Advanced Programming in the UNIX® Environment

ptg10805159

924 Solutions to Selected Exercises Appendix C

Chapter 10

10.1 The program terminates the first time we send it a signal. The reason is that the

pausefunction returns whenever a signal is caught.

10.3 FigureC.11shows the stack frames.

stack frame

formain

stack frame

formain

stack frame

forsleep2

stack frame

forsig_int

stack frame

formain

stack frame

forsleep2

stack frame

forsig_int

stack frame

forsig_alrm

longjmp

stack frame

formain

stack frame

forsleep2

return to

main

bottom

of stack

processing

SIGINT

processing

SIGALRM

after

longjmp

Figure C.11 Stack frames beforeand afterlongjmp

Thelongjmpfromsig_alrmback tosleep2effectively aborts the call to

sig_int.Fromthis point,sleep2returns to main (recall Figure10.8).

10.4 We again have a race condition, this time between the first call toalarmand the

call to setjmp.Ifthe process is blocked by the kernel between these two

function calls, the alarm goes off, the signal handler is called, andlongjmpis

called. But sincesetjmpwas never called, the bufferenv_alrmis not set. The

operation oflongjmpis undefined if its jump buffer has not been initialized by

setjmp.

10.5 See ‘‘Implementing SoftwareTimers’’ by Don Libes (CUsers Journal,vol. 8, no.

11,Nov.1990) for an example. Acopy of this paper is available online at

http://www.kohala.com/start/libes.timers.txt.

10.7 If we simply called_exit,the termination status of the process would not show

that it was terminated by theSIGABRTsignal.

10.8 If the signal was sent by a process owned by some other user,the process has to

be set-user-ID to either root or to the owner of the receiving process, or thekill

attempt won’t work. Therefore, the real user ID provides moreinformation to

the receiver of the signal.