FCNTL(3P) POSIX Programmer's Manual FCNTL(3P)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.
NAMEfcntl - file control
SYNOPSIS
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fildes, int cmd, ...);
DESCRIPTION
The fcntl() function shall perform the operations described below on
open files. The fildes argument is a file descriptor.
The available values for cmd are defined in <fcntl.h> and are as fol‐
lows:
F_DUPFD
Return a new file descriptor which shall be the lowest numbered
available (that is, not already open) file descriptor greater
than or equal to the third argument, arg, taken as an integer of
type 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 exec functions.
F_GETFD
Get the file descriptor flags defined in <fcntl.h> that are
associated with the file descriptor fildes. File descriptor
flags are associated with a single file descriptor and do not
affect other file descriptors that refer to the same file.
F_SETFD
Set the file descriptor flags defined in <fcntl.h>, that are
associated with fildes, to the third argument, arg, taken as
type int. If the FD_CLOEXEC flag in the third argument is 0, the
file shall remain open across the exec functions; otherwise, the
file shall be closed upon successful execution of one of the
exec functions.
F_GETFL
Get the file status flags and file access modes, defined in
<fcntl.h>, for the file description associated with fildes. The
file access modes can be extracted from the return value using
the mask O_ACCMODE, which is defined in <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.
F_SETFL
Set the file status flags, defined in <fcntl.h>, for the file
description associated with fildes from the corresponding bits
in the third argument, arg, taken as type int. Bits correspond‐
ing to the file access mode and the file creation flags, as
defined in <fcntl.h>, that are set in arg shall be ignored. If
any bits in arg other than those mentioned here are changed by
the application, the result is unspecified.
F_GETOWN
If fildes refers to a socket, get the process or process group
ID specified to receive SIGURG signals when out-of-band data is
available. Positive values indicate a process ID; negative val‐
ues, other than -1, indicate a process group ID. If fildes does
not refer to a socket, the results are unspecified.
F_SETOWN
If fildes refers to a socket, set the process or process group
ID specified to receive SIGURG signals when out-of-band data is
available, using the value of the third argument, arg, taken as
type int. Positive values indicate a process ID; negative val‐
ues, other than -1, indicate a process group ID. If fildes does
not refer to a socket, the results are unspecified.
The following values for cmd are available for advisory record locking.
Record locking shall be supported for regular files, and may be sup‐
ported for other files.
F_GETLK
Get the first lock which blocks the lock description pointed to
by the third argument, arg, taken as a pointer to type struct
flock, defined in <fcntl.h>. The information retrieved shall
overwrite the information passed to fcntl() in the structure
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.
F_SETLK
Set or clear a file segment lock according to the lock descrip‐
tion pointed to by the third argument, arg, taken as a pointer
to type struct flock, defined in <fcntl.h>. F_SETLK can estab‐
lish 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
<fcntl.h>. If a shared or exclusive lock cannot be set, fcntl()
shall return immediately with a return value of -1.
F_SETLKW
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 fcntl() is waiting for a
region, fcntl() shall be interrupted. Upon return from the sig‐
nal handler, fcntl() shall return -1 with errno set to [EINTR],
and the lock operation shall not be done.
Additional implementation-defined values for cmd may be defined in
<fcntl.h>. Their names shall start with F_.
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.
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.
The structure flock describes the type ( l_type), starting offset (
l_whence), relative offset ( l_start), size ( l_len), and process ID (
l_pid) of the segment of the file to be affected.
The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END, to indicate
that the relative offset l_start bytes shall be measured from the start
of the file, current position, or end of the file, respectively. The
value of l_len is the number of consecutive bytes to be locked. The
value of l_len may be negative (where the definition of off_t permits
negative values of l_len). The 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 flock structure shall be as follows:
l_type Type of blocking lock found.
l_whence
SEEK_SET.
l_start
Start of the blocking lock.
l_len Length of the blocking lock.
l_pid Process ID of the process that holds the blocking lock.
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 fcntl() function blocks. If the file size or
file descriptor seek offset change while fcntl() is blocked, this shall
not affect the range of bytes locked.
If l_len is positive, the area affected shall start at l_start and end
at l_start+ l_len-1. If l_len is negative, the area affected shall
start at l_start+ l_len and end at 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 l_len to 0.
If such a lock also has l_start set to 0 and l_whence is set to
SEEK_SET, the whole file shall be locked.
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 spec‐
ified in the request.
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 inher‐
ited by a child process.
A potential for deadlock occurs if a process controlling a locked
region is put to sleep by attempting to lock another process' locked
region. If the system detects that sleeping until a locked region is
unlocked would cause a deadlock, fcntl() shall fail with an [EDEADLK]
error.
An unlock (F_UNLCK) request in which 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 off_t, when the process has an existing lock in which
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 l_len equal to 0. Otherwise, an unlock (F_UNLCK)
request shall attempt to unlock only the requested segment.
