The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### NAME

> fcntl.h - file control options

#### SYNOPSIS

> `#include <fcntl.h>`

#### DESCRIPTION

> 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_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 record locking information.
> F_SETLK
>   Set record locking information.
> F_SETLKW
>   Set record locking information; wait if blocked.
> F_GETOWN
>   Get process or process group ID to receive SIGURG signals.
> F_SETOWN
>   Set process or process group ID to receive SIGURG signals.

> The <fcntl.h> header shall define the following symbolic constant used for the fcntl() file descriptor flags, which shall be suitable for use in #if preprocessing directives.

> FD_CLOEXEC
>   Close the file descriptor upon execution of an exec family function.

> 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 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
>   The FD_CLOEXEC flag associated with the new descriptor shall be set to close the file descriptor upon execution of an exec family function.
> 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 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 <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    Process ID of the process holding the lock; returned with F_GETLK.

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

> 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>.

The following sections are informative.

#### APPLICATION USAGE

> Although no existing implementation defines AT_SYMLINK_FOLLOW and AT_SYMLINK_NOFOLLOW as the same numeric value, POSIX.1-2008 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.1-2008.

#### FUTURE DIRECTIONS

> None.

#### 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.

End of informative text.

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