'\" et
.TH FCNTL "3P" 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
\(em file control
.SH SYNOPSIS
.LP
.nf
#include <fcntl.h>
.P
int fcntl(int \fIfildes\fP, int \fIcmd\fP, ...);
.fi
.SH DESCRIPTION
The
\fIfcntl\fR()
function shall perform the operations described below on open files. The
.IR fildes
argument is a file descriptor.
.P
The available values for
.IR cmd
are defined in
.IR <fcntl.h> 
and are as follows:
.IP F_DUPFD 14
Return a new file descriptor which shall be allocated as described in
.IR "Section 2.14" ", " "File Descriptor Allocation",
except that it shall be the lowest numbered available file
descriptor greater than or equal to the third argument,
.IR arg ,
taken as an integer of type
.BR int .
The new file descriptor shall refer to the same open file description as
the original file descriptor, and shall share any locks. The FD_CLOEXEC
flag associated with the new file descriptor shall be cleared to keep
the file open across calls to one of the
.IR exec
functions.
.IP F_DUPFD_CLOEXEC 14
.br
Like F_DUPFD, but the FD_CLOEXEC flag associated with the new file
descriptor shall be set.
.IP F_GETFD 14
Get the file descriptor flags defined in
.IR <fcntl.h> 
that are associated with the file descriptor
.IR fildes .
File descriptor flags are associated with a single file descriptor and
do not affect other file descriptors that refer to the same file.
.IP F_SETFD 14
Set the file descriptor flags defined in
.IR <fcntl.h> ,
that are associated with
.IR fildes ,
to the third argument,
.IR arg ,
taken as type
.BR int .
If the FD_CLOEXEC flag in the third argument
is 0, the file descriptor shall remain open across the
.IR exec
functions; otherwise, the file descriptor shall be closed upon
successful execution of one of the
.IR exec
functions.
.IP F_GETFL 14
Get the file status flags and file access modes, defined in
.IR <fcntl.h> ,
for the file description associated with
.IR fildes .
The file access modes can be extracted from the return value using the
mask O_ACCMODE, which is defined in
.IR <fcntl.h> .
File status flags and file access modes are associated with the file
description and do not affect other file descriptors that refer to the
same file with different open file descriptions. The flags returned may
include non-standard file status flags which the application did not
set, provided that these additional flags do not alter the behavior of
a conforming application.
.IP F_SETFL 14
Set the file status flags, defined in
.IR <fcntl.h> ,
for the file description associated with
.IR fildes
from the corresponding bits in the third argument,
.IR arg ,
taken as type
.BR int .
Bits corresponding to the file access mode and the file creation
flags, as defined in
.IR <fcntl.h> ,
that are set in
.IR arg
shall be ignored. If any bits in
.IR arg
other than those mentioned here are changed by the application, the
result is unspecified. If
.IR fildes
does not support non-blocking operations, it is unspecified whether the
O_NONBLOCK flag will be ignored.
.IP F_GETOWN 14
If
.IR fildes
refers to a socket, get the process ID or process group ID specified to
receive SIGURG signals when out-of-band data is available. Positive
values shall indicate a process ID; negative values, other than \-1,
shall indicate a process group ID; the value zero shall indicate
that no SIGURG signals are to be sent. If
.IR fildes
does not refer to a socket, the results are unspecified.
.IP F_SETOWN 14
If
.IR fildes
refers to a socket, set the process ID or process group ID specified to
receive SIGURG signals when out-of-band data is available, using the
value of the third argument,
.IR arg ,
taken as type
.BR int .
Positive values shall indicate a process ID; negative values,
other than \-1, shall indicate a process group ID; the value zero
shall indicate that no SIGURG signals are to be sent. Each time a
SIGURG signal is sent to the specified process or process group,
permission checks equivalent to those performed by
\fIkill\fR()
shall be performed, as if
\fIkill\fR()
were called by a process with the same real user ID, effective
user ID, and privileges that the process calling
\fIfcntl\fR()
has at the time of the call; if the
\fIkill\fR()
call would fail, no signal shall be sent. These permission checks may
also be performed by the
\fIfcntl\fR()
call. If the process specified by
.IR arg
later terminates, or the process group specified by
.IR arg
later becomes empty, while still being specified to receive
SIGURG signals when out-of-band data is available from
.IR fildes ,
then no signals shall be sent to any subsequently created process
that has the same process ID or process group ID, regardless of
permission; it is unspecified whether this is achieved by the
equivalent of a
.IR fcntl ( fildes ", F_SETOWN, 0)"
call at the time the process terminates or is waited for or the
process group becomes empty, or by other means. If
.IR fildes
does not refer to a socket, the results are unspecified.
