fcntl
Prototype: 
#include <unistd.h>
#include <fcntl.h>

int fcntl(int fd, int cmd);
int fcntl(int fd, int cmd, long arg);
int fcntl(int fd, int cmd, struct flock *flock);
General Description: Manipulate the file or socket handle.
Return Value: On error, -1 is returned, and errno is set appropriately. For a successful call, the return value depends on the operation: F_DUPFD - The new descriptor. F_GETFD - Value of flag. F_GETFL - Value of flags. F_GETOWN - Value of descriptor owner. F_GETSIG - Value of signal sent when read or write becomes possible, or zero for traditional SIGIO behavior. All other commands return zero.
Parameters
fd The descriptor to manipulate.
cmd The operation to perform. Some operations are duplicates to existing functions. Some operation require an operand (arg or flock). Each operation is grouped into specific functions:Duplicate descriptor (F_DUPFD) - Same as dup2(arg, fd), this operation replaces fd with a copy of the descriptor in arg.Manipulate close-on-exec (F_GETFD, F_SETFD) - The kernel does not pass all file descriptors to the exec-child process. With this parameter you can test or set the close-on-exec.Manipulate descriptor flags (F_GETFL, F_SETFL) - Using these commands you can get the flags (set by the open() system call) of the descriptor. You can only set O_APPEND, O_NONBLOCK and O_ASYNC.Manipulate file locks (F_GETLK, F_SETLK, F_SETLKW) - GETLK retrieves the lock structure that currently holds the file. If the file is not locked,Determine who owns I/O signals (F_GETOWN, F_SETOWN) - Return or set the PID of the current owner of the SIGIO signal.Determine the kind of signal to send (F_GETSIG, F_SETSIG) - Gets or sets the signal type when more I/O operations can be performed. Default is SIGIO.
arg The value to set.
flock The locking key.
Possible Errors
EACCES Operation is prohibited by locks held by other processes.
EAGAIN Operation is prohibited because the file has been memory-mapped by another process.
EBADF fd is not an open file descriptor.
EDEADLK It was detected that the specified F_SETLKW command would cause a deadlock.
EFAULT lock is outside your accessible address space.
EINTR For F_SETLKW, the command was interrupted by a signal. For F_GETLK and F_SETLK, the command was interrupted by a signal before the lock was checked or acquired. Most likely when locking a remote file (e.g. locking over NFS), but can sometimes happen locally.
EINVAL For F_DUPFD, arg is negative or is greater than the maximum allowable value. For F_SETSIG, arg is not an allowable signal number.
EMFILE For F_DUPFD, the process already has the maximum number of file descriptors open.
ENOLCK Too many segment locks open, lock table is full, or a remote locking protocol failed (e.g. locking over NFS).
EPERM Attempted to clear the O_APPEND flag on a file that has the append-only attribute set.
Examples 
#include <unistd.h>
#include <fnctl.h>
...
printf("PID which owns SIGIO: %d",
    fnctl(fd, F_GETOWN));


#include <unistd.h>
#include <fnctl.h>
...
if ( fnctl(fd, F_SETSIG, SIGKILL) != 0 )
    perror("Can't set signal");


#include <unistd.h>
#include <fnctl.h>
...
if ( (fd_copy = fcntl(fd, F_DUPFD)) < 0 )
    perror("Can't dup fd");