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1---
2c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
3SPDX-License-Identifier: curl
4Title: curl_multi_perform
5Section: 3
6Source: libcurl
7See-also:
8  - curl_multi_add_handle (3)
9  - curl_multi_cleanup (3)
10  - curl_multi_fdset (3)
11  - curl_multi_info_read (3)
12  - curl_multi_init (3)
13  - curl_multi_wait (3)
14  - libcurl-errors (3)
15---
16
17# NAME
18
19curl_multi_perform - reads/writes available data from easy handles
20
21# SYNOPSIS
22
23~~~c
24#include <curl/curl.h>
25
26CURLMcode curl_multi_perform(CURLM *multi_handle, int *running_handles);
27~~~
28
29# DESCRIPTION
30
31This function performs transfers on all the added handles that need attention
32in a non-blocking fashion. The easy handles have previously been added to the
33multi handle with curl_multi_add_handle(3).
34
35When an application has found out there is data available for the multi_handle
36or a timeout has elapsed, the application should call this function to
37read/write whatever there is to read or write right now etc.
38curl_multi_perform(3) returns as soon as the reads/writes are done. This
39function does not require that there actually is any data available for
40reading or that data can be written, it can be called just in case. It stores
41the number of handles that still transfer data in the second argument's
42integer-pointer.
43
44If the amount of *running_handles* is changed from the previous call (or
45is less than the amount of easy handles you have added to the multi handle),
46you know that there is one or more transfers less "running". You can then call
47curl_multi_info_read(3) to get information about each individual
48completed transfer, and that returned info includes CURLcode and more. If an
49added handle fails quickly, it may never be counted as a running_handle. You
50could use curl_multi_info_read(3) to track actual status of the added
51handles in that case.
52
53When *running_handles* is set to zero (0) on the return of this function,
54there is no longer any transfers in progress.
55
56When this function returns error, the state of all transfers are uncertain and
57they cannot be continued. curl_multi_perform(3) should not be called
58again on the same multi handle after an error has been returned, unless first
59removing all the handles and adding new ones.
60
61# EXAMPLE
62
63~~~c
64int main(void)
65{
66  int still_running;
67  CURL *multi = curl_multi_init();
68  CURL *curl = curl_easy_init();
69  if(curl) {
70    curl_multi_add_handle(multi, curl);
71    do {
72      CURLMcode mc = curl_multi_perform(multi, &still_running);
73
74      if(!mc && still_running)
75        /* wait for activity, timeout or "nothing" */
76        mc = curl_multi_poll(multi, NULL, 0, 1000, NULL);
77
78      if(mc) {
79        fprintf(stderr, "curl_multi_poll() failed, code %d.\n", (int)mc);
80        break;
81      }
82
83    /* if there are still transfers, loop! */
84    } while(still_running);
85  }
86}
87~~~
88
89# AVAILABILITY
90
91Added in 7.9.6
92
93# RETURN VALUE
94
95CURLMcode type, general libcurl multi interface error code.
96
97This function returns errors regarding the whole multi stack. Problems on
98individual transfers may have occurred even when this function returns
99*CURLM_OK*. Use curl_multi_info_read(3) to figure out how individual
100transfers did.
101
102# TYPICAL USAGE
103
104Most applications use curl_multi_poll(3) to make libcurl wait for
105activity on any of the ongoing transfers. As soon as one or more file
106descriptor has activity or the function times out, the application calls
107curl_multi_perform(3).
108