FCNTL(2) System Calls Manual FCNTL(2)
NAME
fcntl – file control
SYNOPSIS
#include <fcntl.h>
int
fcntl(int fildes, int cmd, ...);
DESCRIPTION
fcntl() provides for control over descriptors. The argument fildes is a
descriptor to be operated on by cmd as follows:
F_DUPFD Return a new descriptor as follows:
• Lowest numbered available descriptor
greater than or equal to arg.
• Same object references as the original
descriptor.
• New descriptor shares the same file offset
if the object was a file.
• Same access mode (read, write or
read/write).
• Same file status flags (i.e., both file
descriptors share the same file status
flags).
• The close-on-exec flag associated with the
new file descriptor is cleared so that the
descriptor remains open across an execv(2)
system call.
F_DUPFD_CLOEXEC Like F_DUPFD, except that the close-on-exec flag
associated with the new file descriptor is set.
F_GETFD Get the flags associated with the file descriptor
fildes, as described below (arg is ignored).
F_SETFD Set the file descriptor flags to arg.
F_GETFL Get descriptor status flags, as described below
(arg is ignored).
F_SETFL Set descriptor status flags to arg.
F_GETOWN Get the process ID or process group currently
receiving SIGIO and SIGURG signals; process groups
are returned as negative values (arg is ignored).
F_SETOWN Set the process or process group to receive SIGIO
and SIGURG signals; process groups are specified
by supplying arg as negative, otherwise arg is
interpreted as a process ID.
F_GETPATH Get the path of the file descriptor fildes. The
argument must be a buffer of size MAXPATHLEN or
greater.
F_GETPATH_NOFIRMLINK Get the non firmlinked path of the file descriptor
fildes. The argument must be a buffer of size
MAXPATHLEN or greater.
F_PREALLOCATE Preallocate file storage space. Note: upon
success, the space that is allocated can be the
size requested, larger than the size requested, or
(if the F_ALLOCATEALL flag is not provided)
smaller than the space requested.
F_PUNCHHOLE Deallocate a region and replace it with a hole.
Subsequent reads of the affected region will
return bytes of zeros that are usually not backed
by physical blocks. This will not change the
actual file size. Holes must be aligned to file
system block boundaries. This will fail on file
systems that do not support this interface.
F_SETSIZE Deprecated. In previous releases, this would
allow a process with root privileges to truncate a
file without zeroing space. For security reasons,
this operation is no longer supported and will
instead truncate the file in the same manner as
truncate(2).
F_RDADVISE Issue an advisory read async with no copy to user.
F_RDAHEAD Turn read ahead off/on. A zero value in arg
disables read ahead. A non-zero value in arg
turns read ahead on.
F_NOCACHE Turns data caching off/on. A non-zero value in arg
turns data caching off. A value of zero in arg
turns data caching on.
F_LOG2PHYS Get disk device information. Currently this only
returns the disk device address that corresponds
to the current file offset. Note that the system
may return -1 as the disk device address if the
file is not backed by physical blocks. This is
subject to change.
F_LOG2PHYS_EXT Variant of F_LOG2PHYS that uses the passed in file
offset and length.
F_BARRIERFSYNC Does the same thing as fsync(2) then issues a
barrier command to the drive (arg is ignored).
The barrier applies to I/O that have been flushed
with fsync(2) on the same device before. These
operations are guaranteed to be persisted before
any other I/O that would follow the barrier,
although no assumption should be made on what has
been persisted or not when this call returns.
After the barrier has been issued, operations on
other FDs that have been fsync'd before can still
be re-ordered by the device, but not after the
barrier. This is typically useful to guarantee
valid state on disk when ordering is a concern but
durability is not. A barrier can be used to order
two phases of operations on a set of file
descriptors and ensure that no file can possibly
get persisted with the effect of the second phase
without the effect of the first one. To do so,
execute operations of phase one, then fsync(2)
each FD and issue a single barrier. Finally
execute operations of phase two. This is
currently implemented on HFS and APFS. It requires
hardware support, which Apple SSDs are guaranteed
to provide.
F_FULLFSYNC Does the same thing as fsync(2) then asks the
drive to flush all buffered data to the permanent
storage device (arg is ignored). As this drains
the entire queue of the device and acts as a
barrier, data that had been fsync'd on the same
device before is guaranteed to be persisted when
this call returns. This is currently implemented
on HFS, MS-DOS (FAT), Universal Disk Format (UDF)
and APFS file systems. The operation may take
quite a while to complete. Certain FireWire
drives have also been known to ignore the request
to flush their buffered data.
F_SETNOSIGPIPE Determines whether a SIGPIPE signal will be
generated when a write fails on a pipe or socket
for which there is no reader. If arg is non-zero,
SIGPIPE generation is disabled for descriptor
fildes, while an arg of zero enables it (the
default).
