Event handler function

Mongoose Docs https://www.cesanta.com/docs/#send-and-receive-buffers

User Guide#

Introduction#

Mongoose is a networking library for C/C++. It implements an event-driven, non-blocking APIs for TCP, UDP, HTTP, WebSocket, MQTT. It has been designed for connecting devices and bringing them online. On the market since 2004, used by vast number of open source and commercial products - it even runs on the International Space Station! Mongoose makes embedded network programming fast, robust, and easy.

Features#

• Cross-platform: works on Linux/UNIX, MacOS, QNX, eCos, Windows, Android, iPhone, FreeRTOS, etc
• Supported hardware platforms: TI CC3200, TI MSP432, NRF52, STM32, PIC32, ESP8266, ESP32 and more
• Builtin protocols:
  • plain TCP, plain UDP, SSL/TLS (over TCP, one-way or two-way)
  • HTTP client, HTTP server
  • WebSocket client, WebSocket server
  • MQTT client
  • DNS client, async DNS resolver
• Single-threaded, asynchronous, non-blocking core with simple event-based API
• Native support for LWIP embedded TCP/IP stack
• Tiny static and run-time footprint
• Source code is both ISO C and ISO C++ compliant
• Very easy to integrate: just copy mongoose.c and mongoose.h files to your build tree

Concept#

Mongoose has three basic data structures:

• struct mg_mgr - an event manager that holds all active connections
• struct mg_connection - describes a connection
• struct mg_iobuf - describes data buffer (received or sent data)

Connections could be either listening, outbound or inbound. Outbound connections are created by the mg_connect() call. Listening connections are created by the mg_listen() call. Inbound connections are those accepted by a listening connection. Each connection is described by a struct mg_connection structure, which has a number of fields. All fields are exposed to the application by design, to give an application a full visibility into the Mongoose's internals.

An application that uses mongoose should follow a standard pattern of event-driven application:

1. Declare and initialise an event manager:

   struct mg_mgr mgr;
   mg_mgr_init(&mgr);

2. Create connections. For example, a server application should create listening connections. When any connection is created (listening or outgoing), an event handler function must be specified. An event handler function defines connection's behavior.

   struct mg_connection *c = mg_http_listen(&mgr, "0.0.0.0:8000", fn, arg);

3. Create an event loop by calling mg_mgr_poll():

   for (;;) {
     mg_mgr_poll(&mgr, 1000);
   }

mg_mgr_poll() iterates over all sockets, accepts new connections, sends and receives data, closes connections and calls event handler functions for the respective events.

Since the Mongoose's core is not protected against concurrent accesses, make sure that all mg_* API functions are called from the same thread or RTOS task.

Send and receive buffers#

Each connection has a send and receive buffer:

• struct mg_connection::send - data to be sent to a peer
• struct mg_connection::recv - data received from a peer

When data arrives, Mongoose appends received data to the recv and triggers an MG_EV_RECV event. The user may send data back by calling one of the output functions, like mg_send() or mg_printf(). Output functions append data to the send buffer. When Mongoose successfully writes data to the socket, it discards data from struct mg_connection::send and sends an MG_EV_SEND event.

Event handler function#

Each connection has an event handler function associated with it. That function must be implemented by the user. Event handler is the key element of the Mongoose application, since it defines the connection's behaviour. This is what an event handler function looks like:

static void fn(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
  switch (ev) {
    /* Event handler code that defines behavior of the connection */
    ...
  }
}

• struct mg_connection *c - a connection that received an event
• int ev - an event number, defined in mongoose.h. For example, when data arrives on an inbound connection, ev would be MG_EV_RECV
• void *ev_data - points to the event-specific data, and it has a different meaning for different events. For example, for an MG_EV_RECV event, ev_data is an int * pointing to the number of bytes received from a remote peer and saved into the c->recv IO buffer. The exact meaning of ev_data is described for each event. Protocol-specific events usually have ev_data pointing to structures that hold protocol-specific information
• void *fn_data - a user-defined pointer for the connection, which is a placeholder for application-specific data

Events#

Below is the list of events trigged by Mongoose, taken as-is from mongoose.h. For each event, a comment describes a meaning of the ev_data pointer passed to an event handler:

enum {
  MG_EV_ERROR,      // Error                        char *error_message
  MG_EV_POLL,       // mg_mgr_poll iteration        unsigned long *millis
  MG_EV_RESOLVE,    // Host name is resolved        NULL
  MG_EV_CONNECT,    // Connection established       NULL
  MG_EV_ACCEPT,     // Connection accepted          NULL
  MG_EV_READ,       // Data received from socket    struct mg_str *
  MG_EV_WRITE,      // Data written to socket       int *num_bytes_written
  MG_EV_CLOSE,      // Connection closed            NULL
  MG_EV_HTTP_MSG,   // HTTP request/response        struct mg_http_message *
  MG_EV_WS_OPEN,    // Websocket handshake done     struct mg_http_message *
  MG_EV_WS_MSG,     // Websocket msg, text or bin   struct mg_ws_message *
  MG_EV_WS_CTL,     // Websocket control msg        struct mg_ws_message *
  MG_EV_MQTT_CMD,   // MQTT low-level command       struct mg_mqtt_message *
  MG_EV_MQTT_MSG,   // MQTT PUBLISH received        struct mg_mqtt_message *
  MG_EV_MQTT_OPEN,  // MQTT CONNACK received        int *connack_status_code
  MG_EV_SNTP_TIME,  // SNTP time received           struct timeval *
  MG_EV_USER,       // Starting ID for user events
};

