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1---
2c: Copyright (C) Daniel Stenberg, <daniel.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
13---
14
15# NAME
16
17curl_multi_socket_action - reads/writes available data given an action
18
19# SYNOPSIS
20
21~~~c
22#include <curl/curl.h>
23
24CURLMcode curl_multi_socket_action(CURLM *multi_handle,
25                                   curl_socket_t sockfd,
26                                   int ev_bitmask,
27                                   int *running_handles);
28~~~
29
30# DESCRIPTION
31
32When the application has detected action on a socket handled by libcurl, it
33should call curl_multi_socket_action(3) with the **sockfd** argument
34set to the socket with the action. When the events on a socket are known, they
35can be passed as an events bitmask **ev_bitmask** by first setting
36**ev_bitmask** to 0, and then adding using bitwise OR (|) any combination of
37events to be chosen from CURL_CSELECT_IN, CURL_CSELECT_OUT or
38CURL_CSELECT_ERR. When the events on a socket are unknown, pass 0 instead, and
39libcurl tests the descriptor internally. It is also permissible to pass
40CURL_SOCKET_TIMEOUT to the **sockfd** parameter in order to initiate the
41whole process or when a timeout occurs.
42
43At return, **running_handles** points to the number of running easy handles
44within the multi handle. When this number reaches zero, all transfers are
45complete/done. When you call curl_multi_socket_action(3) on a specific
46socket and the counter decreases by one, it DOES NOT necessarily mean that
47this exact socket/transfer is the one that completed. Use
48curl_multi_info_read(3) to figure out which easy handle that completed.
49
50The curl_multi_socket_action(3) function informs the application about
51updates in the socket (file descriptor) status by doing none, one, or multiple
52calls to the socket callback function set with the
53CURLMOPT_SOCKETFUNCTION(3) option to curl_multi_setopt(3). They
54update the status with changes since the previous time the callback was
55called.
56
57Get the timeout time by setting the CURLMOPT_TIMERFUNCTION(3) option
58with curl_multi_setopt(3). Your application then gets called with
59information on how long to wait for socket actions at most before doing the
60timeout action: call the curl_multi_socket_action(3) function with the
61**sockfd** argument set to CURL_SOCKET_TIMEOUT. You can also use the
62curl_multi_timeout(3) function to poll the value at any given time, but
63for an event-based system using the callback is far better than relying on
64polling the timeout value.
65
66When this function returns error, the state of all transfers are uncertain and
67they cannot be continued. curl_multi_socket_action(3) should not be
68called again on the same multi handle after an error has been returned, unless
69first removing all the handles and adding new ones.
70
71# TYPICAL USAGE
72
731. Create a multi handle
74
752. Set the socket callback with CURLMOPT_SOCKETFUNCTION(3)
76
773. Set the timeout callback with CURLMOPT_TIMERFUNCTION(3), to get to
78know what timeout value to use when waiting for socket activities.
79
804. Add easy handles with curl_multi_add_handle()
81
825. Provide some means to manage the sockets libcurl is using, so you can check
83them for activity. This can be done through your application code, or by way
84of an external library such as libevent or glib.
85
866. Call curl_multi_socket_action(..., CURL_SOCKET_TIMEOUT, 0, ...)
87to kickstart everything. To get one or more callbacks called.
88
897. Wait for activity on any of libcurl's sockets, use the timeout value your
90callback has been told.
91
928, When activity is detected, call curl_multi_socket_action() for the
93socket(s) that got action. If no activity is detected and the timeout expires,
94call curl_multi_socket_action(3) with *CURL_SOCKET_TIMEOUT*.
95
96# EXAMPLE
97
98~~~c
99int main(void)
100{
101  /* the event-library gets told when there activity on the socket 'fd',
102     which we translate to a call to curl_multi_socket_action() */
103  int running;
104  CURLM *multi; /* the stack we work with */
105  int fd; /* the descriptor that had action */
106  int bitmask; /* what activity that happened */
107  CURLMcode mc = curl_multi_socket_action(multi, fd, bitmask, &running);
108  if(mc)
109    printf("error: %s\n", curl_multi_strerror(mc));
110}
111~~~
112
113# AVAILABILITY
114
115This function was added in libcurl 7.15.4, and is deemed stable since 7.16.0.
116
117# RETURN VALUE
118
119CURLMcode type, general libcurl multi interface error code. See
120libcurl-errors(3)
121