Advanced Programming in the UNIX® Environment

ptg10805159

358 Signals Chapter 10

main

signal()

signal()

pr_mask()

sigsetjmp()

pause()

SIGUSR1delivered sig_usr1

pr_mask()

alarm()

time()

time()

time()

SIGALRMdelivered sig_alrm

pr_mask()

return from signal handler return()

pr_mask()

sigsetjmp() siglongjmp()

pr_mask()

exit()

Figure 10.21Timeline for example program handling two signals

Let’s examine the output when the program in Figure10.20 is executed:

$./a.out & start process in background

starting main:

[1] 531 the job-control shell prints its process ID

$kill -USR1 531 send the processSIGUSR1

starting sig_usr1: SIGUSR1

$ in sig_alrm: SIGUSR1 SIGALRM

finishing sig_usr1: SIGUSR1

ending main:

just press RETURN

[1] + Done ./a.out &

The output is what we expect: when a signal handler is invoked, the signal being caught

is added to the current signal mask of the process. The original mask is restored when

the signal handler returns. Also,siglongjmprestores the signal mask that was saved

bysigsetjmp.

If we change the program in Figure10.20 so that the calls tosigsetjmpand

siglongjmparereplaced with calls tosetjmpandlongjmpon Linux (or_setjmp

and_longjmpon FreeBSD), the final line of output becomes

ending main: SIGUSR1

This means that themainfunction is executing with theSIGUSR1signal blocked, after

the call tosetjmp.This probably isn’t what we want.