.P
The following values for
.IR cmd
are available for advisory record locking. Record locking shall be
supported for regular files, and may be supported for other files.
.IP F_GETLK 14
Get any lock which blocks the lock description pointed to by the
third argument,
.IR arg ,
taken as a pointer to type
.BR "struct flock" ,
defined in
.IR <fcntl.h> .
The information retrieved shall overwrite the information passed to
\fIfcntl\fR()
in the structure
.BR flock .
If no lock is found that would prevent this lock from being created,
then the structure shall be left unchanged except for the lock type
which shall be set to F_UNLCK.
.IP F_SETLK 14
Set or clear a file segment lock according to the lock description
pointed to by the third argument,
.IR arg ,
taken as a pointer to type
.BR "struct flock" ,
defined in
.IR <fcntl.h> .
F_SETLK can establish shared (or read) locks (F_RDLCK) or
exclusive (or write) locks (F_WRLCK), as well as to remove either type
of lock (F_UNLCK). F_RDLCK, F_WRLCK, and F_UNLCK are defined in
.IR <fcntl.h> .
If a shared or exclusive lock cannot be set,
\fIfcntl\fR()
shall return immediately with a return value of \-1.
.IP F_SETLKW 14
This command shall be equivalent to F_SETLK except that if a shared or
exclusive lock is blocked by other locks, the thread shall wait until
the request can be satisfied. If a signal that is to be caught is
received while
\fIfcntl\fR()
is waiting for a region,
\fIfcntl\fR()
shall be interrupted. Upon return from the signal handler,
\fIfcntl\fR()
shall return \-1 with
.IR errno
set to
.BR [EINTR] ,
and the lock operation shall not be done.
.P
Additional implementation-defined values for
.IR cmd
may be defined in
.IR <fcntl.h> .
Their names shall start with F_.
.P
When a shared lock is set on a segment of a file, other processes shall
be able to set shared locks on that segment or a portion of it. A
shared lock prevents any other process from setting an exclusive lock
on any portion of the protected area. A request for a shared lock
shall fail if the file descriptor was not opened with read access.
.P
An exclusive lock shall prevent any other process from setting a shared
lock or an exclusive lock on any portion of the protected area. A
request for an exclusive lock shall fail if the file descriptor was not
opened with write access.
.P
The structure
.BR flock
describes the type (\c
.IR l_type ),
starting offset (\c
.IR l_whence ),
relative offset (\c
.IR l_start ),
size (\c
.IR l_len ),
and process ID (\c
.IR l_pid )
of the segment of the file to be affected.
.P
The value of
.IR l_whence
is SEEK_SET, SEEK_CUR, or SEEK_END,
to indicate that the relative offset
.IR l_start
bytes shall be measured from the start of the file, current position,
or end of the file, respectively. The value of
.IR l_len
is the number of consecutive bytes to be locked. The value of
.IR l_len
may be negative (where the definition of
.BR off_t
permits negative values of
.IR l_len ).
The
.IR l_pid
field is only used with F_GETLK to return the process ID of the process
holding a blocking lock. After a successful F_GETLK request, when a
blocking lock is found, the values returned in the
.BR flock
structure shall be as follows:
.IP "\fIl_type\fP" 10
Type of blocking lock found.
.IP "\fIl_whence\fP" 10
SEEK_SET.
.IP "\fIl_start\fP" 10
Start of the blocking lock.
.IP "\fIl_len\fP" 10
Length of the blocking lock.
.IP "\fIl_pid\fP" 10
Process ID of the process that holds the blocking lock.
.P
If the command is F_SETLKW and the process must wait for another
process to release a lock, then the range of bytes to be locked shall
be determined before the
\fIfcntl\fR()
function blocks. If the file size or file descriptor seek offset change
while
\fIfcntl\fR()
is blocked, this shall not affect the range of bytes locked.
.P
If
.IR l_len
is positive, the area affected shall start at
.IR l_start
and end at
.IR l_start +\c
.IR l_len \-1.
If
.IR l_len
is negative, the area affected shall start at
.IR l_start +\c
.IR l_len
and end at
.IR l_start \-1.
