How to stop a process over a consistent time interval in order to execute a measurement function?

Question

I am using the Linux ptrace API in a profiler I am writing.

My pseudo C code looks like this:

setjmp();
measure();
alarm(N);
while(1) {
    waitpid(child, &status, WNOHANG);
    if(child_process_exiting) {
        measure();
        break;
    }
}

The alarm signal handler is as follows:

void sig_handler(int sig) {
     signal(SIGALRM, sig_handler);
     longjmp(env, 0);
 }

I want to repeatedly return to the setjmp call until the child process exits and breaks the loop. The goal is to run the measure function every N seconds until the child process exits.

You do know that you have to call `waitpid` with the `WNOHANG` flag or it will block? — Some programmer dude, Jul 25 '13 at 13:40
By the way, using `setjmp`/`longjmp` in combination with signals my not work on all systems. To be safe you should use `sigsetjmp`/`siglongjmp`. — Some programmer dude, Jul 25 '13 at 14:02

score 3 · Accepted Answer · answered Jul 25 '13 at 13:44

3

Why not simple just sleep for N seconds, then do the measure call? If you install a SIGCHLD handler to catch when the child process exits, then e.g. sleep will be interrupted and return the number of seconds left.

Something like

signal(SIGCHLD, sigchld_handler);

do
{
    measure();
} while (sleep(N) == 0);

waitpid(...);

answered Jul 25 '13 at 13:44

Some programmer dude

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What action would the handler take in this approach? – kobrien Jul 25 '13 at 14:12
@kobrien Unless the child process can be stopped, the signal handled doesn't have to do anything at all. The `sleep` (and other similar functions) will return anyway when a signal is received. – Some programmer dude Jul 25 '13 at 14:18

Lidong Guo · Answer 2 · 2013-07-25T13:49:43.417

0

I only find this need change:

setjmp(env); // env is a jum_buf value

Then

longjmp(env,1);// so if necessary, we can use :int flag = setjum(env);

if flag == 1; I know code call sig_handler and back.

edited Jul 25 '13 at 13:49

answered Jul 25 '13 at 13:43

Lidong Guo

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How to stop a process over a consistent time interval in order to execute a measurement function?

2 Answers2