Files Reference

fcntl.h File

Purpose

Defines file control options.

Description

The /usr/include/fcntl.h file defines the values that can be specified for the Command and Argument parameters of the fcntl subroutine and for the Oflag parameter of the open subroutine. The file-status flags of an open file are described in the following information.

Flag Values for open Subroutine

The following flag values are accessible only to the open subroutine:

O_RDONLY	Read-only
O_WRONLY	Write-only
O_RDWR	Read and write
O_CREAT	Open with file create (uses the third open argument)
O_TRUNC	Open with truncation
O_DIRECT	Open for Direct I/O
O_EXCL	Exclusive open Note The O_EXCL flag is not fully supported for Network File Systems (NFS). The NFS protocol does not guarantee the designed function of the O_EXCL flag.
O_NOCTTY	Do not assign a controlling terminal
O_RSHARE	Read shared open
O_NSHARE	Read shared open

File Access Mode Mask

The O_ACCMODE mask is used to determine the file access mode.

File Status flags for open and fcntl Subroutines

The following file status flags are accessible to both the open and fcntl subroutines:

O_NONBLOCK	POSIX nonblocking I/O
FNONBLOCK	POSIX nonblocking I/O
O_APPEND	An append with writes guaranteed at the end
FAPPEND	An append with writes guaranteed at the end
O_SYNC	Synchronous write option
FSYNC	Synchronous write option
O_DSYNC	Synchronous write option (file data only).
FDATASYNC	Synchronous write option (file data only).
O_RSYNC	Synchronous file attributes on read.
FREADSYNC	Synchronous file attributes on read.
FASYNC	Asynchronous I/O
O_NDELAY	Nonblocking I/O
FNDELAY	Nonblocking I/O
O_LARGEFILE	Access to large files enabled .

File Status Flags for open Subroutine

The following file status flags are accessible to the open subroutine:

O_DEFER	Deferred update
O_DELAY	Open with delay
O_DIRECT	Open for Direct I/O

File Descriptor Flags for fcntl Subroutine

The following file descriptor flag is accessible to the fcntl subroutine:

FD_CLOEXEC

Close this file during an exec.

File flag values corresponding to file access modes are as follows:

FREAD	File is open for read.
FWRITE	File is open for write.

Notes:

The FREAD and FWRITE flags cannot be used unless the _KERNEL flag has been defined.
The ldfcn.h file also assigns values to the FREAD and FWRITE options. If you use the ldfcn.h and fcntl.h files together, directly or indirectly, you should use the #undef statement on the FREAD and FWRITE options of one of the header files. If you do not, the compiler will return a warning about using duplicate definitions.

Command Values for fcntl Subroutine

The Command values for the fcntl subroutine (that is, for fcntl subroutine requests) are:

F_DUPFD	Duplicate the file description.
F_GETFD	Get the file description flags.
F_SETFD	Set the file description flags.
F_GETFL	Get the file status flags and file access modes.
F_SETFL	Set the file flags.
F_GETLK	Return information about an existing file lock.
F_GETLK64	Return information about an existing file lock.
F_SETLK	Set or clear a file lock.
F_SETLK64	Set or clear a file lock.
F_SETLKW	Set or clear a file lock and wait if blocked.
F_SETLKW64	Set or clear a file lock and wait if blocked.
F_GETOWN	Get the descriptor owner.
F_SETOWN	Set the descriptor owner.

Related Information

The fcntl subroutine, open, openx, or creat subroutine.

The sys/types.h file, unistd.h file.

The Header Files Overview defines header files, describes how they are used, and lists several header files for which information is provided.