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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:

                            •   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 sta-tus status
                                tus 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_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 privi-leges. privileges.
                        leges.

     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 implemented 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 automatically closed in the succes-sor successor
                        sor 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; 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.

     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 estab-lish establish
                lish 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 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 hold-ing holding
     ing 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 spec-ified specified
     ified 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 seman-tic semantic
     tic 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(2) 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.

           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 following 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 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, 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 exclu-sive-locked exclusive-locked
                        sive-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.

     [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 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.

     [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 satisfying the lock or unlock request
                        would result in the number of locked regions in the system exceeding a system-imposed systemimposed
                        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) offset
                        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

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