unixdev.net


Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (OpenBSD-5.7)
Page:
Section:
Apropos / Subsearch:
optional field

FCNTL(2)                    BSD System Calls Manual                   FCNTL(2)

NAME
     fcntl -- file control

SYNOPSIS
     #include <&lt;fcntl.h>&gt;

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

DESCRIPTION
     The fcntl() provides control over the properties of a file that is
     already open.  The argument fd is a descriptor to be operated on by cmd
     as described below.  The third parameter is called arg and is technically
     a pointer to void, but is interpreted as an int by some commands, a
     pointer to a struct flock by others (see below), and ignored by the rest.

     The commands are:

     F_DUPFD          Return a new descriptor as follows:

                      o   Lowest numbered available descriptor greater than or
                          equal to arg (interpreted as an int).
                      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 set to remain open across execve(2)
                          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) is called.

     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 (interpreted as an int) is either 0 or
                      FD_CLOEXEC, as described above.

     F_GETFL          Get file status flags associated with the file descrip-
                      tor fd, as described below (arg is ignored).

     F_SETFL          Set file status flags associated with the file descrip-
                      tor fd to arg (interpreted as an int).

     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 supply-
                      ing arg (interpreted as an int) as negative, otherwise
                      arg is taken as a process ID.

     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.

     O_SYNC       Cause writes to be synchronous.  Data will be written to the
                  physical device instead of just being stored in the buffer
                  cache; corresponds to the O_SYNC flag of open(2).

     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
                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 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
     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 data-
     base.  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.

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

           F_DUPFD          A new file descriptor.

           F_DUPFD_CLOEXEC  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
     fcntl() 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.

     [EBADF]            fd 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 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 invalid.

                        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(3)).

                        The argument cmd is F_GETLK, F_SETLK, or F_SETLKW and
                        the data to which arg points is not valid, or fd
                        refers to a file that does not support locking.

     [EMFILE]           The argument cmd is F_DUPFD and the maximum number of
                        open 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.

     [ESRCH]            cmd is F_SETOWN and the process ID given in arg is not
                        in use.

SEE ALSO
     close(2), execve(2), flock(2), open(2), sigaction(2), getdtablesize(3)

STANDARDS
     The fcntl() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').

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

BSD                            December 16, 2014                           BSD