Question

Is there a function in Linux which allows me to see how much CPU did a process use after it finished? I need something similar to bash “time” command. I am fork()ing the process and then waiting using wait() for a child to finish. The way of accurately measuring “real” time (actual time elapsed between fork() and exit()), even when wait() was called a long after the child process became zombie is also welcome, but I’m not sure if its possible.

Answer 1

Sure, wait3 and wait4 have you covered. Alternatively (and more portably) you could use getrusage(2).

The wait3() and wait4() system calls are similar to waitpid(2), but
additionally return resource usage information about the child in the
structure pointed to by rusage.

Example: wait3

struct rusage usage;
wait3(&status, 0, &usage);

Example: getrusage

Of course, wait3 and wait4 are just a convenience. So you could use getrusage:

getrusage(RUSAGE_CHILDREN, &usage);

The disadvantage is that this tells you the resources used by ALL the terminated children.

So, once you get it, what do you do with rusage ? struct rusage has the following form:

struct rusage {
    struct timeval ru_utime; /* user CPU time used */
    struct timeval ru_stime; /* system CPU time used */
    /* More. */
};