focal (3) curl_multi_fdset.3.gz

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  the read_fd_set argument is not a null pointer, it points to an object of type fd_set that on returns
       specifies the file descriptors to be checked for being ready to read.

       If the write_fd_set argument is not a null pointer, it points to an object of type fd_set that on  return
       specifies the file descriptors to be checked for being ready to write.

       If  the  exc_fd_set  argument is not a null pointer, it points to an object of type fd_set that on return
       specifies the file descriptors to be checked for error conditions pending.

       If no file descriptors are set by libcurl, max_fd will contain -1 when this function  returns.  Otherwise
       it  will  contain  the  highest  descriptor  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(). You then need to wait a while before you proceed and  call  curl_multi_perform(3)  anyway.  How
       long  to  wait? Unless curl_multi_timeout(3) 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 curl_multi_timeout(3) to figure out how long to wait for action. Call
       curl_multi_perform(3)  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...

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_wait(3),        curl_multi_timeout(3),
       curl_multi_perform(3), select(2)