FCNTL(2)                  NetBSD System Calls Manual                  FCNTL(2)

NAME
     fcntl -- file descriptor control

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <fcntl.h>

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

DESCRIPTION
     fcntl() provides for control over descriptors.  The argument fd is a
     descriptor to be operated on by cmd as described below.  The third param-
     eter is called arg and is technically a pointer to void, but it is inter-
     preted as an int by some commands and ignored by others.

     Commands are:

     F_DUPFD    Return a new descriptor as follows:

                    o   Lowest numbered available descriptor greater than or
                        equal to arg, which is 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 cleared to remain open across execve(2)
                        system calls.

     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, which is 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 as neg-
                ative, otherwise arg is interpreted as a process ID.  The
                argument arg is interpreted as an int.

     F_CLOSEM   Close all file descriptors greater than or equal to fd.

     F_MAXFD    Return the maximum file descriptor number currently open by
                the process.

     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 l_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).  As specified by the
                value of l_type, 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 AT&T System V
     UNIX 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 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.  Calling 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 chil-
     dren.  Note that flock(2) and fcntl locks may be safely used concur-
     rently.

     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_GETFD    Value of flag (only the low-order bit is defined).

           F_GETFL    Value of flags.

           F_GETOWN   Value of file descriptor owner.

           F_MAXFD    Value of the highest file descriptor open by the
                      process.

           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 arg 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]            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.

     [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(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.

     [ENFILE]           cmd is F_DUPFD and system-wide the maximum allowed
                        number of file descriptors are currently open.

     [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 as argument
                        is not in use.

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

STANDARDS
     The fcntl() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').

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

NetBSD 5.0_STABLE               January 3, 2007              NetBSD 5.0_STABLE
