My Tech notes: Use of Select() System Call In Linux To make a non-blocking connect()

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Wednesday, May 6, 2009

Use of Select() System Call In Linux To make a non-blocking connect()

About Select system call
The  select() function shall examine the file descriptor sets whose addresses are passed in the readfds, writefds,
       and errorfds parameters to see whether some of their descriptors are ready for reading, are ready for  writing,  or
       have an exceptional condition pending, respectively.
  int select(int nfds, fd_set *restrict readfds,
              fd_set *restrict writefds, fd_set *restrict errorfds,
              struct timeval *restrict timeout);
There are many usages of select() system call, Here is one usage in networking applications, The use of select system call is to make non-blocking call in Linux.
how to make a non-blocking connect() in Linux

1. create socket using socket(),

2. set the file descriptor to non-blocking mode using fcntl(2)
   fnctl (fd, SETFL, fcntl(fd, GETFL) | O_NONBLOCK)

3. call connect() - since you have set the socket to non-blocking, it will
return right away with a result of EINPROGRESS.

4.  Now

   a) Go into a loop and keep calling connect() - as long as it
       has not completed it will return EALREADY (see man page for
       connect()).  This is simple but not a good solution. check below solution
       using select to avoid loop wait.
   b) Use select(2), selecting the socket for write.  When the connect is
       finished, the select should return and show the socket as writeable.
       Using select() can be tricky, but will be more efficient, since
       select() will block until the connect has finished and you are not
       wasting CPU in a tight loop.

       /* No loop required */
       fd_set write_set;
       FD_SET(write_set, fd);
       select (fd+1, 0, write_set, 0, 0);

File descriptor masks of type fd_set can be  initialized  and  tested  with  FD_CLR(),  FD_ISSET(),  FD_SET(),  and
     FD_SET(fd,  fdsetp)  shall add the file descriptor fd to the set pointed to by fdsetp. If the file descriptor fd is
       already in this set, there shall be no effect on the set, nor will an error be returned.
     FD_ZERO(fdsetp) shall initialize the descriptor set pointed to by fdsetp to the null set. No error is  returned  if
       the set is not empty at the time FD_ZERO() is invoked.
 The  original  Standard  can  be  obtained  online at

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