curl_multi_fdset - extracts file descriptor information from a multi handle
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.
To be sure to have up-to-date results, you should call curl_multi_perform until it does not return CURLM_CALL_MULTI_PERFORM prior to calling curl_multi_fdset. This will make sure that libcurl has updated the handles’ socket states.
If no file descriptors are set by libcurl, max_fd will contain -1 when this function returns. Otherwise it will contain the higher descriptor number libcurl set.
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 activity 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.
CURLMcode type, general libcurl multi interface error code. See libcurl-errors(3)
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