NAME

fcntl.h — file control options

SYNOPSIS

#include <fcntl.h>

DESCRIPTION

The <fcntl.h> header shall define the f_owner_ex structure, which shall include at least the following members:

int   type      Discriminator for pid.
pid_t pid       Process ID or process group ID.

The <fcntl.h> header shall define the flock structure describing a file lock. It shall include the following members:

short  l_type   Type of lock; F_RDLCK, F_WRLCK, F_UNLCK.
short  l_whence Flag for starting offset.
off_t  l_start  Relative offset in bytes.
off_t  l_len    Size; if 0 then until EOF.
pid_t  l_pid    For a process-owned file lock, ignored on input or the process ID of the
                owning process on output; for an OFD-owned file lock, zero on input or
                (pid_t)-1 on output.

The <fcntl.h> header shall define the mode_t, off_t, and pid_t types as described in <sys/types.h>.

The <fcntl.h> header shall define the following symbolic constants for the cmd argument used by fcntl(). The values shall be unique and shall be suitable for use in #if preprocessing directives.

F_DUPFD

Duplicate file descriptor.

F_DUPFD_CLOEXEC

Duplicate file descriptor with the close-on-exec flag FD_CLOEXEC set.

F_DUPFD_CLOFORK

Duplicate file descriptor with the close-on-fork flag FD_CLOFORK set.

F_GETFD

Get file descriptor flags.

F_SETFD

Set file descriptor flags.

F_GETFL

Get file status flags and file access modes.

F_SETFL

Set file status flags.

F_GETLK

Get information about file locks.

F_SETLK

Set a process-owned file lock.

F_SETLKW

Set a process-owned file lock; wait if blocked.

F_OFD_GETLK

Get information about file locks.

F_OFD_SETLK

Set an OFD-owned file lock.

F_OFD_SETLKW

Set an OFD-owned file lock; wait if blocked.

F_GETOWN

Get process or process group ID to receive SIGURG signals, via int type.

F_GETOWN_EX

Get process or process group ID to receive SIGURG signals, via pid_t type.

F_SETOWN

Set process or process group ID to receive SIGURG signals, via int type.

F_SETOWN_EX

Set process or process group ID to receive SIGURG signals, via pid_t type.

The <fcntl.h> header shall define the following symbolic constants used for the fcntl() file descriptor flags. The values shall be bitwise-distinct and shall be suitable for use in #if preprocessing directives.

FD_CLOEXEC

Close the file descriptor upon execution of an exec family function ^[SPN]  and in the new process image created by posix_spawn() or posix_spawnp().

FD_CLOFORK

Close the file descriptor in any child process created from a process that has the file descriptor open; that is, the child shall not inherit the file descriptor.

The <fcntl.h> header shall also define the following symbolic constants for the l_type argument used for record locking with fcntl(). The values shall be unique and shall be suitable for use in #if preprocessing directives.

F_RDLCK

Shared or read lock.

F_UNLCK

Unlock.

F_WRLCK

Exclusive or write lock.

The <fcntl.h> header shall also define the following symbolic constants for the type member of the f_owner_ex structure. The values shall be unique.

F_OWNER_PID

The pid member of f_owner_ex holds a process ID.

F_OWNER_PGRP

The pid member of f_owner_ex holds a process group ID.

The <fcntl.h> header shall define the values used for l_whence, SEEK_SET, SEEK_CUR, and SEEK_END as described in <stdio.h>.

The <fcntl.h> header shall define the following symbolic constants as file creation flags for use in the oflag value to open() and openat(). The values shall be bitwise-distinct and shall be suitable for use in #if preprocessing directives.

O_CLOEXEC

Atomically set the FD_CLOEXEC flag on the new file descriptor.

O_CLOFORK

Atomically set the FD_CLOFORK flag on the new file descriptor.

O_CREAT

Create file if it does not exist.

O_DIRECTORY

Fail if file is a non-directory file.

O_EXCL

Exclusive use flag.

O_NOCTTY

Do not assign controlling terminal.

O_NOFOLLOW

Do not follow symbolic links.

O_TRUNC

Truncate flag.

O_TTY_INIT

Set the termios structure terminal parameters to a state that provides conforming behavior; see 11.2 Parameters that Can be Set .

The O_TTY_INIT flag can have the value zero and in this case it need not be bitwise-distinct from the other flags.

The <fcntl.h> header shall define the following symbolic constants for use as file status flags for open(), openat(), and fcntl(). The values shall be suitable for use in #if preprocessing directives.

O_APPEND

Set append mode.

O_DSYNC

^[SIO] Write according to synchronized I/O data integrity completion.

O_NONBLOCK

Non-blocking mode.

O_RSYNC

^[SIO] Synchronized read I/O operations.

O_SYNC

Write according to synchronized I/O file integrity completion.

The <fcntl.h> header shall define the following symbolic constant for use as the mask for file access modes. The value shall be suitable for use in #if preprocessing directives.

O_ACCMODE

Mask for file access modes.

The <fcntl.h> header shall define the following symbolic constants for use as the file access modes for open(), openat(), and fcntl(). The values shall be unique, except that O_EXEC and O_SEARCH may have equal values. The values shall be suitable for use in #if preprocessing directives.

O_EXEC

Open for execute only (non-directory files). The result is unspecified if this flag is applied to a directory.

O_RDONLY

Open for reading only.

O_RDWR

Open for reading and writing.

