Advanced Programming in the UNIX® Environment

ptg10805159

362 Signals Chapter 10

err_sys("signal(SIGQUIT) error");

sigemptyset(&zeromask);

sigemptyset(&newmask);

sigaddset(&newmask, SIGQUIT);

/*

*Block SIGQUIT and save current signal mask.

*/

if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)

err_sys("SIG_BLOCK error");

while (quitflag == 0)

sigsuspend(&zeromask);

/*

*SIGQUIT has been caught and is now blocked; do whatever.

*/

quitflag = 0;

/*

*Reset signal mask which unblocks SIGQUIT.

*/

if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)

err_sys("SIG_SETMASK error");

exit(0);

}

Figure 10.23 Usingsigsuspendto wait for a global variable to be set

Sample output from this program is

$./a.out

ˆC type the interrupt character

interrupt

ˆC type the interrupt character again

interrupt

ˆC and again

interrupt

ˆ\$ now terminate with the quit character

For portability between non-POSIX systems that support ISO C and POSIX.1 systems, the only

thing we should do within a signal handler is assign a value to a variable of type

sig_atomic_t—nothing else. POSIX.1 goes further and specifies a list of functions that are

safe to call from within a signal handler (Figure10.4), but if we do this, our code may not run

correctly on non-POSIX systems.

Example

As another example of signals, we show how signals can be used to synchronize a

parent and child. Figure10.24 shows implementations of the five routinesTELL_WAIT,

TELL_PARENT,TELL_CHILD,WAIT_PARENT,andWAIT_CHILDfrom Section 8.9.