When the file descriptor fildes refers to a shared memory object, the
behavior of fcntl() shall be the same as for a regular file except the
effect of the following values for the argument cmd shall be unspeci‐
fied: F_SETFL, F_GETLK, F_SETLK, and F_SETLKW.
If fildes refers to a typed memory object, the result of the fcntl()
function is unspecified.
RETURN VALUE
Upon successful completion, the value returned shall depend on cmd as
follows:
F_DUPFD
A new file descriptor.
F_GETFD
Value of flags defined in <fcntl.h>. The return value shall not
be negative.
F_SETFD
Value other than -1.
F_GETFL
Value of file status flags and access modes. The return value is
not negative.
F_SETFL
Value other than -1.
F_GETLK
Value other than -1.
F_SETLK
Value other than -1.
F_SETLKW
Value other than -1.
F_GETOWN
Value of the socket owner process or process group; this will
not be -1.
F_SETOWN
Value other than -1.
Otherwise, -1 shall be returned and errno set to indicate the error.
ERRORS
The fcntl() function shall fail if:
EACCES or EAGAIN
The cmd argument is F_SETLK; the type of lock ( 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.
EBADF The fildes argument is not a valid open file descriptor, or the
argument cmd is F_SETLK or F_SETLKW, the type of lock, l_type,
is a shared lock (F_RDLCK), and fildes is not a valid file
descriptor open for reading, or the type of lock, l_type, is an
exclusive lock (F_WRLCK), and fildes is not a valid file
descriptor open for writing.
EINTR The cmd argument is F_SETLKW and the function was interrupted by
a signal.
EINVAL The cmd argument is invalid, or the cmd argument is F_DUPFD and
arg is negative or greater than or equal to {OPEN_MAX}, or the
cmd argument is F_GETLK, F_SETLK, or F_SETLKW and the data
pointed to by arg is not valid, or fildes refers to a file that
does not support locking.
EMFILE The argument cmd is F_DUPFD and {OPEN_MAX} file descriptors are
currently open in the calling process, or no file descriptors
greater than or equal to arg are available.
ENOLCK The argument 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.
EOVERFLOW
One of the values to be returned cannot be represented cor‐
rectly.
EOVERFLOW
The cmd argument is F_GETLK, F_SETLK, or F_SETLKW and the small‐
est or, if l_len is non-zero, the largest offset of any byte in
the requested segment cannot be represented correctly in an
object of type off_t.
The fcntl() function may fail if:
EDEADLK
The 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.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
The ellipsis in the SYNOPSIS is the syntax specified by the ISO C stan‐
dard for a variable number of arguments. It is used because System V
uses pointers for the implementation of file locking functions.
The arg values to F_GETFD, F_SETFD, F_GETFL, and F_SETFL all represent
flag values to allow for future growth. Applications using these func‐
tions should do a read-modify-write operation on them, rather than
assuming that only the values defined by this volume of
IEEE Std 1003.1-2001 are valid. It is a common error to forget this,
particularly in the case of F_SETFD.
This volume of IEEE Std 1003.1-2001 permits concurrent read and write
access to file data using the fcntl() 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.
Data losses occur in several ways. One case occurs when several pro‐
cesses try to update the same record, without sequencing controls; sev‐
eral updates may occur in parallel and the last writer "wins". Another
case is a bit-tree or other internal list-based database that is under‐
going 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 fcntl() is useful for many applications, it is not
intended to be overly general and does not handle the bit-tree example
well.
This facility is only required for regular files because it is not
appropriate for many devices such as terminals and network connections.
Since fcntl() works with "any file descriptor associated with that
file, however it is obtained", the file descriptor may have been inher‐
ited through a fork() or exec operation and thus may affect a file that
another process also has open.
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 IEEE Std 1003.1-2001 to use dif‐
ferent specifications.
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 fork(), locks
are not inherited through fork(). Yet locks may be inherited through
one of the exec functions.
The identification of a machine in a network environment is outside the
scope of this volume of IEEE Std 1003.1-2001. Thus, an l_sysid member,
such as found in System V, is not included in the locking structure.
Changing of lock types can result in a previously locked region being
split into smaller regions.
Mandatory locking was a major feature of the 1984 /usr/group standard.
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 impor‐
tant 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.
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.
Mandatory locks were omitted for several reasons:
1. Mandatory lock setting was done by multiplexing the set-group-ID
bit in most implementations; this was confusing, at best.
2. The relationship to file truncation as supported in 4.2 BSD was not
well specified.
3. Any publicly readable file could be locked by anyone. Many histori‐
cal implementations keep the password database in a publicly read‐
able file. A malicious user could thus prohibit logins. Another
possibility would be to hold open a long-distance telephone line.
4. Some demand-paged historical implementations offer memory mapped
files, and enforcement cannot be done on that type of file.
Since sleeping on a region is interrupted with any signal, alarm() 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 [EDEADLK] error was made
optional.
FUTURE DIRECTIONS
None.
SEE ALSOalarm(), close(), exec(), open(), sigaction(), the Base Definitions
volume of IEEE Std 1003.1-2001, <fcntl.h>, <signal.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 FCNTL(3P)
