curl_multi_fdset(3) libcurl Manual curl_multi_fdset(3)NAMEcurl_multi_fdset - extracts file descriptor information from a multi
CURLMcode curl_multi_fdset(CURLM *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.
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 currently does something that isn't possible for your applica‐
tion 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 par‐
ticular 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 stor‐
ing it will possibly save you from the crash, but will make your pro‐
gram NOT wait for sockets it should wait for...
CURLMcode type, general libcurl multi interface error code. See
libcurl-errors(3)SEE ALSOcurl_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)