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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 (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.
NAME
fcntl - 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 descrip-
tor.
The available values for cmd are defined in <fcntl.h> and are as follows:
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 corre-
sponding bits in the third argument, arg, taken as type int. Bits corresponding 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 values, 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 values, 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 supported 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 description pointed to by the third argument, arg, taken as
a pointer to type struct flock, defined in <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 <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 signal 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 con-
flicts with the type specified 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 inherited 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 reg-
ular file except the effect of the following values for the argument cmd shall be unspecified: 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 correctly.
EOVERFLOW
The cmd argument is F_GETLK, F_SETLK, or F_SETLKW and the smallest 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 standard 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. Applica-
tions using these functions 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 processes try to update the same record, without sequenc-
ing 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 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 inherited 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 pro-
cesses 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 different 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. Equiva-
lently, 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 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 sec-
ond user executes another process that updates the same file and if only one of the two processes is using advisory lock-
ing, 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 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.
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 applica-
tions 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 ALSO
alarm(), close(), exec(), open(), sigaction(), the Base Definitions volume of IEEE Std 1003.1-2001, <fcntl.h>, <sig-
nal.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, Copy-
right (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.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 FCNTL(3P)
