Advanced Programming in the UNIX® Environment

ptg10805159

922 Solutions to Selected Exercises Appendix C

tovforkand does a return fromf1.The return information is often stored in the

stack frame, and that information has probably been modified by the child. After

the parent resumes, what happens with this example depends on many

implementation features of your UNIX system (whereinthe stack frame the

return information is stored, what information in the stack frame is wiped out

when the automatic variables aremodified, and so on). The normal result is a

corefile, but your results may differ.

stack frame

formain

stack frame

forf1

bottom of stack

direction of

stack growth

Figure C.9 Stack frames whenvforkis called

8.4 In Figure8.13, we have the parent write its output first. When the parent is done,

the child writes its output, but we let the parent terminate. Whether the parent

terminates or whether the child finishes its output first depends on the kernel’s

scheduling of the two processes (another race condition). When the parent

terminates, the shell starts up the next program, and this next program can

interferewith the output from the previous child.

We can prevent this from happening by not letting the parent terminate until the

child has also finished its output. Replace the code following theforkwith the

following:

else if (pid == 0) {

WAIT_PARENT(); /* parent goes first */

charatatime("output from child\n");

TELL_PARENT(getppid()); /* tell parent we’re done */

}else {

charatatime("output from parent\n");

TELL_CHILD(pid); /* tell child we’re done */

WAIT_CHILD(); /* wait for child to finish */

}

We won’t see this happen if we let the child go first, since the shell doesn’t start

the next program until the parent terminates.

8.5 The same value (/home/sar/bin/testinterp)isprinted forargv[2].The

reason is thatexeclpends up callingexecvewith the samepathnameas when

we callexecldirectly.Recall Figure8.15.