx_conf_t *cf)
{
    ngx_http_handler_pt        *h;
    ngx_http_core_main_conf_t  *cmcf;

    cmcf = ngx_http_conf_get_module_main_conf(cf, ngx_http_core_module);

    h = ngx_array_push(&cmcf->phases[NGX_HTTP_PREACCESS_PHASE].handlers);
    if (h == NULL) {
        return NGX_ERROR;
    }

    *h = ngx_http_foo_handler;

    return NGX_OK;
}

Phase handlers are expected to return specific codes:

• NGX_OK — proceed to the next phase.

• NGX_DECLINED — proceed to the next handler of the current phase. If the current handler is the last in the current phase, move to the next phase.

• NGX_AGAIN, NGX_DONE — suspend phase handling until some future event, which could be an asynchronous I/O operation or just a delay, for example. It is assumed that phase handling will be resumed later by calling ngx_http_core_run_phases().

• Any other value returned by the phase handler is treated as a request finalization code, in particular, an HTTP response code. The request is finalized with the provided code.

For some phases, return codes are treated in a slightly different way. At the content phase, any return code other than NGX_DECLINED is considered a finalization code. Any return code from the location content handlers is considered a finalization code. At the access phase, in satisfy any mode, returning a code other than NGX_OK, NGX_DECLINED, NGX_AGAIN, NGX_DONE is considered a denial. If no subsequent access handlers allow or deny access with a different code, the denial code will become the finalization code.

Examples

The nginx-dev-examples repository provides examples of nginx modules suitable for Angie as well.

Code style

General rules

• maximum text width is 80 characters

• indentation is 4 spaces

• no tabs, no trailing spaces

• list elements on the same line are separated with spaces

• hexadecimal literals are lowercase

• file names, function and type names, and global variables have the ngx_ prefix or a more specific prefix such as ngx_http_ and ngx_mail_

size_t
ngx_utf8_length(u_char *p, size_t n)
{
    u_char  c, *last;
    size_t  len;

    last = p + n;

    for (len = 0; p < last; len++) {

        c = *p;

        if (c < 0x80) {
            p++;
            continue;
        }

        if (ngx_utf8_decode(&p, last - p) > 0x10ffff) {
            /* invalid UTF-8 */
            return n;
        }
    }

    return len;
}

Files

A typical source file may contain the following sections, separated by two blank lines:

• copyright statements

• includes

• preprocessor definitions

• type definitions

• function prototypes

• variable definitions

• function definitions

Copyright statements look like this:

/*
 * Copyright (C) Author Name
 * Copyright (C) Organization, Inc.
 */

If the file is modified significantly, the list of authors should be updated, the new author is added to the top.

The ngx_config.h and ngx_core.h files are always included first, followed by one of ngx_http.h, ngx_stream.h, or ngx_mail.h. Then follow optional external header files:

#include <ngx_config.h>
#include <ngx_core.h>
#include <ngx_http.h>

#include <libxml/parser.h>
#include <libxml/tree.h>
#include <libxslt/xslt.h>

#if (NGX_HAVE_EXSLT)
#include <libexslt/exslt.h>
#endif

Header files should include the so-called "header guard":

#ifndef _NGX_PROCESS_CYCLE_H_INCLUDED_
#define _NGX_PROCESS_CYCLE_H_INCLUDED_
...
#endif /* _NGX_PROCESS_CYCLE_H_INCLUDED_ */

Comments

• // comments are not used

• text is in English, American spelling is preferred

• multi-line comments are formatted like this:

  /*
   * The red-black tree code is based on the algorithm described in
   * the "Introduction to Algorithms" by Cormen, Leiserson and Rivest.
   */
  

  /* find the server configuration for the address:port */
  

Preprocessor

Macro names start with the ngx_ or NGX_ prefix (or more specific). Macro names for constants are uppercase. Parameterized macros and macros for initializers are lowercase. The macro name and value are separated by at least two spaces:

#define NGX_CONF_BUFFER  4096

#define ngx_buf_in_memory(b)  (b->temporary || b->memory || b->mmap)

#define ngx_buf_size(b)                                                      \
    (ngx_buf_in_memory(b) ? (off_t) (b->last - b->pos):                      \
                            (b->file_last - b->file_pos))

#define ngx_null_string  { 0, NULL }

Conditions are inside parentheses, negation is outside:

#if (NGX_HAVE_KQUEUE)
...
#elif ((NGX_HAVE_DEVPOLL && !(NGX_TEST_BUILD_DEVPOLL)) \
       || (NGX_HAVE_EVENTPORT && !(NGX_TEST_BUILD_EVENTPORT)))
...
#elif (NGX_HAVE_EPOLL && !(NGX_TEST_BUILD_EPOLL))
...
#elif (NGX_HAVE_POLL)
...
#else /* select */
...
#endif /* NGX_HAVE_KQUEUE */

