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(libc.info)Descriptor Flags

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File Descriptor Flags
=====================

   "File descriptor flags" are miscellaneous attributes of a file
descriptor.  These flags are associated with particular file
descriptors, so that if you have created duplicate file descriptors
from a single opening of a file, each descriptor has its own set of
flags.

   Currently there is just one file descriptor flag: `FD_CLOEXEC',
which causes the descriptor to be closed if you use any of the
`exec...' functions (Note: Executing a File).

   The symbols in this section are defined in the header file `fcntl.h'.

 - Macro: int F_GETFD
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should return the file descriptor flags associated with
     the FILEDES argument.

     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 (except that currently there is only one flag
     to use).

     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_SETFD
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should set the file descriptor flags associated with the
     FILEDES argument.  This requires a third `int' argument to specify
     the new flags, so the form of the call is:

          fcntl (FILEDES, F_SETFD, NEW-FLAGS)

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

   The following macro is defined for use as a file descriptor flag with
the `fcntl' function.  The value is an integer constant usable as a bit
mask value.

 - Macro: int FD_CLOEXEC
     This flag specifies that the file descriptor should be closed when
     an `exec' function is invoked; see Note: Executing a File.  When
     a file descriptor is allocated (as with `open' or `dup'), this bit
     is initially cleared on the new file descriptor, meaning that
     descriptor will survive into the new program after `exec'.

   If you want to modify the file descriptor flags, you should get the
current flags with `F_GETFD' 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 `FD_CLOEXEC'
without altering any other flags:

     /* Set the `FD_CLOEXEC' 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_cloexec_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFD, 0);
       /* If reading the flags failed, return error indication now.
       if (oldflags < 0)
         return oldflags;
       /* Set just the flag we want to set. */
       if (value != 0)
         oldflags |= FD_CLOEXEC;
       else
         oldflags &= ~FD_CLOEXEC;
       /* Store modified flag word in the descriptor. */
       return fcntl (desc, F_SETFD, oldflags);
     }