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curl_multi_fdset(3) [debian man page]

curl_multi_fdset(3)						  libcurl Manual					       curl_multi_fdset(3)

NAME
curl_multi_fdset - extracts file descriptor information from a multi handle SYNOPSIS
#include <curl/curl.h> CURLMcode curl_multi_fdset(CURLM *multi_handle, fd_set *read_fd_set, fd_set *write_fd_set, fd_set *exc_fd_set, int *max_fd); DESCRIPTION
This function extracts file descriptor information from a given multi_handle. libcurl returns its fd_set sets. The application can use these to select() on, but be sure to FD_ZERO them before calling this function as curl_multi_fdset(3) only adds its own descriptors, it doesn't zero or otherwise remove any others. The curl_multi_perform(3) function should be called as soon as one of them is ready to be read from or written to. If no file descriptors are set by libcurl, max_fd will contain -1 when this function returns. Otherwise it will contain the higher descrip- tor number libcurl set. When libcurl returns -1 in max_fd, it is because libcurl currently does something that isn't possible for your application to monitor with a socket and unfortunately you can then not know exactly when the current action is completed using select(). When max_fd returns with -1, you need to wait a while and then proceed and call curl_multi_perform anyway. How long to wait? I would sug- gest 100 milliseconds at least, but you may want to test it out in your own particular conditions to find a suitable value. When doing select(), you should use curl_multi_timeout to figure out how long to wait for action. Call curl_multi_perform even if no activ- ity has been seen on the fd_sets after the timeout expires as otherwise internal retries and timeouts may not work as you'd think and want. If one of the sockets used by libcurl happens to be larger than what can be set in an fd_set, which on POSIX systems means that the file descriptor is larger than FD_SETSIZE, then libcurl will try to not set it. Setting a too large file descriptor in an fd_set implies an out of bounds write which can cause crashes, or worse. The effect of NOT storing it will possibly save you from the crash, but will make your program NOT wait for sockets it should wait for... RETURN VALUE
CURLMcode type, general libcurl multi interface error code. See libcurl-errors(3) SEE ALSO
curl_multi_cleanup(3), curl_multi_init(3), curl_multi_timeout(3), curl_multi_perform(3) libcurl 7.16.0 2 Jan 2006 curl_multi_fdset(3)

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curl_multi_perform(3)						  libcurl Manual					     curl_multi_perform(3)

NAME
curl_multi_perform - reads/writes available data from each easy handle SYNOPSIS
#include <curl/curl.h> CURLMcode curl_multi_perform(CURLM *multi_handle, int *running_handles); DESCRIPTION
This function handles transfers on all the added handles that need attention in an non-blocking fashion. When an application has found out there's data available for the multi_handle or a timeout has elapsed, the application should call this function to read/write whatever there is to read or write right now etc. curl_multi_perform(3) returns as soon as the reads/writes are done. This function does not require that there actually is any data available for reading or that data can be written, it can be called just in case. It will write the number of handles that still transfer data in the second argument's integer-pointer. If the amount of running_handles is changed from the previous call (or is less than the amount of easy handles you've added to the multi handle), you know that there is one or more transfers less "running". You can then call curl_multi_info_read(3) to get information about each individual completed transfer, and that returned info includes CURLcode and more. If an added handle fails very quickly, it may never be counted as a running_handle. When running_handles is set to zero(0) on the return of this function, there is no longer any transfers in progress. EXAMPLE
#ifdef _WIN32 #define SHORT_SLEEP Sleep(100) #else #define SHORT_SLEEP usleep(100000) #endif fd_set fdread; fd_set fdwrite; fd_set fdexcep; int maxfd = -1; long curl_timeo; curl_multi_timeout(multi_handle, &curl_timeo); if(curl_timeo < 0) curl_timeo = 1000; timeout.tv_sec = curl_timeo / 1000; timeout.tv_usec = (curl_timeo % 1000) * 1000; FD_ZERO(&fdread); FD_ZERO(&fdwrite); FD_ZERO(&fdexcep); /* get file descriptors from the transfers */ mc = curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd); if(maxfd == -1) { SHORT_SLEEP; rc = 0; } else rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout); switch(rc) { case -1: /* select error */ break; case 0: default: /* timeout or readable/writable sockets */ curl_multi_perform(multi_handle, &still_running); break; } /* if there are still transfers, loop! */ RETURN VALUE
CURLMcode type, general libcurl multi interface error code. Before version 7.20.0: If you receive CURLM_CALL_MULTI_PERFORM, this basically means that you should call curl_multi_perform(3) again, before you select() on more actions. You don't have to do it immediately, but the return code means that libcurl may have more data avail- able to return or that there may be more data to send off before it is "satisfied". Do note that curl_multi_perform(3) will return CURLM_CALL_MULTI_PERFORM only when it wants to be called again immediately. When things are fine and there is nothing immediate it wants done, it'll return CURLM_OK and you need to wait for "action" and then call this function again. This function only returns errors etc regarding the whole multi stack. Problems still might have occurred on individual transfers even when this function returns CURLM_OK. Use curl_multi_info_read(3) to figure out how individual transfers did. TYPICAL USAGE
Most applications will use curl_multi_fdset(3) to get the multi_handle's file descriptors, and curl_multi_timeout(3) to get a suitable timeout period, then it'll wait for action on the file descriptors using select(3). As soon as one or more file descriptor is ready, curl_multi_perform(3) gets called. SEE ALSO
curl_multi_cleanup(3), curl_multi_init(3), curl_multi_wait(3), curl_multi_fdset(3), curl_multi_info_read(3), libcurl-errors(3) libcurl 7.54.0 February 03, 2016 curl_multi_perform(3)
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