Locks may start and extend beyond the current end of a file, but shall
not extend before the beginning of the file. A lock shall be set to
extend to the largest possible value of the file offset for that file
by setting
.IR l_len
to 0. If such a lock also has
.IR l_start
set to 0 and
.IR l_whence
is set to SEEK_SET, the whole file shall be locked.
.P
There shall be at most one type of lock set for each byte in the file.
Before a successful return from an F_SETLK or an F_SETLKW request when
the calling process has previously existing locks on bytes in the
region specified by the request, the previous lock type for each byte
in the specified region shall be replaced by the new lock type. As
specified above under the descriptions of shared locks and exclusive
locks, an F_SETLK or an F_SETLKW request (respectively) shall fail or
block when another process has existing locks on bytes in the specified
region and the type of any of those locks conflicts with the type
specified in the request.
.P
All locks associated with a file for a given process shall be removed
when a file descriptor for that file is closed by that process or the
process holding that file descriptor terminates. Locks are not
inherited by a child process.
.P
A potential for deadlock occurs if a process controlling a locked
region is put to sleep by attempting to lock the locked region of
another process. If the system detects that sleeping until a locked
region is unlocked would cause a deadlock,
\fIfcntl\fR()
shall fail with an
.BR [EDEADLK] 
error.
.P
An unlock (F_UNLCK) request in which
.IR l_len
is non-zero and the offset of the last byte of the requested segment is
the maximum value for an object of type
.BR off_t ,
when the process has an existing lock in which
.IR l_len
is 0 and which includes the last byte of the requested segment, shall be
treated as a request to unlock from the start of the requested segment
with an
.IR l_len
equal to 0. Otherwise, an unlock (F_UNLCK) request shall attempt to
unlock only the requested segment.
.P
When the file descriptor
.IR fildes
refers to a shared memory object, the behavior of
\fIfcntl\fR()
shall be the same as for a regular file except the effect of the
following values for the argument
.IR cmd
shall be unspecified: F_SETFL, F_GETLK, F_SETLK, and F_SETLKW.
.P
If
.IR fildes
refers to a typed memory object, the result of the
\fIfcntl\fR()
function is unspecified.
.SH "RETURN VALUE"
Upon successful completion, the value returned shall depend on
.IR cmd
as follows:
.IP F_DUPFD 12
A new file descriptor.
.IP F_DUPFD_CLOEXEC 12
.br
A new file descriptor.
.IP F_GETFD 12
Value of flags defined in
.IR <fcntl.h> .
The return value shall not be negative.
.IP F_SETFD 12
Value other than \-1.
.IP F_GETFL 12
Value of file status flags and access modes. The return value is not
negative.
.IP F_SETFL 12
Value other than \-1.
.IP F_GETLK 12
Value other than \-1.
.IP F_SETLK 12
Value other than \-1.
.IP F_SETLKW 12
Value other than \-1.
.IP F_GETOWN 12
Value of the socket owner process or process group; this will not be
\-1.
.IP F_SETOWN 12
Value other than \-1.
.P
Otherwise, \-1 shall be returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIfcntl\fR()
function shall fail if:
.TP
.BR EACCES " or " EAGAIN
.br
The
.IR cmd
argument is F_SETLK; the type of lock (\c
.IR l_type )
is a shared (F_RDLCK) or exclusive (F_WRLCK) lock and the segment of a
file to be locked is already exclusive-locked by another process, or the
type is an exclusive lock and some portion of the segment of a file to
be locked is already shared-locked or exclusive-locked by another process.
.TP
.BR EBADF
The
.IR fildes
argument is not a valid open file descriptor, or the argument
.IR cmd
is F_SETLK or F_SETLKW, the type of lock,
.IR l_type ,
is a shared lock (F_RDLCK), and
.IR fildes
is not a valid file descriptor open for reading, or the type of lock,
.IR l_type ,
is an exclusive lock (F_WRLCK), and
.IR fildes
is not a valid file descriptor open for writing.
.TP
.BR EINTR
The
.IR cmd
argument is F_SETLKW and the function was interrupted by a signal.
.TP
.BR EINVAL
The
.IR cmd
argument is invalid, or the
.IR cmd
argument is F_DUPFD or F_DUPFD_CLOEXEC and
.IR arg
is negative or greater than or equal to
{OPEN_MAX},
or the
.IR cmd
argument is F_GETLK, F_SETLK, or F_SETLKW and the data pointed to by
.IR arg
is not valid, or
.IR fildes
refers to a file that does not support locking.
