Technical Reference: Base Operating System and Extensions, Volume 1

The following code sample illustrates the use of the fcntl subroutine to set and clear the close-on-exec flag:

/* set the close-on-exec flag for fd1 */
fcntl(fd1,F_SETFD,FD_CLOEXEC);
/* clear the close-on-exec flag for fd2 */
fcntl(fd2,F_SETFD,0);

Although the fcntl subroutine is normally used to set the close-on-exec flag, the ioctl subroutine may be used if the application program is linked with the Berkeley Compatibility Library (libbsd.a) or the Berkeley Thread Safe Library (libbsd_r.a) (4.2.1 and later versions). The following ioctl code fragment is equivalent to the preceding fcntl fragment:

/* set the close-on-exec flag for fd1 */
ioctl(fd1,FIOCLEX,0);
/* clear the close-on-exec flag for fd2 */
ioctl(fd2,FIONCLEX,0);

The third parameter to the ioctl subroutine is not used for the FIOCLEX and FIONCLEX ioctl commands.

int flag;
/* enable NBIO for fd1 */
flag = 1;
ioctl(fd1,FIONBIO,&flag);
/* disable NBIO for fd2 */
flag = 0;
ioctl(fd2,FIONBIO,&flag);

int nbytes;

ioctl(fd,FIONREAD,&nbytes);

The third parameter of the ioctl subroutine for this command is a pointer to an integer variable that indicates whether the asynchronous I/O notification should be enabled or disabled. A value of 0 disables I/O notification; any nonzero value enables I/O notification. A sample code segment follows:

int flag;
/* enable ASYNC on fd1 */
flag = 1;
ioctl(fd, FIOASYNC,&flag);
/* disable ASYNC on fd2 */
flag = 0;
ioctl(fd,FIOASYNC,&flag);

Sockets support both process groups and individual process recipients, while ttys and psuedo-ttys support only process groups. Attempts to specify an individual process as the recipient will be converted to the process group to which the process belongs. The following code example illustrates how to set the recipient identifier:

int owner;
owner = -getpgrp();
ioctl(fd,FIOSETOWN,&owner);

Note: In this example, the asynchronous I/O signals are being enabled on a process group basis. Therefore, the value passed through the owner parameter must be a negative number.

The following code sample illustrates enabling asynchronous I/O signals to an individual process:

int owner;
owner = getpid();
ioctl(fd,FIOSETOWN,&owner);

int owner;
ioctl(fd,FIOGETOWN,&owner);

If the owner of the asynchronous I/O capability is a process group, the value returned in the reference parameter is negative. If the owner is an individual process, the value is positive.