Types

Type names end with the _t suffix. A defined type name is separated by at least two spaces:

typedef ngx_uint_t  ngx_rbtree_key_t;

Structure types are defined using typedef. Inside structures, member types and names are aligned:

typedef struct {
    size_t      len;
    u_char     *data;
} ngx_str_t;

Keep the same alignment between different structures in the file. A structure that points to itself has a name ending with _s. Adjacent structure definitions are separated by two blank lines:

typedef struct ngx_list_part_s  ngx_list_part_t;

struct ngx_list_part_s {
    void             *elts;
    ngx_uint_t        nelts;
    ngx_list_part_t  *next;
};


typedef struct {
    ngx_list_part_t  *last;
    ngx_list_part_t   part;
    size_t            size;
    ngx_uint_t        nalloc;
    ngx_pool_t       *pool;
} ngx_list_t;

Each structure member is declared on its own line:

typedef struct {
    ngx_uint_t        hash;
    ngx_str_t         key;
    ngx_str_t         value;
    u_char           *lowcase_key;
} ngx_table_elt_t;

Function pointers inside structures have defined types ending with _pt:

typedef ssize_t (*ngx_recv_pt)(ngx_connection_t *c, u_char *buf, size_t size);
typedef ssize_t (*ngx_recv_chain_pt)(ngx_connection_t *c, ngx_chain_t *in,
    off_t limit);
typedef ssize_t (*ngx_send_pt)(ngx_connection_t *c, u_char *buf, size_t size);
typedef ngx_chain_t *(*ngx_send_chain_pt)(ngx_connection_t *c, ngx_chain_t *in,
    off_t limit);

typedef struct {
    ngx_recv_pt        recv;
    ngx_recv_chain_pt  recv_chain;
    ngx_recv_pt        udp_recv;
    ngx_send_pt        send;
    ngx_send_pt        udp_send;
    ngx_send_chain_pt  udp_send_chain;
    ngx_send_chain_pt  send_chain;
    ngx_uint_t         flags;
} ngx_os_io_t;

Enumerations have types ending with _e:

typedef enum {
    ngx_http_fastcgi_st_version = 0,
    ngx_http_fastcgi_st_type,
    ...
    ngx_http_fastcgi_st_padding
} ngx_http_fastcgi_state_e;

Variables

Variables are declared sorted by length of the base type, then alphabetically. Type names and variable names are aligned. The type and name "columns" are separated by two spaces. Large arrays are put at the end of a declaration block:

u_char                      *rv, *p;
ngx_conf_t                  *cf;
ngx_uint_t                   i, j, k;
unsigned int                 len;
struct sockaddr             *sa;
const unsigned char         *data;
ngx_peer_connection_t       *pc;
ngx_http_core_srv_conf_t   **cscfp;
ngx_http_upstream_srv_conf_t *us, *uscf;
u_char                       text[NGX_SOCKADDR_STRLEN];

Static and global variables may be initialized on declaration:

static ngx_str_t  ngx_http_memcached_key = ngx_string("memcached_key");

static ngx_uint_t  mday[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

static uint32_t  ngx_crc32_table16[] = {
    0x00000000, 0x1db71064, 0x3b6e20c8, 0x26d930ac,
    ...
    0x9b64c2b0, 0x86d3d2d4, 0xa00ae278, 0xbdbdf21c
};

There is a bunch of commonly used type/name combinations:

u_char                        *rv;
ngx_int_t                      rc;
ngx_conf_t                    *cf;
ngx_connection_t              *c;
ngx_http_request_t            *r;
ngx_peer_connection_t         *pc;
ngx_http_upstream_srv_conf_t  *us, *uscf;

Functions

All functions (even static ones) should have prototypes. Prototypes include argument names. Long prototypes are wrapped with a single indentation on continuation lines:

static char *ngx_http_block(ngx_conf_t *cf, ngx_command_t *cmd, void *conf);
static ngx_int_t ngx_http_init_phases(ngx_conf_t *cf,
    ngx_http_core_main_conf_t *cmcf);

static char *ngx_http_merge_servers(ngx_conf_t *cf,
    ngx_http_core_main_conf_t *cmcf, ngx_http_module_t *module,
    ngx_uint_t ctx_index);

