#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:
 15
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.
 15
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. 
 15
F_SETFD
Set the close-on-exec flag associated with
 fd to the low order bit of
 arg (0 or 1 as above).
 15
F_GETFL
Get descriptor status flags, as described below.
 15
F_SETFL
Set descriptor status flags.
 15
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:
 15
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:
 15
[EBADF]
 Fildes is not a valid open file descriptor.
 15
[EMFILE]
 Cmd is F_DUPFD and the maximum allowed number of file descriptors are currently
open.
 15
[EINVAL]
 Cmd is F_DUPFD and
 arg is negative or greater than the maximum allowable number
(see
 getdtablesize (2)). 
 15
[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.