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FCNTL(2)                  FreeBSD System Calls Manual                 FCNTL(2)

NAME
     fcntl - file control

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <fcntl.h>

     int
     fcntl(int fd, int cmd, ...);

DESCRIPTION
     The fcntl() system call provides for control over descriptors.  The
     argument fd is a descriptor to be operated on by cmd as described below.
     Depending on the value of cmd, fcntl() can take an additional third
     argument int arg.

     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 FD_CLOEXEC associated
                               with the new file descriptor is cleared, so the
                               file descriptor is to remain open across
                               execve(2) system calls.

     F_DUPFD_CLOEXEC   Like F_DUPFD, but the FD_CLOEXEC flag associated with
                       the new file descriptor is set, so the file descriptor
                       is closed when execve(2) system call executes.

     F_DUP2FD          It is functionally equivalent to

                             dup2(fd, arg)

     F_DUP2FD_CLOEXEC  Like F_DUP2FD, but the FD_CLOEXEC flag associated with
                       the new file descriptor is set.

                       The F_DUP2FD and F_DUP2FD_CLOEXEC constants are not
                       portable, so they should not be used if portability is
                       needed.  Use dup2() instead of F_DUP2FD.

     F_GETFD           Get the close-on-exec flag associated with the file
                       descriptor fd as FD_CLOEXEC.  If the returned value
                       ANDed with FD_CLOEXEC is 0, the file will remain open
                       across exec(), otherwise the file will be closed upon
                       execution of exec() (arg is ignored).

     F_SETFD           Set the close-on-exec flag associated with fd to arg,
                       where arg is either 0 or FD_CLOEXEC, as described
                       above.

     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_READAHEAD       Set or clear the read ahead amount for sequential
                       access to the third argument, arg, which is rounded up
                       to the nearest block size.  A zero value in arg turns
                       off read ahead, a negative value restores the system
                       default.

     F_RDAHEAD         Equivalent to Darwin counterpart which sets read ahead
                       amount of 128KB when the third argument, arg is non-
                       zero.  A zero value in arg turns off read ahead.

     The flags for the F_GETFL and F_SETFL flags are as follows:

     O_NONBLOCK   Non-blocking I/O; if no data is available to a read(2)
                  system 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_DIRECT     Minimize or eliminate the cache effects of reading and
                  writing.  The system will attempt to avoid caching the data
                  you read or write.  If it cannot avoid caching the data, it
                  will minimize the impact the data has on the cache.  Use of
                  this flag can drastically reduce performance if not used
                  with care.

     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 */
             int     l_sysid;        /* remote system id or zero for local */
     };
     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 system 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
                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, l_start means end edge of the region.  The l_pid and l_sysid
     fields are only used with F_GETLK to return the process ID of the process
     holding a blocking lock and the system ID of the system that owns that
     process.  Locks created by the local system will have a system ID of
     zero.  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
     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)
     system call.  The flock(2) interface has much more rational last close
     semantics and allows locks to be inherited by child processes.  The
     flock(2) system call 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.

     The fcntl(), flock(2), and lockf(3) locks are compatible.  Processes
     using different locking interfaces can cooperate over the same file
     safely.  However, only one of such interfaces should be used within the
     same process.  If a file is locked by a process through flock(2), any
     record within the file will be seen as locked from the viewpoint of
     another process using fcntl() or lockf(3), and vice versa.  Note that
     fcntl(F_GETLK) returns -1 in l_pid if the process holding a blocking lock
     previously locked the file descriptor by flock(2).

     All locks associated with a file for a given process are removed when the
     process terminates.

     All locks obtained before a call to execve(2) remain in effect until the
     new program releases them.  If the new program does not know about the
     locks, they will not be released until the program exits.

     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.

RETURN VALUES
     Upon successful completion, the value returned depends on cmd as follows:

           F_DUPFD    A new file descriptor.

           F_DUP2FD   A file descriptor equal to arg.

           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
                        segment of a file to be locked is already shared-
                        locked or exclusive-locked by another process.

     [EBADF]            The fd argument is not a valid open file descriptor.

                        The argument cmd is F_DUP2FD, and arg is not a valid
                        file descriptor.

                        The argument cmd is F_SETLK or F_SETLKW, the type of
                        lock (l_type) is a shared lock (F_RDLCK), and fd 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 fd
                        is not a valid file descriptor open for writing.

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

     [EINTR]            The argument cmd is F_SETLKW, and the system call was
                        interrupted by a signal.

     [EINVAL]           The cmd argument 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.

     [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 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 limit.

     [EOPNOTSUPP]       The argument cmd is F_GETLK, F_SETLK or F_SETLKW and
                        fd refers to a file for which locking is not
                        supported.

     [EOVERFLOW]        The argument cmd is F_GETLK, F_SETLK or F_SETLKW and
                        an off_t calculation overflowed.

     [EPERM]            The cmd argument is F_SETOWN and the process ID or
                        process group given as an argument is in a different
                        session than the caller.

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

     In addition, if fd refers to a descriptor open on a terminal device (as
     opposed to a descriptor open on a socket), a cmd of F_SETOWN can fail for
     the same reasons as in tcsetpgrp(3), and a cmd of F_GETOWN for the
     reasons as stated in tcgetpgrp(3).

SEE ALSO
     close(2), dup2(2), execve(2), flock(2), getdtablesize(2), open(2),
     sigaction(2), lockf(3), tcgetpgrp(3), tcsetpgrp(3)

STANDARDS
     The F_DUP2FD constant is non portable.  It is provided for compatibility
     with AIX and Solaris.

HISTORY
     The fcntl() system call appeared in 4.2BSD.

     The F_DUP2FD constant first appeared in FreeBSD 7.1.

FreeBSD 11.0-PRERELEASE        February 8, 2013        FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY

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