The function name in a definition starts on a new line. The opening and closing braces for the function body are on separate lines. The function body is indented. There are two empty lines between functions:

static ngx_int_t
ngx_http_find_virtual_server(ngx_http_request_t *r, u_char *host, size_t len)
{
    ...
}


static ngx_int_t
ngx_http_add_addresses(ngx_conf_t *cf, ngx_http_core_srv_conf_t *cscf,
    ngx_http_conf_port_t *port, ngx_http_listen_opt_t *lsopt)
{
    ...
}

There is no space after the function name and opening parenthesis. Long function calls are wrapped so that continuation lines start at the position of the first function argument. If this is impossible, format the first continuation line so that it ends at position 79:

ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
               "http header: \"%V: %V\"",
               &h->key, &h->value);

hc->busy = ngx_palloc(r->connection->pool,
                  cscf->large_client_header_buffers.num * sizeof(ngx_buf_t *));

The ngx_inline macro should be used instead of inline:

static ngx_inline void ngx_cpuid(uint32_t i, uint32_t *buf);

Expressions

Binary operators except . and -> should be separated from their operands by one space. Unary operators and subscripts are not separated from their operands by spaces:

width = width * 10 + (*fmt++ - '0');

ch = (u_char) ((decoded << 4) + (ch - '0'));

r->exten.data = &r->uri.data[i + 1];

Type casts are separated by one space from casted expressions. An asterisk inside a type cast is separated by a space from the type name:

len = ngx_sock_ntop((struct sockaddr *) sin6, p, len, 1);

If an expression does not fit into a single line, it is wrapped. The preferred point to break a line is a binary operator. The continuation line is aligned with the start of expression:

if (status == NGX_HTTP_MOVED_PERMANENTLY
    || status == NGX_HTTP_MOVED_TEMPORARILY
    || status == NGX_HTTP_SEE_OTHER
    || status == NGX_HTTP_TEMPORARY_REDIRECT
    || status == NGX_HTTP_PERMANENT_REDIRECT)
{
    ...
}

p->temp_file->warn = "an upstream response is buffered "
                     "to a temporary file";

As a last resort, it is possible to wrap an expression so that the continuation line ends at position 79:

hinit->hash = ngx_pcalloc(hinit->pool, sizeof(ngx_hash_wildcard_t)
                                     + size * sizeof(ngx_hash_elt_t *));

The above rules also apply to sub-expressions, where each sub-expression has its own indentation level:

if (((u->conf->cache_use_stale & NGX_HTTP_UPSTREAM_FT_UPDATING)
     || c->stale_updating) && !r->background
    && u->conf->cache_background_update)
{
    ...
}

Sometimes it is convenient to wrap an expression after a type cast. In this case, the continuation line is indented:

node = (ngx_rbtree_node_t *)
           ((u_char *) lr - offsetof(ngx_rbtree_node_t, color));

Pointers are explicitly compared with NULL (not 0):

if (ptr != NULL) {
    ...
}

Conditionals and Loops

The if keyword is separated from the condition by one space. The opening brace is located on the same line, or on a dedicated line if the condition takes several lines. The closing brace is located on a dedicated line, optionally followed by else if / else. Usually, there is an empty line before the else if / else part:

if (node->left == sentinel) {
    temp = node->right;
    subst = node;

} else if (node->right == sentinel) {
    temp = node->left;
    subst = node;

} else {
    subst = ngx_rbtree_min(node->right, sentinel);

    if (subst->left != sentinel) {
        temp = subst->left;

    } else {
        temp = subst->right;
    }
}

Similar formatting rules apply to do and while loops:

while (p < last && *p == ' ') {
    p++;
}

while (p < last && *p == ' ') {
    p++;
}

do {
    ctx->node = rn;
    ctx = ctx->next;
} while (ctx);

The switch keyword is separated from the condition by one space. The opening brace is located on the same line. The closing brace is located on a dedicated line. The case keywords are aligned with switch:

switch (ch) {
case '!':
    looked = 2;
    state = ssi_comment0_state;
    break;

case '<':
    copy_end = p;
    break;

default:
    copy_end = p;
    looked = 0;
    state = ssi_start_state;
    break;
}

Most for loops are formatted as follows:

for (i = 0; i < ccf->env.nelts; i++) {
    ...
}

for (q = ngx_queue_head(locations);
     q != ngx_queue_sentinel(locations);
     q = ngx_queue_next(q))
{
    ...
}