F_GETNOSIGPIPE Returns whether a SIGPIPE signal will be generated
when a write fails on a pipe or socket for which
there is no reader. The semantics of the return
value match those of the arg of F_SETNOSIGPIPE.
F_TRANSFEREXTENTS Transfer any extra space in the file past the
logical EOF (as previously allocated via
F_PREALLOCATE) to another file. The other file is
specified via a file descriptor as the lone extra
argument. Both descriptors must reference regular
files in the same volume.
The flags for the F_GETFD and F_SETFD commands are as follows:
FD_CLOEXEC Close-on-exec; the given file descriptor will be
automatically closed in the successor process image
when one of the execv(2) or posix_spawn(2) family of
system calls is invoked.
The flags for the F_GETFL and F_SETFL commands are as follows:
O_NONBLOCK Non-blocking I/O; if no data is available to a read(2)
call, or if a write(2) operation would block, the read
or write call returns -1 with the error EAGAIN.
O_APPEND Force each write to append at the end of file;
corresponds to the O_APPEND flag of open(2).
O_ASYNC Enable the SIGIO signal to be sent to the process
group when I/O is possible, e.g., upon availability of
data to be read.
Several commands are available for doing advisory file locking; they all
operate on the following structure:
struct flock {
off_t l_start; /* starting offset */
off_t l_len; /* len = 0 means until end of file */
pid_t l_pid; /* lock owner */
short l_type; /* lock type: read/write, etc. */
short l_whence; /* type of l_start */
};
The commands available for advisory record locking are as follows:
F_GETLK Get the first lock that blocks the lock description pointed to
by the third argument, arg, taken as a pointer to a struct
flock (see above). The information retrieved overwrites the
information passed to fcntl in the flock structure. If no
lock is found that would prevent this lock from being created,
the structure is left unchanged by this function call except
for the lock type which is set to F_UNLCK. If a lock that
does not support the discovery of lock ownership by process
(such as an OFD lock (see below), one created by the flock(2)
system call or the open(2) system call with the O_SHLOCK or
O_EXLOCK flag) is found, l_pid is set to -1.
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 a struct flock (see above). F_SETLK is used to
establish shared (or read) locks (F_RDLCK) or exclusive (or
write) locks, (F_WRLCK), as well as remove either type of lock
(F_UNLCK). If a shared or exclusive lock cannot be set, fcntl
returns immediately with EAGAIN.
F_SETLKW This command is the same as F_SETLK except that if a shared or
exclusive lock is blocked by other locks, the thread waits
until the request can be satisfied. If a signal that is to be
caught is received while fcntl is waiting for a region, the
fcntl will be interrupted if the signal handler has not
specified the SA_RESTART (see sigaction(2)).
When a shared lock has been set on a segment of a file, other processes
can 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 fails if the file
descriptor was not opened with read access.
An exclusive lock prevents 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 fails if the file was not opened with write access.
The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that
the relative offset, l_start bytes, will 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. If l_len is
negative, the result is undefined. 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, the value of l_whence is SEEK_SET.
Locks may start and extend beyond the current end of a file, but may not
start or extend before the beginning of the file. A lock is set to
extend to the largest possible value of the file offset for that file if
l_len is set to zero. If l_whence and l_start point to the beginning of
the file, and l_len is zero, the entire file is locked. If an
application wishes only to do entire file locking, the flock(2) system
call is more efficient.
There is 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 is 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 fails or blocks respectively 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.
This interface follows the completely stupid semantics of System V and
IEEE Std 1003.1-1988 (“POSIX.1”) that require that all locks associated
with a file for a given process are removed when any file descriptor for
that file is closed by that process. This semantic means that
applications must be aware of any files that a subroutine library may
access. For example if an application for updating the password file
locks the password file database while making the update, and then calls
getpwnam(3) to retrieve a record, the lock will be lost because
getpwnam(3) opens, reads, and closes the password database. The database
close will release all locks that the process has associated with the
database, even if the library routine never requested a lock on the
database. Another minor semantic problem with this interface is that
locks are not inherited by a child process created using the fork(2)
function. The flock(2) interface has much more rational last close
semantics and allows locks to be inherited by child processes. Flock(2)
is recommended for applications that want to ensure the integrity of
their locks when using library routines or wish to pass locks to their
children. Note that flock(2) and fcntl locks may be safely used
concurrently.
All locks associated with a file for a given process are removed when the
process terminates.
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. This implementation detects that sleeping until a locked region
is unlocked would cause a deadlock and fails with an EDEADLK error.
