'\" et
.TH fcntl.h "0P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH 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.
.\"
.SH NAME
fcntl.h
\(em file control options
.SH SYNOPSIS
.LP
.nf
#include <fcntl.h>
.fi
.SH DESCRIPTION
The
.IR <fcntl.h> 
header shall define the following symbolic constants for the
.IR cmd
argument used by
\fIfcntl\fR().
The values shall be unique and shall be suitable for use in
.BR #if
preprocessing directives.
.IP F_DUPFD 12
Duplicate file descriptor.
.IP F_DUPFD_CLOEXEC 12
.br
Duplicate file descriptor with the close-on-\c
.IR exec
flag FD_CLOEXEC set.
.IP F_GETFD 12
Get file descriptor flags.
.IP F_SETFD 12
Set file descriptor flags.
.IP F_GETFL 12
Get file status flags and file access modes.
.IP F_SETFL 12
Set file status flags.
.IP F_GETLK 12
Get record locking information.
.IP F_SETLK 12
Set record locking information.
.IP F_SETLKW 12
Set record locking information; wait if blocked.
.IP F_GETOWN 12
Get process or process group ID to receive SIGURG signals.
.IP F_SETOWN 12
Set process or process group ID to receive SIGURG signals.
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constant used for the
\fIfcntl\fR()
file descriptor flags, which shall be suitable for use in
.BR #if
preprocessing directives.
.IP FD_CLOEXEC 12
Close the file descriptor upon execution of an
.IR exec
family function.
.P
The
.IR <fcntl.h> 
header shall also define the following symbolic constants for the
.IR l_type
argument used for record locking with
\fIfcntl\fR().
The values shall be unique and shall be suitable for use in
.BR #if
preprocessing directives.
.IP F_RDLCK 12
Shared or read lock.
.IP F_UNLCK 12
Unlock.
.IP F_WRLCK 12
Exclusive or write lock.
.P
The
.IR <fcntl.h> 
header shall define the values used for
.IR l_whence ,
SEEK_SET, SEEK_CUR, and SEEK_END
as described in
.IR <stdio.h> .
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constants as file creation
flags for use in the
.IR oflag
value to
\fIopen\fR()
and
\fIopenat\fR().
The values shall be bitwise-distinct and shall be suitable for use in
.BR #if
preprocessing directives.
.IP O_CLOEXEC 12
The FD_CLOEXEC flag associated with the new descriptor shall be
set to close the file descriptor upon execution of an
.IR exec
family function.
.IP O_CREAT 12
Create file if it does not exist.
.IP O_DIRECTORY 12
Fail if file is a non-directory file.
.IP O_EXCL 12
Exclusive use flag.
.IP O_NOCTTY 12
Do not assign controlling terminal.
.IP O_NOFOLLOW 12
Do not follow symbolic links.
.IP O_TRUNC 12
Truncate flag.
.IP O_TTY_INIT 12
Set the
.BR termios
structure terminal parameters to a state that provides conforming
behavior; see
.IR "Section 11.2" ", " "Parameters that Can be Set".
.P
The O_TTY_INIT flag can have the value zero and in this case it need
not be bitwise-distinct from the other flags.
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constants for use as
file status flags for
\fIopen\fR(),
\fIopenat\fR(),
and
\fIfcntl\fR().
The values shall be suitable for use in
.BR #if
preprocessing directives.
.IP O_APPEND 12
Set append mode.
.IP O_DSYNC 12
Write according to synchronized I/O data integrity completion.
.IP O_NONBLOCK 12
Non-blocking mode.
.IP O_RSYNC 12
Synchronized read I/O operations.
.IP O_SYNC 12
Write according to synchronized I/O file integrity completion.
.P
The
.IR <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
.BR #if
preprocessing directives.
.IP O_ACCMODE 12
Mask for file access modes.
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constants for use as
the file access modes for
\fIopen\fR(),
\fIopenat\fR(),
and
\fIfcntl\fR().
The values shall be unique, except that O_EXEC and O_SEARCH may have
equal values. The values shall be suitable for use in
.BR #if
preprocessing directives.
.IP O_EXEC 12
Open for execute only (non-directory files). The result is unspecified
if this flag is applied to a directory.
.IP O_RDONLY 12
Open for reading only.
.IP O_RDWR 12
Open for reading and writing.
