fcntl.h(7POSIX)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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 Section 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
    Write according to synchronized I/O data integrity completion.

O_NONBLOCK
    Non-blocking mode.

O_RSYNC
    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.

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, ...);
    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>

The System Interfaces volume of POSIX.1-2017, creat(), exec, fcntl(), futimens(), open(), posix_fadvise(), posix_fallocate(), posix_madvise()

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .