1curl_multi_fdset(3)             libcurl Manual             curl_multi_fdset(3)
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NAME

6       curl_multi_fdset  -  extracts  file descriptor information from a multi
7       handle
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SYNOPSIS

10       #include <curl/curl.h>
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12       CURLMcode curl_multi_fdset(CURLM *multi_handle,
13                                  fd_set *read_fd_set,
14                                  fd_set *write_fd_set,
15                                  fd_set *exc_fd_set,
16                                  int *max_fd);
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DESCRIPTION

19       This  function  extracts  file  descriptor  information  from  a  given
20       multi_handle.  libcurl returns its fd_set sets. The application can use
21       these to select() on, but be sure to FD_ZERO them before  calling  this
22       function  as  curl_multi_fdset(3)  only  adds  its  own descriptors, it
23       doesn't zero or otherwise remove any others. The  curl_multi_perform(3)
24       function  should  be  called as soon as one of them is ready to be read
25       from or written to.
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27       If no file descriptors are set by libcurl, max_fd will contain -1  when
28       this  function returns. Otherwise it will contain the higher descriptor
29       number libcurl set. When libcurl returns -1 in max_fd,  it  is  because
30       libcurl  currently does something that isn't possible for your applica‐
31       tion to monitor with a socket and unfortunately you can then  not  know
32       exactly  when  the  current  action  is  completed using select(). When
33       max_fd returns with -1, you need to wait a while and then  proceed  and
34       call  curl_multi_perform  anyway. How long to wait? I would suggest 100
35       milliseconds at least, but you may want to test it out in your own par‐
36       ticular conditions to find a suitable value.
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38       When  doing  select(),  you should use curl_multi_timeout to figure out
39       how long to wait for action. Call curl_multi_perform even if no  activ‐
40       ity has been seen on the fd_sets after the timeout expires as otherwise
41       internal retries and timeouts may not work as you'd think and want.
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43       If one of the sockets used by libcurl happens to be  larger  than  what
44       can  be  set  in  an fd_set, which on POSIX systems means that the file
45       descriptor is larger than FD_SETSIZE, then libcurl will try to not  set
46       it.  Setting a too large file descriptor in an fd_set implies an out of
47       bounds write which can cause crashes, or worse. The effect of NOT stor‐
48       ing  it  will possibly save you from the crash, but will make your pro‐
49       gram NOT wait for sockets it should wait for...
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RETURN VALUE

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

56       curl_multi_cleanup(3),    curl_multi_init(3),    curl_multi_timeout(3),
57       curl_multi_perform(3)
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61libcurl 7.16.0                    2 Jan 2006               curl_multi_fdset(3)
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