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(libc.info)Control Operations

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Control Operations on Files
===========================

   This section describes how you can perform various other operations
on file descriptors, such as inquiring about or setting flags describing
the status of the file descriptor, manipulating record locks, and the
like.  All of these operations are performed by the function `fcntl'.

   The second argument to the `fcntl' function is a command that
specifies which operation to perform.  The function and macros that name
various flags that are used with it are declared in the header file
`fcntl.h'.  Many of these flags are also used by the `open' function;
see Note: Opening and Closing Files.

 - Function: int fcntl (int FILEDES, int COMMAND, ...)
     The `fcntl' function performs the operation specified by COMMAND
     on the file descriptor FILEDES.  Some commands require additional
     arguments to be supplied.  These additional arguments and the
     return value and error conditions are given in the detailed
     descriptions of the individual commands.

     Briefly, here is a list of what the various commands are.

    `F_DUPFD'
          Duplicate the file descriptor (return another file descriptor
          pointing to the same open file).  Note: Duplicating
          Descriptors.

    `F_GETFD'
          Get flags associated with the file descriptor.  Note:
          Descriptor Flags.

    `F_SETFD'
          Set flags associated with the file descriptor.  Note:
          Descriptor Flags.

    `F_GETFL'
          Get flags associated with the open file.  Note: File Status
          Flags.

    `F_SETFL'
          Set flags associated with the open file.  Note: File Status
          Flags.

    `F_GETLK'
          Get a file lock.  Note: File Locks.

    `F_SETLK'
          Set or clear a file lock.  Note: File Locks.

    `F_SETLKW'
          Like `F_SETLK', but wait for completion.  Note: File Locks.

    `F_GETOWN'
          Get process or process group ID to receive `SIGIO' signals.
          Note: Interrupt Input.

    `F_SETOWN'
          Set process or process group ID to receive `SIGIO' signals.
          Note: Interrupt Input.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `fcntl' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `fcntl' should be protected using cancellation handlers.