NAME
fcntl - file control

SYNOPSIS
#include <fcntl.h>

res = fcntl(fd, cmd, arg)
int res;
int fd, cmd, 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:

Lowest numbered available descriptor greater than or
equal to arg.

Same object references as the original descriptor.

New descriptor shares the same file pointer if the
object 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(2)
system calls.

F_GETFD Get the close-on-exec flag associated with the file
descriptor fd. If the low-order bit is 0, the file
will remain open across exec, otherwise the file
will be closed upon execution of exec.

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.

F_SETFL Set descriptor status flags.

F_GETOWN Get the process ID or process group currently
receiving SIGIO and SIGURG signals; process groups
are returned as negative values.

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:

FNDELAY Non-blocking I/O; if no data is available to a read
call, or if a write operation would block, the call
returns -1 with the error EWOULDBLOCK.

FAPPEND Force each write to append at the end of file;
corresponds to the O_APPEND flag of open(2).

FASYNC 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 VALUE
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 one or more of the following are true:

[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.