Connection flags#

struct mg_connection has a bitfield with connection flags. Flags are binary, they can be either 0 or 1. Some flags are set by Mongoose and must be not changed by an application code, for example is_udp flag tells application if that connection is UDP or not. Some flags can be changed by application, for example, is_drainig flag, if set by an application, tells Mongoose to send the remaining data to peer, and when everything is sent, close the connection.

User-changeable flags are: is_hexdumping, is_drainig, is_closing.

This is taken from mongoose.h as-is:

struct mg_connection {
  ...
  unsigned is_listening : 1;   // Listening connection
  unsigned is_client : 1;      // Outbound (client) connection
  unsigned is_accepted : 1;    // Accepted (server) connection
  unsigned is_resolving : 1;   // Non-blocking DNS resolv is in progress
  unsigned is_connecting : 1;  // Non-blocking connect is in progress
  unsigned is_tls : 1;         // TLS-enabled connection
  unsigned is_tls_hs : 1;      // TLS handshake is in progress
  unsigned is_udp : 1;         // UDP connection
  unsigned is_websocket : 1;   // WebSocket connection
  unsigned is_hexdumping : 1;  // Hexdump in/out traffic
  unsigned is_draining : 1;    // Send remaining data, then close and free
  unsigned is_closing : 1;     // Close and free the connection immediately
  unsigned is_readable : 1;    // Connection is ready to read
  unsigned is_writable : 1;    // Connection is ready to write
};

Build options#

Mongoose source code ships in two files:

• mongoose.h - API definitions
• mongoose.c - implementation

Therefore to integrate Mongoose into an application, simply copy these two files to the application's source tree.

The mongoose.c and mongoose.h files are, actually, an amalgamation - a non-amalgamated sources can be found at https://github.com/cesanta/mongoose/tree/master/src

Mongoose source code uses a bunch of build constants defined at https://github.com/cesanta/mongoose/blob/master/src/config.h, together with their default values.

In order to change the constant during build time, use the -D <PREPROCESSOR_FLAG> compiler option. For example, to disable both MQTT, compile the application my_app.c like this (assumed UNIX system):

$ cc my_app.c mongoose.c -D MG_MQTT_ENABLE=0

Here is a list of build constants and their default values:

Name	Default	Description
MG_ENABLE_LWIP	0	Use LWIP low-level API instead of BSD sockets
MG_ENABLE_SOCKET	1	Use BSD socket low-level API
MG_ENABLE_MBEDTLS	0	Enable Mbed TLS library
MG_ENABLE_OPENSSL	0	Enable OpenSSL library
MG_ENABLE_FS	1	Enable API that use filesystem, like mg_http_send_file()
MG_ENABLE_IPV6	0	Enable IPv6
MG_ENABLE_LOG	1	Enable LOG() macro
MG_ENABLE_MD5	0	Use native MD5 implementation
MG_ENABLE_DIRECTORY_LISTING	0	Enable directory listing for HTTP server
MG_ENABLE_HTTP_DEBUG_ENDPOINT	0	Enable /debug/info debug URI
MG_ENABLE_SOCKETPAIR	0	Enable mg_socketpair() for multi-threading
MG_ENABLE_SSI	0	Enable serving SSI files by mg_http_serve_dir()
MG_IO_SIZE	512	Granularity of the send/recv IO buffer growth
MG_MAX_RECV_BUF_SIZE	(3 * 1024 * 1024)	Maximum recv buffer size
MG_MAX_HTTP_HEADERS	40	Maximum number of HTTP headers

NOTE: MG_IO_SIZE controls the maximum UDP message size, see https://github.com/cesanta/mongoose/issues/907 for details. If application uses large UDP messages, increase the MG_IO_SIZE limit accordingly.

Minimal HTTP server#

This example is a simple static HTTP server that serves current directory:

#include "mongoose.h"

static const char *s_web_root_dir = ".";
static const char *s_listening_address = "http://localhost:8000";

static void cb(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
  struct mg_http_serve_opts opts = {.root_dir = s_web_root_dir};
  if (ev == MG_EV_HTTP_MSG) mg_http_serve_dir(c, ev_data, &opts);
}

int main(int argc, char *argv[]) {
  struct mg_mgr mgr;
  mg_mgr_init(&mgr);
  mg_http_listen(&mgr, s_listening_address, cb, &mgr);
  for (;;) mg_mgr_poll(&mgr, 1000);
  mg_mgr_free(&mgr);
  return 0;
}

Minimal TCP echo server#

This example is a simple TCP echo server that listens on port 1234:

