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13.11 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 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). See Duplicating Descriptors.
F_GETFD
Get flags associated with the file descriptor. See Descriptor Flags.
F_SETFD
Set flags associated with the file descriptor. See Descriptor Flags.
F_GETFL
Get flags associated with the open file. See File Status Flags.
F_SETFL
Set flags associated with the open file. See File Status Flags.
F_GETLK
Get a file lock. See File Locks.
F_SETLK
Set or clear a file lock. See File Locks.
F_SETLKW
Like F_SETLK, but wait for completion. See File Locks.
F_GETOWN
Get process or process group ID to receive SIGIO signals. See Interrupt Input.
F_SETOWN
Set process or process group ID to receive SIGIO signals. See 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.