CODEWITHSUNDEEP
cmacospthreadskernelsignals
TheDarkKnight
TheDarkKnight

Reputation: 27611

Underlying mechanism when pausing a process

I have a program that requires it to pause and resume another program. To do this, I use the kill function, either from code with: -

kill(pid, SIGSTOP); // to pause
kill(pid, SIGCONT); // to resume

Or from the command line with the similar

kill -STOP <pid>
kill -CONT <pid>

I can trace what's going on with the threads using this code, taken from Mac OS X Internals.

If a program is paused and immediately resumed, the state of threads can show as UNINTERRUPTIBLE. Most of the time, they report as WAITING, which is not surprising and if a thread is doing work, it will show as RUNNING.

What I don't understand is when I pause a program and view the states of the threads, they still show as WAITING. I would have expected their state to be either STOPPED or HALTED

Can someone explain why they still show as WAITING and when would they be STOPPED or HALTED. In addition, is there another structure somewhere that shows the state of the program and its threads being halted in this way?

Upvotes: 2

Views: 97

Answers (2)

TheDarkKnight
TheDarkKnight

Reputation: 27611

After researching and experimenting with the available structures, I've discovered that it is possible to show the state of a program being halted by looking at the process suspend count. This is my solution: -

int ProcessIsSuspended(unsigned int pid)
{
    kern_return_t kr;
    mach_port_t task;
    mach_port_t mytask;

    mytask = mach_task_self();

    kr = task_for_pid(mytask, pid, &task);
    // handle error here...

    char infoBuf[TASK_INFO_MAX];
    struct task_basic_info_64 *tbi;

    int infoSize = TASK_INFO_MAX;
    kr = task_info(task, TASK_BASIC_INFO_64, (task_info_t)infoBuf, (mach_msg_type_number_t *)&infoSize);
    // handle error here....

    tbi = (struct task_basic_info_64 *) infoBuf;
    if(tbi->suspend_count > 0) // process is suspended
        return 1;

    return 0;
}

If suspend_count is 0, the program is running, else it is in a paused state, waiting to be resumed.

Upvotes: 0

user3007735
user3007735

Reputation: 159

"Waiting" is shown in your case because you did not terminate the program rather paused it, where as Stopped or Halted state usually occurs when the program immediately stopped working due to some runtime error. As far as your second question is concerned, I do not think there is some other structure out there to show the state of the program. Cheers

Upvotes: 1

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