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curl_multi_fdset(3)             libcurl Manual             curl_multi_fdset(3)

       curl_multi_fdset - extracts file descriptor information from a multi handle

       #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);

       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 descriptor number libcurl set. When libcurl returns -1 in max_fd, it is because libcurl cur-
       rently  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 suggest 100
       milliseconds at least, but you may want to test it out in your own particular conditions  to  find  a  suitable

       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.

       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...

       CURLMcode type, general libcurl multi interface error code. See libcurl-errors(3)

       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)