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(libc.info)Getting File Status Flags

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Getting and Setting File Status Flags
-------------------------------------

   The `fcntl' function can fetch or change file status flags.

 - Macro: int F_GETFL
     This macro is used as the COMMAND argument to `fcntl', to read the
     file status flags for the open file with descriptor FILEDES.

     The normal return value from `fcntl' with this command is a
     nonnegative number which can be interpreted as the bitwise OR of
     the individual flags.  Since the file access modes are not
     single-bit values, you can mask off other bits in the returned
     flags with `O_ACCMODE' to compare them.

     In case of an error, `fcntl' returns -1.  The following `errno'
     error conditions are defined for this command:

    `EBADF'
          The FILEDES argument is invalid.

 - Macro: int F_SETFL
     This macro is used as the COMMAND argument to `fcntl', to set the
     file status flags for the open file corresponding to the FILEDES
     argument.  This command requires a third `int' argument to specify
     the new flags, so the call looks like this:

          fcntl (FILEDES, F_SETFL, NEW-FLAGS)

     You can't change the access mode for the file in this way; that is,
     whether the file descriptor was opened for reading or writing.

     The normal return value from `fcntl' with this command is an
     unspecified value other than -1, which indicates an error.  The
     error conditions are the same as for the `F_GETFL' command.

   If you want to modify the file status flags, you should get the
current flags with `F_GETFL' and modify the value.  Don't assume that
the flags listed here are the only ones that are implemented; your
program may be run years from now and more flags may exist then.  For
example, here is a function to set or clear the flag `O_NONBLOCK'
without altering any other flags:

     /* Set the `O_NONBLOCK' flag of DESC if VALUE is nonzero,
        or clear the flag if VALUE is 0.
        Return 0 on success, or -1 on error with `errno' set. */
     
     int
     set_nonblock_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFL, 0);
       /* If reading the flags failed, return error indication now. */
       if (oldflags == -1)
         return -1;
       /* Set just the flag we want to set. */
       if (value != 0)
         oldflags |= O_NONBLOCK;
       else
         oldflags &= ~O_NONBLOCK;
       /* Store modified flag word in the descriptor. */
       return fcntl (desc, F_SETFL, oldflags);
     }