FCNTL(2)                  OpenBSD Programmer's Manual                 FCNTL(2)

NAME
     fcntl - file control



SYNOPSIS
     #include <fcntl.h>

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

DESCRIPTION
     The fcntl() provides control over the properties of a file that is al-
     ready 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:

                    -   Lowest numbered available descriptor greater than or
                        equal to arg (interpreted as an int).
                    -   Same object references as the original descriptor.
                    -   New descriptor shares the same file offset if the ob-
                        ject 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 set to remain open across execv(3)
                        calls.

     F_GETFD    Get the close-on-exec flag associated with the file descriptor
                fd. If the low-order bit of the returned value 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 the low-order
                bit of arg (interpreted as an int). The flag should be speci-
                fied as 0 (do not close-on-exec) or 1 (do close-on-exec).

     F_GETFL    Get descriptor status flags, as described below (arg is ig-
                nored).

     F_SETFL    Set descriptor status flags 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 supplying arg (inter-
                preted as an int) as negative, otherwise arg is taken as a
                process ID.

     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) 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.

     F_SETLK    Set or clear a file segment lock according to the lock de-
                scription pointed to by the third argument, arg, taken as a
                pointer to a struct flock (see above).  F_SETLK is used to es-
                tablish 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 de-
     scriptor 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 ex-
     tend 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 Std1003.1-1988 (``POSIX'') 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 getpw-
     name(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 semantic problem with this interface is that locks are not inherit-
     ed 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 applica-
     tions 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 pro-
     cess.  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_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 er-
     ror.

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

     [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.

     [EMFILE]      cmd is F_DUPFD and the maximum allowed number of file de-
                   scriptors are currently open.

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

     [EINTR]       The argument cmd is F_SETLKW, and the function was inter-
                   rupted by a signal.

     [EINVAL]      cmd is F_DUPFD and arg is negative or greater than the max-
                   imum 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 fildes refers to
                   a file that does not support locking.

     [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.

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

SEE ALSO
     close(2),  execve(2),  flock(2),  open(2),  sigaction(2),  getdtable-
     size(3)

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

OpenBSD 2.6                    January 12, 1994                              4