#include "mongoose.h"

static const char *s_listening_address = "tcp://0.0.0.0:1234";

static void cb(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
  if (ev == MG_EV_RECV) {
    mg_send(c, c->recv.buf, c->recv.len);     // Echo received data back
    mg_iobuf_delete(&c->recv, c->recv.len);   // And discard it
  }
}

int main(int argc, char *argv[]) {
  struct mg_mgr mgr;
  mg_mgr_init(&mgr);
  mg_listen(&mgr, s_listening_address, cb, &mgr);
  for (;;) mg_mgr_poll(&mgr, 1000);
  mg_mgr_free(&mgr);
  return 0;
}

API Reference#

Core#

struct mg_mgr#

struct mg_mgr {
  struct mg_connection *conns;  // List of active connections
  struct mg_connection *dnsc;   // DNS resolver connection
  const char *dnsserver;        // DNS server URL
  int dnstimeout;               // DNS resolve timeout in milliseconds
};

Event management structure that holds a list of active connections, together with some housekeeping information.

struct mg_connection#

struct mg_connection {
  struct mg_connection *next;  // Linkage in struct mg_mgr :: connections
  struct mg_mgr *mgr;          // Our container
  struct mg_addr peer;         // Remote peer address
  void *fd;                    // Connected socket, or LWIP data
  struct mg_iobuf recv;        // Incoming data
  struct mg_iobuf send;        // Outgoing data
  mg_event_handler_t fn;       // User-specified event handler function
  void *fn_data;               // User-speficied function parameter
  mg_event_handler_t pfn;      // Protocol-specific handler function
  void *pfn_data;              // Protocol-specific function parameter
  char label[32];              // Arbitrary label
  void *tls;                   // TLS specific data
  unsigned is_listening : 1;   // Listening connection
  unsigned is_client : 1;      // Outbound (client) connection
  unsigned is_accepted : 1;    // Accepted (server) connection
  unsigned is_resolving : 1;   // Non-blocking DNS resolv is in progress
  unsigned is_connecting : 1;  // Non-blocking connect is in progress
  unsigned is_tls : 1;         // TLS-enabled connection
  unsigned is_tls_hs : 1;      // TLS handshake is in progress
  unsigned is_udp : 1;         // UDP connection
  unsigned is_websocket : 1;   // WebSocket connection
  unsigned is_hexdumping : 1;  // Hexdump in/out traffic
  unsigned is_draining : 1;    // Send remaining data, then close and free
  unsigned is_closing : 1;     // Close and free the connection immediately
  unsigned is_readable : 1;    // Connection is ready to read
  unsigned is_writable : 1;    // Connection is ready to write
};

A connection - either a listening connection, or an accepted connection, or an outbout connection.

mg_mgr_init()#

void mg_mgr_init(struct mg_mgr *);

Initialise event manager structure: set a list of active connections to NULL, set DNS server and timeout to their default values, etc.

mg_mgr_poll()#

void mg_mgr_poll(struct mg_mgr *mgr, int ms);

Perform a single poll iteration. For each connection in the mgr->conns list,

• See if there is incoming data. If it is, read it into the c->recv buffer, send MG_EV_RECV event
• See if there is data in the c->send buffer, and write it, send MG_EV_WRITE event
• If a connection is listening, accept an incoming connection if any, and send MG_EV_ACCEPT event to it
• Send MG_EV_POLL event

Each connection has two event handler functions: c->fn and c->pfn. The c->fn is a user-specified event handler function. The c->pfn is a protocol-specific handler function that is set implicitly. For example, a mg_http_listen() sets c->pfn to a Mongoose's HTTP event handler. A protocol-specific handler is called before user-specific handler. It parses incoming data and may invoke protocol-specific events like MG_EV_HTTP_MSG.

mg_mgr_free()#

void mg_mgr_free(struct mg_mgr *mgr);

Close all connections, and free all resources.

mg_listen()#

struct mg_connection *mg_listen(struct mg_mgr *mgr, const char *url,
                                mg_event_handler_t fn, void *fn_data);

Create a listening connection, append this connection to mgr->conns.

• url - specifies local IP address and port to listen on, e.g. tcp://127.0.0.1:1234 or udp://0.0.0.0:9000
• fn - an event handler function
• fn_data - an arbitrary pointer, which will be passed as fn_data when an event handler is called. This pointer is also stored in a connection structure as c->fn_data

Return value: created connection, or NULL on error.

mg_connect()#

struct mg_connection *mg_connect(struct mg_mgr *mgr, const char *url,
                                 mg_event_handler_t fn, void *fn_data);

Create an outbout connection, append this connection to mgr->conns.

• url - specifies remote IP address/port to connect to, e.g. http://a.com
• fn - an event handler function
• fn_data - an arbitrary pointer, which will be passed as fn_data when an event handler is called. This pointer is also stored in a connection structure as c->fn_data

Return value: created connection, or NULL on error.

mg_send()#

int mg_send(struct mg_connection *c, const void *data, size_t size);

Append data of size size to the c->send buffer. Return number of bytes appended.

Note: this function does not push data to the network! It only appends data to the output buffer. The data is being sent when mg_mgr_poll() is called. If mg_send() is called multiple times, the output buffer grows.