An alternative set of commands is available for advisory record locking;
they operate on the same flock structure described above, the fields of
the structure have the same semantics, and the commands behave similarly
to the traditional record locks described above. The primary difference
is that open file description (OFD) locks are locks on the file
associated with the open file description used to acquire them, and not
with the process that created them. OFD locks are conceptually similar
to locks managed by flock(2), with the addition of record locking
capabilities.
A new open file description can be obtained from e.g., open(2). However,
file descriptors that have been duplicated using e.g., dup(2) or fork(2)
do not result in multiple instances of a lock, but create additional
references to the same open file description, and thus reference the same
lock. For example, if a process holding an OFD lock on a file forks, and
the child explicitly unlocks a record, the parent will also lose that
lock on the same record.
Only the last close of the last file descriptor in any process still
referencing the open file description causes an automatic unlock to
occur, so this type of record lock avoids the more unfortunate close(2)
semantics of traditional advisory record locks.
The commands used for OFD locks are direct analogs of traditional record
locking commands:
F_OFD_GETLK Get the first lock that blocks the lock description
pointed to by the third argument, arg, taken as a pointer
to a struct flock (see above). The information retrieved
overwrites the information passed to fcntl in the flock
structure. If no lock is found that would prevent this
lock from being created, the structure is left unchanged
by this function call except for the lock type which is
set to F_UNLCK. If a lock that does not support the
discovery of lock ownership by process (such as an OFD
lock, one created by the flock(2) system call or the
open(2) system call with the O_SHLOCK or O_EXLOCK flag) is
found, l_pid is set to -1.
F_OFD_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 a struct flock (see above). F_SETLK is
used to establish shared (or read) locks (F_RDLCK) or
exclusive (or write) locks, (F_WRLCK), as well as remove
either type of lock (F_UNLCK). If a shared or exclusive
lock cannot be set, fcntl returns immediately with a
return value of -1.
F_OFD_SETLKW This command is the same as F_OFD_SETLK except that if a
shared or exclusive lock is blocked by other locks, the
thread waits until the request can be satisfied. If a
signal that is to be caught is received while fcntl is
waiting for a region, the fcntl will be interrupted if the
signal handler has not specified the SA_RESTART (see
sigaction(2)).
No deadlock detection is performed for OFD file locks.
The F_PREALLOCATE command operates on the following structure:
typedef struct fstore {
u_int32_t fst_flags; /* IN: flags word */
int fst_posmode; /* IN: indicates offset field */
off_t fst_offset; /* IN: start of the region */
off_t fst_length; /* IN: size of the region */
off_t fst_bytesalloc; /* OUT: number of bytes allocated */
} fstore_t;
The flags (fst_flags) for the F_PREALLOCATE command are as follows:
F_ALLOCATECONTIG Allocate contiguous space. (Note that the file
system may ignore this request if fst_length is
very large.)
F_ALLOCATEALL Allocate all requested space or no space at all.
F_ALLOCATEPERSIST Allocate space that is not freed when close(2)
is called. (Note that the file system may ignore
this request.)
The position modes (fst_posmode) for the F_PREALLOCATE command indicate
how to use the offset field. The modes are as follows:
F_PEOFPOSMODE Allocate from the physical end of file. In this
case, fst_length indicates the number of newly
allocated bytes desired.
F_VOLPOSMODE Allocate from the volume offset.
The F_PUNCHHOLE command operates on the following structure:
typedef struct fpunchhole {
u_int32_t fp_flags; /* unused */
u_int32_t reserved; /* (to maintain 8-byte alignment) */
off_t fp_offset; /* IN: start of the region */
off_t fp_length; /* IN: size of the region */
} fpunchhole_t;
The F_RDADVISE command operates on the following structure which holds
information passed from the user to the system:
struct radvisory {
off_t ra_offset; /* offset into the file */
int ra_count; /* size of the read */
};
The F_LOG2PHYS command operates on the following structure:
struct log2phys {
u_int32_t l2p_flags; /* unused so far */
off_t l2p_contigbytes; /* unused so far */
off_t l2p_devoffset; /* bytes into device */
};
The F_LOG2PHYS_EXT command operates on the same structure as F_LOG2PHYS
but treats it as an in/out:
struct log2phys {
u_int32_t l2p_flags; /* unused so far */
off_t l2p_contigbytes; /* IN: number of bytes to be queried;
OUT: number of contiguous bytes allocated at this position */
off_t l2p_devoffset; /* IN: bytes into file;
OUT: bytes into device */
};
If fildes is a socket, then the F_SETNOSIGPIPE and F_GETNOSIGPIPE
commands are directly analogous, and fully interoperate with the
SO_NOSIGPIPE option of setsockopt(2) and getsockopt(2) respectively.
RETURN VALUES
Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD A new file descriptor.
