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fcntl(2)                    BSD 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:

                            o   Lowest numbered available descriptor greater
                                than or equal to arg.
                            o   Same object references as the original
                                descriptor.
                            o   New descriptor shares the same file offset if
                                the object was a file.
                            o   Same access mode (read, write or read/write).
                            o   Same file status flags (i.e., both file
                                descriptors share the same file status flags).
                            o   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 asso-
                        ciated 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 receiv-
                        ing 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 sup-
                        plying arg as negative, otherwise arg is interpreted
                        as a process ID.

     F_GETPATH          Get the path of the file descriptor Fildes.  The argu-
                        ment 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 same size or
                        larger than the space requested.

     F_SETSIZE          Truncate a file without zeroing space.  The calling
                        process must have root privileges.

     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_READBOOTSTRAP    Read bootstrap from disk.

     F_WRITEBOOTSTRAP   Write bootstrap on disk.  The calling process must
                        have root privileges.

     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
                        includes the disk device address that corresponds to
                        the current file offset. Note that if the file offset
                        is not backed by physical blocks we can return -1 as
                        the offset. This is subject to change.

     F_LOG2PHYS_EXT     Variant of F_LOG2PHYS that uses the passed in file
                        offset and length.

     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).  This is currently imple-
                        mented on HFS, MS-DOS (FAT), and Universal Disk Format
                        (UDF) 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.

     The flags for the F_GETFD and F_SETFD commands are as follows:

           FD_CLOEXEC   Close-on-exec; the given file descriptor will be auto-
                        matically 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
                        call, or if a write 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; corre-
                        sponds 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.

     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 process 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 speci-
                fied 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 applica-
     tion 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 call-
     ing process has previously existing locks on bytes in the region speci-
     fied by the request, the previous lock type for each byte in the speci-
     fied 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 applica-
     tions 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 data-
     base 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 fcntl(2) locks may be safely used con-
     currently.

     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.

           F_ALLOCATEALL      Allocate all requested space or no space at all.

     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.

           F_VOLPOSMODE    Allocate from the volume offset.

     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_READBOOTSTRAP and F_WRITEBOOTSTRAP commands operate on the follow-

     ing structure.

             typedef struct fbootstraptransfer {
                 off_t fbt_offset;       /* IN: offset to start read/write */
                 size_t fbt_length;      /* IN: number of bytes to transfer */
                 void *fbt_buffer;       /* IN: buffer to be read/written */
             } fbootstraptransfer_t;

     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 com-
     mands 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, 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 seg-
                        ment of a file to be locked is already shared-locked
                        or exclusive-locked by another process.

     [EACCESS]          The argument cmd is either F_SETSIZE or
                        F_WRITEBOOTSTRAP and the calling process does not have
                        root privileges.

     [EBADF]            Fildes is not a valid open file descriptor.

                        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.

                        The argument cmd is F_SETLK or F_SETLKW, the type of
                        lock (l_type) is an exclusive lock (F_WRLCK), and
                        fildes is not a valid file descriptor open for writ-
                        ing.

                        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 read-
                        ing.

     [EDEADLK]          The argument cmd is F_SETLKW, and a deadlock condition
                        was detected.

     [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, or F_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 either F_READBOOTSTRAP or
                        F_WRITEBOOTSTRAP and the operation was attempted on a
                        non-HFS disk type.

     [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 or F_SETLKW, and satisfy-
                        ing the lock or unlock request would result in the
                        number of locked regions in the system exceeding a
                        system-imposed limit.

     [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) off-
                        set of a byte in the requested segment will not fit in
                        an object of type off_t.

     [ESRCH]            Cmd is F_SETOWN and the process ID given as argument
                        is not in use.


SEE ALSO

     close(2), execve(2), flock(2), getdtablesize(2), open(2), pipe(2),
     socket(2), setsockopt(2), sigaction(3)


HISTORY

     The fcntl() function call appeared in 4.2BSD.

4.2 Berkeley Distribution      February 17, 2011     4.2 Berkeley Distribution

Mac OS X 10.9.1 - Generated Sun Jan 5 20:33:10 CST 2014