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| .TH curl_multi_fdset 3 "2 Jan 2006" "libcurl" "libcurl" |
| .SH NAME |
| curl_multi_fdset - extracts file descriptor information from a multi handle |
| .SH SYNOPSIS |
| .nf |
| #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); |
| .ad |
| .SH DESCRIPTION |
| This function extracts file descriptor information from a given multi_handle. |
| libcurl returns its \fIfd_set\fP sets. The application can use these to |
| select() on, but be sure to \fIFD_ZERO\fP them before calling this function as |
| \fIcurl_multi_fdset(3)\fP only adds its own descriptors, it does not zero or |
| otherwise remove any others. The \fIcurl_multi_perform(3)\fP function should |
| be called as soon as one of them is ready to be read from or written to. |
| |
| If the \fIread_fd_set\fP argument is not a null pointer, it points to an |
| object of type \fBfd_set\fP that on returns specifies the file descriptors to |
| be checked for being ready to read. |
| |
| If the \fIwrite_fd_set\fP argument is not a null pointer, it points to an |
| object of type \fBfd_set\fP that on return specifies the file descriptors to |
| be checked for being ready to write. |
| |
| If the \fIexc_fd_set\fP argument is not a null pointer, it points to an object |
| of type \fBfd_set\fP that on return specifies the file descriptors to be |
| checked for error conditions pending. |
| |
| If no file descriptors are set by libcurl, \fImax_fd\fP will contain -1 when |
| this function returns. Otherwise it will contain the highest descriptor number |
| libcurl set. When libcurl returns -1 in \fImax_fd\fP, it is because libcurl |
| currently does something that is not 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(). You then need to wait a while before you |
| proceed and call \fIcurl_multi_perform(3)\fP anyway. How long to wait? Unless |
| \fIcurl_multi_timeout(3)\fP gives you a lower number, we suggest 100 |
| milliseconds or so, but you may want to test it out in your own particular |
| conditions to find a suitable value. |
| |
| When doing select(), you should use \fIcurl_multi_timeout(3)\fP to figure out |
| how long to wait for action. Call \fIcurl_multi_perform(3)\fP even if no |
| activity has been seen on the \fBfd_sets\fP after the timeout expires as |
| otherwise internal retries and timeouts may not work as you would think and |
| want. |
| |
| If one of the sockets used by libcurl happens to be larger than what can be |
| set in an \fBfd_set\fP, which on POSIX systems means that the file descriptor |
| is larger than \fBFD_SETSIZE\fP, then libcurl will try to not set it. Setting |
| a too large file descriptor in an \fBfd_set\fP 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... |
| .SH EXAMPLE |
| .nf |
| /* get file descriptors from the transfers */ |
| mc = curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd); |
| |
| if(mc != CURLM_OK) { |
| fprintf(stderr, "curl_multi_fdset() failed, code %d.\\n", mc); |
| break; |
| } |
| |
| /* wait for activity on one of the sockets */ |
| rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout); |
| .fi |
| .SH AVAILABILITY |
| Added in 7.9.6 |
| .SH RETURN VALUE |
| \fBCURLMcode\fP type, general libcurl multi interface error code. See |
| \fIlibcurl-errors(3)\fP |
| .SH "SEE ALSO" |
| .BR curl_multi_cleanup "(3), " curl_multi_init "(3), " |
| .BR curl_multi_wait "(3), " |
| .BR curl_multi_timeout "(3), " curl_multi_perform "(3), " select "(2) " |