F_GETFD Value of flag (only the low-order bit is defined).
F_GETFL Value of flags.
F_GETOWN Value of file descriptor owner.
other Value other than -1.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
ERRORS
The fcntl() system call will fail if:
[EAGAIN] The argument cmd is F_SETLK or F_OFD_SETLK, the type
of lock (l_type) is a shared lock (F_RDLCK) or
exclusive lock (F_WRLCK), 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.
[EACCES] The argument cmd is F_SETSIZE and the calling process
does not have root privileges.
[EBADF] Fildes is not a valid open file descriptor.
The argument cmd is F_SETLK, F_SETLKW, F_OFD_SETLK or
F_OFD_SETLKW, the type of lock (l_type) is a shared
lock (F_RDLCK), and fildes is not a valid file
descriptor open for reading.
The argument cmd is F_SETLK, F_SETLKW, F_OFD_SETLK or
F_OFD_SETLKW, the type of lock (l_type) is an
exclusive lock (F_WRLCK), and fildes is not a valid
file descriptor open for writing.
The argument cmd is F_PREALLOCATE and the calling
process does not have file write permission.
The argument cmd is F_LOG2PHYS or F_LOG2PHYS_EXT and
fildes is not a valid file descriptor open for
reading.
The argument cmd is F_TRANSFEREXTENTS and either file
descriptor does not correspond to a valid regular
file, or either file is not open for writing.
[EDEADLK] The argument cmd is F_SETLKW, and a deadlock condition
was detected.
[EFBIG] The argument cmd is F_PREALLOCATE, F_PEOFPOSMODE is
set and preallocating fst_length bytes on fildes would
exceed the maximum file size.
[EINTR] The argument cmd is F_SETLKW or F_OFD_SETLKW, and the
function was interrupted by a signal.
[EINVAL] Cmd is F_DUPFD and arg is negative or greater than the
maximum allowable number (see getdtablesize(2)).
The argument cmd is F_GETLK, F_SETLK, F_SETLKW,
F_OFD_GETLK, F_OFD_SETLK or F_OFD_SETLKW, and the data
to which arg points is not valid, or fildes refers to
a file that does not support locking.
The argument cmd is F_PREALLOCATE and the fst_posmode
is not a valid mode, or when F_PEOFPOSMODE is set and
fst_offset is a non-zero value, or when F_VOLPOSMODE
is set and fst_offset is a negative or zero value.
The argument cmd is F_PUNCHHOLE and either fp_offset
or fp_length are negative, or when both fp_offset and
fp_length are not multiples of the file system block
size, or when either fp_flags or reserved is non-zero
value.
The argument cmd is F_TRANSFEREXTENTS and the
additional file descriptor is negative or both file
descriptors reference the same file.
[EMFILE] Cmd is F_DUPFD and the maximum allowed number of file
descriptors are currently open.
[EMFILE] The argument cmd is F_DUPFD and the maximum number of
file descriptors permitted for the process are already
in use, or no file descriptors greater than or equal
to arg are available.
[ENOLCK] The argument cmd is F_SETLK, F_SETLKW, F_OFD_SETLK or
F_OFD_SETLKW, and satisfying the lock or unlock
request would result in the number of locked regions
in the system exceeding a system-imposed limit.
[ENOSPC] The argument cmd is F_PREALLOCATE and either there is
no space available on the volume containing fildes or
fst_flags contains F_ALLOCATEALL and there is not
enough space available on the volume containing fildes
to satisfy the entire request.
The argument cmd is F_PUNCHHOLE and there is not
enough space available on the volume containing fildes
to satisfy the request. As an example, a filesystem
that supports cloned files may return this error if
punching a hole requires the creation of a clone and
there is not enough space available to do so.
[EOVERFLOW] A return value would overflow its representation. For
example, cmd is F_GETLK, F_SETLK, or F_SETLKW and the
smallest (or, if l_len is non-zero, the largest)
offset of a byte in the requested segment will not fit
in an object of type off_t.
[EPERM] The argument cmd is F_PUNCHHOLE and the calling
process does not have file write permission.
[ESRCH] Cmd is F_SETOWN and the process ID given as argument
is not in use.
[ENOTSUP] Cmd is F_TRANSFEREXTENTS and the given files aren't on
an APFS volume.
[EXDEV] Cmd is F_TRANSFEREXTENTS and the referenced files are
not in the same volume.
SEE ALSO
close(2), execve(2), flock(2), fork(2), getdtablesize(2), open(2),
pipe(2), setsockopt(2), socket(2), sigaction(3)
HISTORY
The fcntl() function call appeared in 4.2BSD.
Open file description locks first appeared in Linux 3.15
BSD 4.2 August 12, 2021 BSD 4.2