.IP O_SEARCH 12
Open directory for search only. The result is unspecified if this flag
is applied to a non-directory file.
.IP O_WRONLY 12
Open for writing only.
.P
The
.IR <fcntl.h> 
header shall define the symbolic constants for file modes for use as
values of
.BR mode_t
as described in
.IR <sys/stat.h> .
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constant as a special value
used in place of a file descriptor for the
.IR *at (\|)
functions which take a directory file descriptor as a parameter:
.IP AT_FDCWD 12
Use the current working directory to determine the target of relative
file paths.
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constant as a value for the
.IR flag
used by
\fIfaccessat\fR():
.IP AT_EACCESS 12
Check access using effective user and group ID.
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constant as a value for the
.IR flag
used by
\fIfstatat\fR(),
\fIfchmodat\fR(),
\fIfchownat\fR(),
and
\fIutimensat\fR():
.IP AT_SYMLINK_NOFOLLOW 12
.br
Do not follow symbolic links.
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constant as a value for
the flag used by
\fIlinkat\fR():
.IP AT_SYMLINK_FOLLOW 12
.br
Follow symbolic link.
.br
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constant as a value
for the flag used by
\fIunlinkat\fR():
.IP AT_REMOVEDIR 12
.br
Remove directory instead of file.
.P
The
.IR <fcntl.h> 
header shall define the following symbolic constants for the
.IR advice
argument used by
\fIposix_fadvise\fR():
.IP POSIX_FADV_DONTNEED 6
.br
The application expects that it will not access the specified data in
the near future.
.IP POSIX_FADV_NOREUSE 6
.br
The application expects to access the specified data once and then not
reuse it thereafter.
.IP POSIX_FADV_NORMAL 6
.br
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.
.IP POSIX_FADV_RANDOM 6
.br
The application expects to access the specified data in a random
order.
.IP POSIX_FADV_SEQUENTIAL 6
.br
The application expects to access the specified data sequentially from
lower offsets to higher offsets.
.IP POSIX_FADV_WILLNEED 6
.br
The application expects to access the specified data in the near
future.
.P
The
.IR <fcntl.h> 
header shall define the
.BR flock
structure describing a file lock. It shall include the following members:
.sp
.RS 4
.nf

short  l_type   \fRType of lock; F_RDLCK, F_WRLCK, F_UNLCK.\fR
short  l_whence \fRFlag for starting offset.\fR
off_t  l_start  \fRRelative offset in bytes.\fR
off_t  l_len    \fRSize; if 0 then until EOF.\fR
pid_t  l_pid    \fRProcess ID of the process holding the lock; returned with F_GETLK.\fR
.fi
.P
.RE
.P
The
.IR <fcntl.h> 
header shall define the
.BR mode_t ,
.BR off_t ,
and
.BR pid_t
types as described in
.IR <sys/types.h> .
.P
The following shall be declared as functions and may also be defined as
macros. Function prototypes shall be provided.
.sp
.RS 4
.nf

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);
.fi
.P
.RE
.P
Inclusion of the
.IR <fcntl.h> 
header may also make visible all symbols from
.IR <sys/stat.h> 
and
.IR <unistd.h> .
.LP
.IR "The following sections are informative."
.SH "APPLICATION USAGE"
Although no existing implementation defines AT_SYMLINK_FOLLOW and
AT_SYMLINK_NOFOLLOW as the same numeric value, POSIX.1\(hy2008 does not prohibit
that as the two constants are not used with the same interfaces.
.SH RATIONALE
While many of the symbolic constants introduced in the
.IR <fcntl.h> 
header do not strictly need to be used in
.BR #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\(hy2008.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fB<stdio.h>\fP",
.IR "\fB<sys_stat.h>\fP",
.IR "\fB<sys_types.h>\fP",
.IR "\fB<unistd.h>\fP"
.P
The System Interfaces volume of POSIX.1\(hy2017,
.IR "\fIcreat\fR\^(\|)",
.IR "\fIexec\fR\^",
.IR "\fIfcntl\fR\^(\|)",
.IR "\fIfutimens\fR\^(\|)",
.IR "\fIopen\fR\^(\|)",
.IR "\fIposix_fadvise\fR\^(\|)",
.IR "\fIposix_fallocate\fR\^(\|)",
.IR "\fIposix_madvise\fR\^(\|)"
.\"
.SH 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 .
.PP
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 .