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