.TP
.BR EMFILE
The argument
.IR cmd
is F_DUPFD or F_DUPFD_CLOEXEC and all file descriptors available to
the process are currently open, or no file descriptors greater than or
equal to
.IR arg
are available.
.TP
.BR ENOLCK
The argument
.IR cmd
is F_SETLK or F_SETLKW and satisfying the lock or unlock request would
result in the number of locked regions in the system exceeding a
system-imposed limit.
.TP
.BR EOVERFLOW
One of the values to be returned cannot be represented correctly.
.TP
.BR EOVERFLOW
The
.IR cmd
argument is F_GETLK, F_SETLK, or F_SETLKW and the smallest or, if
.IR l_len
is non-zero, the largest offset of any byte in the requested segment
cannot be represented correctly in an object of type
.BR off_t .
.TP
.BR ESRCH
The
.IR cmd
argument is F_SETOWN and no process or process group can be found
corresponding to that specified by
.IR arg .
.br
.P
The
\fIfcntl\fR()
function may fail if:
.TP
.BR EDEADLK
The
.IR cmd
argument is F_SETLKW, the lock is blocked by a lock from another
process, and putting the calling process to sleep to wait for that lock
to become free would cause a deadlock.
.TP
.BR EINVAL
The
.IR cmd
argument is F_SETOWN and the value of the argument is not valid as a
process or process group identifier.
.TP
.BR EPERM
The
.IR cmd
argument is F_SETOWN and the calling process does not have
permission to send a SIGURG signal to any process specified by
.IR arg .
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Locking and Unlocking a File"
.P
The following example demonstrates how to place a lock on bytes 100 to
109 of a file and then later remove it. F_SETLK is used to perform a
non-blocking lock request so that the process does not have to wait if
an incompatible lock is held by another process; instead the process
can take some other action.
.sp
.RS 4
.nf

#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
.P
int
main(int argc, char *argv[])
{
    int fd;
    struct flock fl;
.P
    fd = open("testfile", O_RDWR);
    if (fd == -1)
        /* Handle error */;
.P
    /* Make a non-blocking request to place a write lock
       on bytes 100-109 of testfile */
.P
    fl.l_type = F_WRLCK;
    fl.l_whence = SEEK_SET;
    fl.l_start = 100;
    fl.l_len = 10;
.P
    if (fcntl(fd, F_SETLK, &fl) == -1) {
        if (errno == EACCES || errno == EAGAIN) {
            printf("Already locked by another process\en");
.P
            /* We cannot get the lock at the moment */
.P
        } else {
            /* Handle unexpected error */;
        }
    } else { /* Lock was granted... */
.P
        /* Perform I/O on bytes 100 to 109 of file */
.P
        /* Unlock the locked bytes */
.P
        fl.l_type = F_UNLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = 100;
        fl.l_len = 10;
        if (fcntl(fd, F_SETLK, &fl) == -1)
            /* Handle error */;
    }
    exit(EXIT_SUCCESS);
} /* main */
.fi
.P
.RE
.SS "Setting the Close-on-Exec Flag"
.P
The following example demonstrates how to set the close-on-exec flag
for the file descriptor
.IR fd .
.sp
.RS 4
.nf

#include <unistd.h>
#include <fcntl.h>
\&...
    int flags;
.P
    flags = fcntl(fd, F_GETFD);
    if (flags == -1)
        /* Handle error */;
    flags |= FD_CLOEXEC;
    if (fcntl(fd, F_SETFD, flags) == -1)
        /* Handle error */;"
.fi
.P
.RE
.SH "APPLICATION USAGE"
The
.IR arg
values to F_GETFD, F_SETFD, F_GETFL, and F_SETFL all represent flag
values to allow for future growth. Applications using these functions
should do a read-modify-write operation on them, rather than assuming
that only the values defined by this volume of POSIX.1\(hy2017 are valid. It is a common error to
forget this, particularly in the case of F_SETFD. Some implementations
set additional file status flags to advise the application of default
behavior, even though the application did not request these flags.
.P
On systems which do not perform permission checks at the time of an
\fIfcntl\fR()
call with F_SETOWN, if the permission checks performed at the time
the signal is sent disallow sending the signal to any process,
the process that called
\fIfcntl\fR()
has no way of discovering that this has happened. A call to
\fIkill\fR()
with signal 0 can be used as a prior check of permissions, although
this is no guarantee that permission will be granted at the time
a signal is sent, since the target process(es) could change
user IDs or privileges in the meantime.
