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 from: @(#)fcntl.2 6.7 (Berkeley) 3/10/91
 $Id: fcntl.2,v 1.4 1993/12/15 17:36:56 jtc Exp $

.Dd March 10, 1991
.Dt FCNTL 2
.Os BSD 4.2
.Sh NAME
.Nm fcntl
.Nd file control
.Sh SYNOPSIS
.Fd #include <fcntl.h>
.Ft int
.Fn fcntl "int fd" "int cmd" "int arg"
.Sh DESCRIPTION
.Fn Fcntl
provides for control over descriptors.
The argument
.Fa fd
is a descriptor to be operated on by
.Fa cmd
as follows:
l -tag -width F_GETOWNX t Dv F_DUPFD Return a new descriptor as follows:

p
l -bullet -compact -offset 4n t Lowest numbered available descriptor greater than or equal to
.Fa arg .
t Same object references as the original descriptor.
t New descriptor shares the same file offset if the object
was a file.
t Same access mode (read, write or read/write).
t Same file status flags (i.e., both file descriptors
share the same file status flags).
t The close-on-exec flag associated with the new file descriptor
is set to remain open across
.Xr execv 2
system calls.
.El
t Dv F_GETFD Get the close-on-exec flag associated with the file descriptor
.Fa fd .
If the low-order bit of the returned value is 0,
the file will remain open across
.Fn exec ,
otherwise the file will be closed upon execution of
.Fn exec
.Fa ( arg
is ignored).
t Dv F_SETFD Set the close-on-exec flag associated with
.Fa fd
to the low order bit of
.Fa arg
(0 or 1 as above).
t Dv F_GETFL Get descriptor status flags, as described below
.Fa ( arg
is ignored).
t Dv F_SETFL Set descriptor status flags to
.Fa arg .
t Dv F_GETOWN Get the process ID or process group
currently receiving
.Dv SIGIO
and
.Dv SIGURG
signals; process groups are returned
as negative values
.Fa ( arg
is ignored).
t Dv F_SETOWN Set the process or process group
to receive
.Dv SIGIO
and
.Dv SIGURG
signals;
process groups are specified by supplying
.Fa arg
as negative, otherwise 
.Fa arg
is interpreted as a process ID.
.El

p
The flags for the
.Dv F_GETFL
and
.Dv F_SETFL
flags are as follows:
l -tag -width F_GETOWNX t Dv O_NDELAY Non-blocking I/O; if no data is available to a
.Xr read
call, or if a
.Xr write
operation would block,
the read or write call returns -1 with the error
.Er EWOULDBLOCK .
t Dv O_APPEND Force each write to append at the end of file;
corresponds to the
.Dv O_APPEND
flag of
.Xr open 2 .
t Dv O_ASYNC Enable the
.Dv SIGIO
signal to be sent to the process group
when I/O is possible, e.g.,
upon availability of data to be read.
.El
.Sh RETURN VALUES
Upon successful completion, the value returned depends on
.Fa cmd
as follows:
l -tag -width F_GETOWNX -offset indent t Dv F_DUPFD A new file descriptor.
t Dv F_GETFD Value of flag (only the low-order bit is defined).
t Dv F_GETFL Value of flags.
t Dv F_GETOWN Value of file descriptor owner.
t other Value other than -1.
.El

p
Otherwise, a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn Fcntl
will fail if:
l -tag -width Er t Bq Er EBADF .Fa Fildes
is not a valid open file descriptor.
t Bq Er EMFILE .Fa Cmd
is
.Dv F_DUPFD
and the maximum allowed number of file descriptors are currently
open.
t Bq Er EINVAL .Fa Cmd
is
.Dv F_DUPFD
and
.Fa arg
is negative or greater than the maximum allowable number
(see
.Xr getdtablesize 2 ) .
t Bq Er ESRCH .Fa Cmd
is
.Dv F_SETOWN
and
the process ID given as argument is not in use.
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr execve 2 ,
.Xr getdtablesize 2 ,
.Xr open 2 ,
.Xr sigvec 2
.Sh BUGS
The asynchronous I/O facilities of
.Dv FNDELAY
and
.Dv FASYNC
are currently available only for tty and socket operations.
.Sh HISTORY
The
.Fn fcntl
function call appeared in
x 4.2 .