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
andSIGURG
signals; process groups are returned as negative values (arg is ignored). F_SETOWN
- Set the process or process group to receive
SIGIO
andSIGURG
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
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 ofF_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 toF_UNLCK
. If a lock that does not support the discovery of lock ownership by process (one created by the flock(2) system call or the open(2) system call with theO_SHLOCK
orO_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 withEAGAIN
. F_SETLKW
- This command is the same as
F_SETLK
except that if a shared or exclusive lock is blocked by other locks, the process waits until the request can be satisfied. If a signal that is to be caught is received whilefcntl
is waiting for a region, thefcntl
will be interrupted if the signal handler has not specified theSA_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 much 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
getpwname(3) to retrieve a record, the lock will be lost because
getpwname(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.
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:
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
, 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
orF_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
orF_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
orF_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
, 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
, orF_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 whenF_PEOFPOSMODE
is set and fst_offset is a non-zero value, or whenF_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_DUPED
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
orF_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 containsF_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), getdtablesize(2), open(2), pipe(2), setsockopt(2), socket(2), sigaction(3)
HISTORY
The fcntl
() function call appeared in
4.2BSD.