If some part of the for statement is omitted, this is indicated by the /* void */ comment:

for (i = 0; /* void */ ; i++) {
    ...
}

A loop with an empty body is also indicated by the /* void */ comment which may be placed on the same line:

for (cl = *busy; cl->next; cl = cl->next) { /* void */ }

An endless loop looks like this:

for ( ;; ) {
    ...
}

Labels

Labels are surrounded by empty lines and are indented at the previous level:

    if (i == 0) {
        u->err = "host not found";
        goto failed;
    }

    u->addrs = ngx_pcalloc(pool, i * sizeof(ngx_addr_t));
    if (u->addrs == NULL) {
        goto failed;
    }

    u->naddrs = i;

    ...

    return NGX_OK;

failed:

    freeaddrinfo(res);
    return NGX_ERROR;

Debugging Memory Issues

To debug memory issues such as buffer overruns or use-after-free errors, you can use AddressSanitizer (ASan), supported by some modern compilers. To enable ASan with gcc and clang, use the -fsanitize=address compiler and linker option. When building Angie, this can be done by adding the option to --with-cc-opt and --with-ld-opt parameters of the configure script.

Since most allocations in Angie are made from Angie internal pool, enabling ASan may not always be enough to debug memory issues. The internal pool allocates a big chunk of memory from the system and cuts smaller allocations from it. However, this mechanism can be disabled by setting the NGX_DEBUG_PALLOC macro to 1. In this case, allocations are passed directly to the system allocator, giving it full control over buffer boundaries.

The following configuration line summarizes the information provided above. It is recommended while developing third-party modules and testing Angie on different platforms.

auto/configure --with-cc-opt='-fsanitize=address -DNGX_DEBUG_PALLOC=1'
               --with-ld-opt=-fsanitize=address

Common Pitfalls

Writing a C module

The most common pitfall is an attempt to write a full-fledged C module when it can be avoided. In most cases your task can be accomplished by creating a proper configuration. If writing a module is inevitable, try to make it as small and simple as possible. For example, a module can only export some variables.

Before starting a module, consider the following questions:

• Is it possible to implement a desired feature using already available modules?

• Is it possible to solve an issue using built-in scripting languages, such as Perl or NJS?

C Strings

The most used string type in Angie, ngx_str_t is not a C-style zero-terminated string. You cannot pass the data to standard C library functions such as strlen() or strstr(). Instead, Angie counterparts that accept either ngx_str_t should be used or pointer to data and a length. However, there is a case when ngx_str_t holds a pointer to a zero-terminated string: strings that come as a result of configuration file parsing are zero-terminated.

Global Variables

Avoid using global variables in your modules. Most likely it is an error to have a global variable. Any global data should be tied to a configuration cycle and be allocated from the corresponding memory pool. This allows Angie to perform graceful configuration reloads. An attempt to use global variables will likely break this feature, because it will be impossible to have two configurations at the same time and get rid of them. Sometimes global variables are required. In this case, special attention is needed to manage reconfiguration properly. Also, check if libraries used by your code have implicit global state that may be broken on reload.

Manual Memory Management

Instead of dealing with malloc/free approach which is error prone, learn how to use Angie pools. A pool is created and tied to an object - configuration, cycle, connection <#http_connection>, or HTTP request. When the object is destroyed, the associated pool is destroyed too. So when working with an object, it is possible to allocate the amount needed from the corresponding pool and not worry about freeing memory even in case of errors.

Threads

It is recommended to avoid using threads in Angie because it will definitely break things: most Angie functions are not thread-safe. It is expected that a thread will be executing only system calls and thread-safe library functions. If you need to run some code that is not related to client request processing, the proper way is to schedule a timer in the init_process module handler and perform required actions in timer handler. Internally Angie makes use of threads to boost IO-related operations, but this is a special case with a lot of limitations.

Blocking Libraries

A common mistake is to use libraries that are blocking internally. Most libraries out there are synchronous and blocking by nature. In other words, they perform one operation at a time and waste time waiting for response from other peer. As a result, when a request is processed with such library, whole Angie worker is blocked, thus destroying performance. Use only libraries that provide asynchronous interface and don't block whole process.

HTTP Requests to External Services

Often modules need to perform an HTTP call to some external service. A common mistake is to use some external library, such as libcurl, to perform the HTTP request. It is absolutely unnecessary to bring a huge amount of external (probably blocking!) code for the task which can be accomplished by Angie itself.

There are two basic usage scenarios when an external request is needed:

• in the context of processing a client request (for example, in content handler)

• in the context of a worker process (for example, timer handler)

In the first case, the best is to use subrequests API. Instead of directly accessing external service, you declare a location in Angie configuration and direct your subrequest to this location. This location is not limited to proxying requests, but may contain other Angie directives. An example of such approach is the auth_request directive implemented in Auth Request.

For the second case, it is possible to use basic HTTP client functionality available in Angie. For example, OCSP module implements simple HTTP client.