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I am using socketpair() system call for IPC and it returns 0,1 or 2 file descriptor if any of them is available. Using write() system call for fd 0 writes to the STDIN thereby messing up the output of my application. Is there something I can do to prevent the socketpair() from returning 0,1 or 2 as fd ?

Shekhar Bhandakkar
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    Using `write()` to FD 0 writes to whaever 0 is connected to. If it's a socket, it isn't `stdin`, and it can't 'mess up the output of your application'. It isn't clear that you have a problem to solve. – user207421 Feb 19 '17 at 06:59
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    The returnvalue from socketpair is **not** a file descriptor, it is a status code (-1 or 0), meaning failure or success. – wildplasser Feb 19 '17 at 15:18
  • My bad, I meant to say that the socket is allocated the file descriptor 0,1 or 2. – Shekhar Bhandakkar Feb 20 '17 at 08:08

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Don't close standard input, standard output or standard error before using socketpair(). If necessary, open /dev/null for those file descriptors.

When file descriptors are allocated (by any system call — open(), socket(), socketpair(), accept(), pipe(), dup(), etc), the number used is always the lowest available (unopen) number. If you're getting 0, 1 or 2 allocated by socketpair(), it means you must have closed the corresponding descriptor — but why did you do that? Never mind; don't do it.

Or fix it by opening /dev/null. Remember that 0 should be readable and 1 and 2 should be writable (and that it doesn't matter if 0 is writable and 1 and 2 are readable; indeed, when a shell is started in a terminal, all three are often readable and writable).

Jonathan Leffler
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