1curl_multi_perform(3) libcurl Manual curl_multi_perform(3)
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6 curl_multi_perform - reads/writes available data from each easy handle
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9 #include <curl/curl.h>
10 CURLMcode curl_multi_perform(CURLM *multi_handle, int *running_han‐
12 dles);
13
15 This function handles transfers on all the added handles that need
16 attention in an non-blocking fashion.
17 When an application has found out there's data available for the
19 multi_handle or a timeout has elapsed, the application should call this
20 function to read/write whatever there is to read or write right now
21 etc. curl_multi_perform(3) returns as soon as the reads/writes are
22 done. This function does not require that there actually is any data
23 available for reading or that data can be written, it can be called
24 just in case. It will write the number of handles that still transfer
25 data in the second argument's integer-pointer.
26 If the amount of running_handles is changed from the previous call (or
28 is less than the amount of easy handles you've added to the multi han‐
29 dle), you know that there is one or more transfers less "running". You
30 can then call curl_multi_info_read(3) to get information about each
31 individual completed transfer, and that returned info includes CURLcode
32 and more. If an added handle fails very quickly, it may never be
33 counted as a running_handle. You could use curl_multi_info_read(3) to
34 track actual status of the added handles in that case.
35 When running_handles is set to zero (0) on the return of this function,
37 there is no longer any transfers in progress.
38
40 #ifdef _WIN32
41 #define SHORT_SLEEP Sleep(100)
42 #else
43 #define SHORT_SLEEP usleep(100000)
44 #endif
45 fd_set fdread;
47 fd_set fdwrite;
48 fd_set fdexcep;
49 int maxfd = -1;
50 long curl_timeo;
52 curl_multi_timeout(multi_handle, &curl_timeo);
54 if(curl_timeo < 0)
55 curl_timeo = 1000;
56 timeout.tv_sec = curl_timeo / 1000;
58 timeout.tv_usec = (curl_timeo % 1000) * 1000;
59 FD_ZERO(&fdread);
61 FD_ZERO(&fdwrite);
62 FD_ZERO(&fdexcep);
63 /* get file descriptors from the transfers */
65 mc = curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);
66 if(maxfd == -1) {
68 SHORT_SLEEP;
69 rc = 0;
70 }
71 else
72 rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);
73 switch(rc) {
75 case -1:
76 /* select error */
77 break;
78 case 0:
79 default:
80 /* timeout or readable/writable sockets */
81 curl_multi_perform(multi_handle, &still_running);
82 break;
83 }
84 /* if there are still transfers, loop! */
86
88 CURLMcode type, general libcurl multi interface error code.
89 Before version 7.20.0: If you receive CURLM_CALL_MULTI_PERFORM, this
91 basically means that you should call curl_multi_perform(3) again,
92 before you select() on more actions. You don't have to do it immedi‐
93 ately, but the return code means that libcurl may have more data avail‐
94 able to return or that there may be more data to send off before it is
95 "satisfied". Do note that curl_multi_perform(3) will return
96 CURLM_CALL_MULTI_PERFORM only when it wants to be called again immedi‐
97 ately. When things are fine and there is nothing immediate it wants
98 done, it'll return CURLM_OK and you need to wait for "action" and then
99 call this function again.
100 This function only returns errors etc regarding the whole multi stack.
102 Problems still might have occurred on individual transfers even when
103 this function returns CURLM_OK. Use curl_multi_info_read(3) to figure
104 out how individual transfers did.
105
107 Most applications will use curl_multi_fdset(3) to get the multi_han‐
108 dle's file descriptors, and curl_multi_timeout(3) to get a suitable
109 timeout period, then it'll wait for action on the file descriptors
110 using select(3). As soon as one or more file descriptor is ready,
111 curl_multi_perform(3) gets called.
112
114 curl_multi_cleanup(3), curl_multi_init(3), curl_multi_wait(3),
115 curl_multi_fdset(3), curl_multi_info_read(3), libcurl-errors(3)
116libcurl 7.69.1 October 31, 2019 curl_multi_perform(3)