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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FCNTL(3P) POSIX Programmer's Manual FCNTL(3P)
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.
fcntl — file control
#include <fcntl.h>
int fcntl(int fildes, int cmd, ...);
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
follows:
F_DUPFD Return a new file descriptor which shall be
allocated as described in 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, 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_DUPFD_CLOEXEC
Like F_DUPFD, but the FD_CLOEXEC flag associated
with the new file descriptor shall be set.
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 descriptor shall remain open across the exec
functions; otherwise, the file descriptor 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. 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.
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 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. If fildes does not support
non-blocking operations, it is unspecified whether
the O_NONBLOCK flag will be ignored.
F_GETOWN If 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 fildes
does not refer to a socket, the results are
unspecified.
F_SETOWN If 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, arg, taken as type
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 kill() shall be performed, as if
kill() were called by a process with the same real
user ID, effective user ID, and privileges that the
process calling fcntl() has at the time of the
call; if the kill() call would fail, no signal
shall be sent. These permission checks may also be
performed by the fcntl() call. If the process
specified by arg later terminates, or the process
group specified by arg later becomes empty, while
still being specified to receive SIGURG signals
when out-of-band data is available from 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 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 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 any 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 conflicts 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 the locked region of
another process. 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 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.
Upon successful completion, the value returned shall depend on
cmd as follows:
F_DUPFD A new file descriptor.
F_DUPFD_CLOEXEC
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.
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 or F_DUPFD_CLOEXEC 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 or F_DUPFD_CLOEXEC and all
file descriptors available to the process are currently
open, 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.
ESRCH The cmd argument is F_SETOWN and no process or process
group can be found corresponding to that specified by arg.
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.
EINVAL The cmd argument is F_SETOWN and the value of the argument
is not valid as a process or process group identifier.
EPERM The cmd argument is F_SETOWN and the calling process does
not have permission to send a SIGURG signal to any process
specified by arg.
The following sections are informative.
Locking and Unlocking a File
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.
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
int fd;
struct flock fl;
fd = open("testfile", O_RDWR);
if (fd == -1)
/* Handle error */;
/* Make a non-blocking request to place a write lock
on bytes 100-109 of testfile */
fl.l_type = F_WRLCK;
fl.l_whence = SEEK_SET;
fl.l_start = 100;
fl.l_len = 10;
if (fcntl(fd, F_SETLK, &fl) == -1) {
if (errno == EACCES || errno == EAGAIN) {
printf("Already locked by another process\n");
/* We cannot get the lock at the moment */
} else {
/* Handle unexpected error */;
}
} else { /* Lock was granted... */
/* Perform I/O on bytes 100 to 109 of file */
/* Unlock the locked bytes */
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 */
Setting the Close-on-Exec Flag
The following example demonstrates how to set the close-on-exec
flag for the file descriptor fd.
#include <unistd.h>
#include <fcntl.h>
...
int flags;
flags = fcntl(fd, F_GETFD);
if (flags == -1)
/* Handle error */;
flags |= FD_CLOEXEC;
if (fcntl(fd, F_SETFD, flags) == -1)
/* Handle error */;"
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 functions should do a read-modify-write operation on
them, rather than assuming that only the values defined by this
volume of POSIX.1‐2017 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.
On systems which do not perform permission checks at the time of
an fcntl() 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 fcntl() has no way of
discovering that this has happened. A call to kill() 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.
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.
This volume of POSIX.1‐2017 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 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
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 processes
are sharing an open file description, but there are too many
implementations of the existing mechanism for this volume of
POSIX.1‐2017 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. 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 POSIX.1‐2017. 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 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
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 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.
None.
alarm(3p), close(3p), exec(1p), kill(3p), open(3p), sigaction(3p)
The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),
signal.h(0p)
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 .
IEEE/The Open Group 2017 FCNTL(3P)
Pages that refer to this page: fcntl.h(0p), stropts.h(0p), aio_fsync(3p), dup(3p), exec(3p), fchmod(3p), fdatasync(3p), fork(3p), fstatvfs(3p), ioctl(3p), lockf(3p), mmap(3p), open(3p), pipe(3p), posix_spawn(3p), posix_typed_mem_open(3p), pselect(3p), read(3p), shm_open(3p), write(3p)
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