.SH RATIONALE
The ellipsis in the SYNOPSIS is the syntax specified by the ISO\ C standard
for a variable number of arguments. It is used because System V uses
pointers for the implementation of file locking functions.
.P
This volume of POSIX.1\(hy2017 permits concurrent read and write access to file data using the
\fIfcntl\fR()
function; this is a change from the 1984 /usr/group standard and early proposals. Without
concurrency controls, this feature may not be fully utilized without
occasional loss of data.
.P
Data losses occur in several ways. One case occurs when several
processes try to update the same record, without sequencing controls;
several updates may occur in parallel and the last writer ``wins''.
Another case is a bit-tree or other internal list-based database that
is undergoing reorganization. Without exclusive use to the tree segment
by the updating process, other reading processes chance getting lost in
the database when the index blocks are split, condensed, inserted, or
deleted. While
\fIfcntl\fR()
is useful for many applications, it is not intended to be overly
general and does not handle the bit-tree example well.
.P
This facility is only required for regular files because it is not
appropriate for many devices such as terminals and network
connections.
.P
Since
\fIfcntl\fR()
works with ``any file descriptor associated with that file, however it
is obtained'', the file descriptor may have been inherited through a
\fIfork\fR()
or
.IR exec
operation and thus may affect a file that another process also has
open.
.P
The use of the open file description to identify what to lock requires
extra calls and presents problems if several processes are sharing an
open file description, but there are too many implementations of the
existing mechanism for this volume of POSIX.1\(hy2017 to use different specifications.
.P
Another consequence of this model is that closing any file descriptor
for a given file (whether or not it is the same open file description
that created the lock) causes the locks on that file to be relinquished
for that process. Equivalently, any close for any file/process pair
relinquishes the locks owned on that file for that process. But note
that while an open file description may be shared through
\fIfork\fR(),
locks are not inherited through
\fIfork\fR().
Yet locks may be inherited through one of the
.IR exec
functions.
.P
The identification of a machine in a network environment is outside
the scope of this volume of POSIX.1\(hy2017. Thus, an
.IR l_sysid
member, such as found in System V, is not included in the locking
structure.
.P
Changing of lock types can result in a previously locked region being
split into smaller regions.
.P
Mandatory locking was a major feature of the 1984 /usr/group standard.
.P
For advisory file record locking to be effective, all processes that
have access to a file must cooperate and use the advisory mechanism
before doing I/O on the file. Enforcement-mode record locking is
important when it cannot be assumed that all processes are cooperating.
For example, if one user uses an editor to update a file at the same
time that a second user executes another process that updates the same
file and if only one of the two processes is using advisory locking,
the processes are not cooperating. Enforcement-mode record locking
would protect against accidental collisions.
.P
Secondly, advisory record locking requires a process using locking to
bracket each I/O operation with lock (or test) and unlock operations.
With enforcement-mode file and record locking, a process can lock the
file once and unlock when all I/O operations have been completed.
Enforcement-mode record locking provides a base that can be enhanced;
for example, with sharable locks. That is, the mechanism could be
enhanced to allow a process to lock a file so other processes could
read it, but none of them could write it.
.P
Mandatory locks were omitted for several reasons:
.IP " 1." 4
Mandatory lock setting was done by multiplexing the set-group-ID
bit in most implementations; this was confusing, at best.
.IP " 2." 4
The relationship to file truncation as supported in 4.2 BSD
was not well specified.
.IP " 3." 4
Any publicly readable file could be locked by anyone. Many historical
implementations keep the password database in a publicly readable
file. A malicious user could thus prohibit logins. Another
possibility would be to hold open a long-distance telephone line.
.IP " 4." 4
Some demand-paged historical implementations offer memory mapped files,
and enforcement cannot be done on that type of file.
.P
Since sleeping on a region is interrupted with any signal,
\fIalarm\fR()
may be used to provide a timeout facility in applications requiring
it. This is useful in deadlock detection. Since implementation of
full deadlock detection is not always feasible, the
.BR [EDEADLK] 
error was made optional.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIalarm\fR\^(\|)",
.IR "\fIclose\fR\^(\|)",
.IR "\fIexec\fR\^",
.IR "\fIkill\fR\^(\|)",
.IR "\fIopen\fR\^(\|)",
.IR "\fIsigaction\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<fcntl.h>\fP",
.IR "\fB<signal.h>\fP"
.\"
.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 .