The waitpid() parameters

Question

I have a question about the waitpid parameters. I'm supposed to start p[1] (process 1) when p[0] is done.

This is what starts p0:

if(p[0] == 0){
    process(0,1); //(process, duration(time))
    return 0;
}

Now I want p1 to start as soon as p0 ends (after 1 sec)

if(p[1] == 0){
    process(1,2);
    return 0;
}
waitpid(p[0], NULL, 0);

Here's my question: what do the parameters in waitpid mean? should the last parameter be set to 1, since p[0] ends after 1 sec and this is when I want p[1] to start?

Answer 1

what does the parameters in waitpid means?

You can look up the manual of waitpid(3) for the meanings of its arguments.

In your case,

waitpid(p[0], NULL, 0);

means

• p[0]: wait for the pid hold on p[0]
• NULL: don't care about status
• 0: no flags

should the last parameter be set to 1, since p[0] ends after 1 sec and this is when I want p[1] to start?

To achieve your goal, starting process 1 as soon as process 0 ends, you should put the waitpid(...); statement before the fork() used to create process 1, and use right parameters in your call to waitpid().

Answer 2

Full documentation is on the manpage or here: http://linux.die.net/man/2/waitpid

Basically you have 3 parameters:

pid_t waitpid(pid_t pid, int *status, int options);

pid is the process you are waiting for. Unless you are waiting for multiple children (in which case specify 0, -1, or a number less than -1 - details in the manpage), specify the pid of your process here.

status is a pointer to an integer which will be filled in with the exit status. This is a combination of the process's exit status and a description of how it has (or has not) exited. The manpage gives you macros you can use to understand this.

options can be filled in with a number of flags, ored together. The most useful of these is the somewhat oddly named W_NOHANG, which makes waitpid simply tell you whether the process has finished (and if so what its exit status was) rather than wait for it to finish.