FCNTL(2)                  386BSD Programmer's Manual                  FCNTL(2)

NAME
     fcntl - file control

SYNOPSIS
     #include <sys/fcntl.h>

     int
     fcntl(int fd, int cmd, int arg)

DESCRIPTION
     Fcntl() provides for control over descriptors.  The argument fd is a
     descriptor to be operated on by cmd as follows:

     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 associated with the new file
                        descriptor is set to remain open across execv(2)
                        system 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 (0 or 1 as 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.

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

     O_NDELAY   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 EWOULDBLOCK.

     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.

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
     Fcntl() will fail if:

     [EBADF]       Fildes is not a valid open file descriptor.

     [EMFILE]      Cmd is F_DUPFD and the maximum allowed number of file
                   descriptors are currently open.

     [EINVAL]      Cmd is F_DUPFD and arg is negative or greater than the
                   maximum allowable number (see getdtablesize(2)).

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

SEE ALSO
     close(2),  execve(2),  getdtablesize(2),  open(2),  sigvec(2)

BUGS
     The asynchronous I/O facilities of FNDELAY and FASYNC are currently
     available only for tty and socket operations.

HISTORY
     The fcntl function call appeared in 4.2BSD.

4.2 Berkeley Distribution       March 10, 1991                               2