O_SEARCH

Open directory for search only. The result is unspecified if this flag is applied to a non-directory file.

O_WRONLY

Open for writing only.

The <fcntl.h> header shall define the symbolic constants for file modes for use as values of mode_t as described in <sys/stat.h>.

The <fcntl.h> header shall define the following symbolic constant as a special value used in place of a file descriptor for the *at() functions which take a directory file descriptor as a parameter:

AT_FDCWD

Use the current working directory to determine the target of relative file paths.

The <fcntl.h> header shall define the following symbolic constant as a value for the flag used by faccessat():

AT_EACCESS

Check access using effective user and group ID.

The <fcntl.h> header shall define the following symbolic constant as a value for the flag used by fstatat(), fchmodat(), fchownat(), and utimensat():

AT_SYMLINK_NOFOLLOW

Do not follow symbolic links.

The <fcntl.h> header shall define the following symbolic constant as a value for the flag used by linkat():

AT_SYMLINK_FOLLOW

Follow symbolic link.

The <fcntl.h> header shall define the following symbolic constant as a value for the flag used by unlinkat():

AT_REMOVEDIR

Remove directory instead of file.

^[ADV] The <fcntl.h> header shall define the following symbolic constants for the advice argument used by posix_fadvise():

POSIX_FADV_DONTNEED

The application expects that it will not access the specified data in the near future.

POSIX_FADV_NOREUSE

The application expects to access the specified data once and then not reuse it thereafter.

POSIX_FADV_NORMAL

The application has no advice to give on its behavior with respect to the specified data. It is the default characteristic if no advice is given for an open file.

POSIX_FADV_RANDOM

The application expects to access the specified data in a random order.

POSIX_FADV_SEQUENTIAL

The application expects to access the specified data sequentially from lower offsets to higher offsets.

POSIX_FADV_WILLNEED

The application expects to access the specified data in the near future.

The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.

int  creat(const char *, mode_t);
int  fcntl(int, int, ...);
int  open(const char *, int, ...);
int  openat(int, const char *, int, ...);
[ADV]
int  posix_fadvise(int, off_t, off_t, int);
int  posix_fallocate(int, off_t, off_t);

Inclusion of the <fcntl.h> header may also make visible all symbols from <sys/stat.h> and <unistd.h>.

APPLICATION USAGE

Although no existing implementation defines AT_SYMLINK_FOLLOW and AT_SYMLINK_NOFOLLOW as the same numeric value, POSIX.1-2024 does not prohibit that as the two constants are not used with the same interfaces.

RATIONALE

While many of the symbolic constants introduced in the <fcntl.h> header do not strictly need to be used in #if preprocessor directives, widespread historic practice has defined them as macros that are usable in such constructs, and examination of existing applications has shown that they are occasionally used in such a way. Therefore it was decided to retain this requirement on an implementation in POSIX.1-2024.

FUTURE DIRECTIONS

A future version of this standard may add an O_NOCLOBBER file creation flag. See the FUTURE DIRECTIONS section for open .

SEE ALSO

<stdio.h> , <sys/stat.h> , <sys/types.h> , <unistd.h>

XSH creat , exec , fcntl , futimens , open , posix_fadvise , posix_fallocate , posix_madvise

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Realtime Extension.

Issue 6

The following changes are made for alignment with the ISO POSIX-1:1996 standard:

• O_DSYNC and O_RSYNC are marked as part of the Synchronized Input and Output option.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• The definition of the mode_t, off_t, and pid_t types is mandated.

The F_GETOWN and F_SETOWN values are added for sockets.

The posix_fadvise(), posix_fallocate(), and posix_madvise() functions are added for alignment with IEEE Std 1003.1d-1999.

IEEE PASC Interpretation 1003.1 #102 is applied, moving the prototype for posix_madvise() to <sys/mman.h>.

IEEE Std 1003.1-2001/Cor 2-2004, item XBD/TC2/D6/18 is applied, updating the prototypes for posix_fadvise() and posix_fallocate() to be large file-aware, using off_t instead of size_t.

Issue 7

Austin Group Interpretation 1003.1-2001 #144 is applied, adding the O_TTY_INIT flag.

Austin Group Interpretation 1003.1-2001 #171 is applied, adding support to set the FD_CLOEXEC flag atomically at open(), and adding the F_DUPFD_CLOEXEC flag.

The openat() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 2.

Additional flags are added to support faccessat(), fchmodat(), fchownat(), fstatat(), linkat(), open(), openat(), and unlinkat().

This reference page is clarified with respect to macros and symbolic constants.

Changes are made related to support for finegrained timestamps.

Changes are made to allow a directory to be opened for searching.

POSIX.1-2008, Technical Corrigendum 1, XBD/TC1-2008/0044 [274] and XBD/TC1-2008/0045 [78,432] are applied.

POSIX.1-2008, Technical Corrigendum 2, XBD/TC2-2008/0060 [847] is applied.

Issue 8

Austin Group Defect 768 is applied, adding OFD-owned file locks.

Austin Group Defect 1016 is applied, changing the FUTURE DIRECTIONS section.

Austin Group Defect 1274 is applied, adding the f_owner_ex structure and related symbolic constants.

Austin Group Defect 1318 is applied, adding FD_CLOFORK and O_CLOFORK, and changing O_CLOEXEC.

Austin Group Defect 1351 is applied, adding F_DUPFD_CLOFORK.