Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.15 sigsetjmpandsiglongjmpFunctions 357

{

time_t starttime;

if (canjump == 0)

return; /* unexpected signal, ignore */

pr_mask("starting sig_usr1: ");

alarm(3); /* SIGALRM in 3 seconds */

starttime = time(NULL);

for ( ; ; ) /* busy wait for 5 seconds */

if (time(NULL) > starttime + 5)

break;

pr_mask("finishing sig_usr1: ");

canjump = 0;

siglongjmp(jmpbuf, 1); /* jump back to main, don’t return */

}

static void

sig_alrm(int signo)

{

pr_mask("in sig_alrm: ");

}

Figure 10.20Example of signal masks,sigsetjmp,andsiglongjmp

This program demonstrates another technique that should be used whenever

siglongjmpis called from a signal handler.Weset the variablecanjumpto a nonzero

value only after we’ve calledsigsetjmp.This variable is examined in the signal

handler,andsiglongjmpis called only if the flagcanjumpis nonzero. This technique

provides protection against the signal handler being called at some earlier or later time,

when the jump buffer hasn’t been initialized bysigsetjmp.(In this trivial program,

we terminate quickly after thesiglongjmp,but in larger programs, the signal handler

may remain installed long after thesiglongjmp.) Providing this type of protection

usually isn’t required withlongjmpin normal C code (as opposed to a signal handler).

Since a signal can occur atanytime, however, we need the added protection in a signal

handler.

Here, we use the data typesig_atomic_t,which is defined by the ISO C standard

to be the type of variable that can be written without being interrupted. By this we

mean that a variable of this type should not extend across page boundaries on a system

with virtual memory and can be accessed with a single machine instruction, for

example. Wealways include the ISO type qualifiervolatilefor these data types as

well, since the variable is being accessed by two different threads of control: themain

function and the asynchronously executing signal handler.Figure10.21 shows a

timeline for this program. Wecan divide Figure10.21 into three parts: the left part

(corresponding tomain), the center part (sig_usr1), and the right part (sig_alrm).

While the process is executing in the left part, its signal mask is 0 (no signals are

blocked). While executing in the center part, its signal mask is SIGUSR1.While

executing in the right part, its signal mask isSIGUSR1|SIGALRM.