1---
2c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
3SPDX-License-Identifier: curl
4Title: curl_multi_socket_action
5Section: 3
6Source: libcurl
7See-also:
8  - curl_multi_cleanup (3)
9  - curl_multi_fdset (3)
10  - curl_multi_info_read (3)
11  - curl_multi_init (3)
12  - the hiperfifo.c example
13Protocol:
14  - All
15---
16
17# NAME
18
19curl_multi_socket_action - reads/writes available data given an action
20
21# SYNOPSIS
22
23~~~c
24#include <curl/curl.h>
25
26CURLMcode curl_multi_socket_action(CURLM *multi_handle,
27                                   curl_socket_t sockfd,
28                                   int ev_bitmask,
29                                   int *running_handles);
30~~~
31
32# DESCRIPTION
33
34When the application has detected action on a socket handled by libcurl, it
35should call curl_multi_socket_action(3) with the **sockfd** argument
36set to the socket with the action. When the events on a socket are known, they
37can be passed as an events bitmask **ev_bitmask** by first setting
38**ev_bitmask** to 0, and then adding using bitwise OR (|) any combination of
39events to be chosen from CURL_CSELECT_IN, CURL_CSELECT_OUT or
40CURL_CSELECT_ERR. When the events on a socket are unknown, pass 0 instead, and
41libcurl tests the descriptor internally. It is also permissible to pass
42CURL_SOCKET_TIMEOUT to the **sockfd** parameter in order to initiate the
43whole process or when a timeout occurs.
44
45At return, **running_handles** points to the number of running easy handles
46within the multi handle. When this number reaches zero, all transfers are
47complete/done. When you call curl_multi_socket_action(3) on a specific
48socket and the counter decreases by one, it DOES NOT necessarily mean that
49this exact socket/transfer is the one that completed. Use
50curl_multi_info_read(3) to figure out which easy handle that completed.
51
52The curl_multi_socket_action(3) function informs the application about
53updates in the socket (file descriptor) status by doing none, one, or multiple
54calls to the socket callback function set with the
55CURLMOPT_SOCKETFUNCTION(3) option to curl_multi_setopt(3). They
56update the status with changes since the previous time the callback was
57called.
58
59Get the timeout time by setting the CURLMOPT_TIMERFUNCTION(3) option
60with curl_multi_setopt(3). Your application then gets called with
61information on how long to wait for socket actions at most before doing the
62timeout action: call the curl_multi_socket_action(3) function with the
63**sockfd** argument set to CURL_SOCKET_TIMEOUT. You can also use the
64curl_multi_timeout(3) function to poll the value at any given time, but
65for an event-based system using the callback is far better than relying on
66polling the timeout value.
67
68When this function returns error, the state of all transfers are uncertain and
69they cannot be continued. curl_multi_socket_action(3) should not be
70called again on the same multi handle after an error has been returned, unless
71first removing all the handles and adding new ones.
72
73# TYPICAL USAGE
74
751. Create a multi handle
76
772. Set the socket callback with CURLMOPT_SOCKETFUNCTION(3)
78
793. Set the timeout callback with CURLMOPT_TIMERFUNCTION(3), to get to
80know what timeout value to use when waiting for socket activities.
81
824. Add easy handles with curl_multi_add_handle()
83
845. Provide some means to manage the sockets libcurl is using, so you can check
85them for activity. This can be done through your application code, or by way
86of an external library such as libevent or glib.
87
886. Call curl_multi_socket_action(..., CURL_SOCKET_TIMEOUT, 0, ...)
89to kickstart everything. To get one or more callbacks called.
90
917. Wait for activity on any of libcurl's sockets, use the timeout value your
92callback has been told.
93
948, When activity is detected, call curl_multi_socket_action() for the
95socket(s) that got action. If no activity is detected and the timeout expires,
96call curl_multi_socket_action(3) with *CURL_SOCKET_TIMEOUT*.
97
98# EXAMPLE
99
100~~~c
101int main(void)
102{
103  /* the event-library gets told when there activity on the socket 'fd',
104     which we translate to a call to curl_multi_socket_action() */
105  int running;
106  CURLM *multi; /* the stack we work with */
107  int fd; /* the descriptor that had action */
108  int bitmask; /* what activity that happened */
109  CURLMcode mc = curl_multi_socket_action(multi, fd, bitmask, &running);
110  if(mc)
111    printf("error: %s\n", curl_multi_strerror(mc));
112}
113~~~
114
115# AVAILABILITY
116
117This function was added in libcurl 7.15.4, and is deemed stable since 7.16.0.
118
119# RETURN VALUE
120
121CURLMcode type, general libcurl multi interface error code. See
122libcurl-errors(3)
123