Calculating process run time of child process fork()

Question

I am having a program with 1 parent process and 3 children process. And I want to calculate run time of all children processes.

int run_time[3];    // Variable to save the running time of the children process
time_t start[3];        // Variable to help measure the actual time
for ( i = 0; i < 3; i++ )
{
    if( fork() == 0 )           // 3 Children process running
    {
        start[i] = time(NULL);  // Start time of child process
        usleep(1000000);
        run_time[i] = time(NULL) - start[i];        // Calculate run time
        printf("Running time: %d from child\n",run_time[i]);
        exit(0);
    }
}
for ( i2 = 0; i2 < 3; i2++ )    // Waiting all 3 children process finish
    waitpid(-1, NULL, 0);
for ( i3 = 0; i3 < 3; i3++ )    // Printing out run time of children from parent process
    printf("Running time: %d from parent\n",run_time[i3]);

As I know that I can not save the calculate data from child process ( run_time[] in my code) to the parent process, even with global variable and pointer (I tried). There is only one way which is using pipe() . Something like this: int fd[2] and then pipe(fd) . But then I can not use pipe() for more than 1 child process. So I would like to as if there is another way to calculate run time of child process without using pipe() ? And how can I use pipe() for multiple children process.

Answer 1

If second-level granularity is sufficient for you and the run-time is expected to be on the order of seconds, you can encapsulate the runtime in the return code of the child process, like this:

// return the runtime as an 8-bit integer to the parent
exit(run_time[i] & 0xff);

Then, in parent process, use the macro WEXITSTATUS to get the exit code. See the documentation of the wait() system call:

If the value of WIFEXITED(stat_val) is non-zero, this macro evaluates to the low-order 8 bits of the status argument that the child process passed to _exit() or exit(), or the value the child process returned from main().

Since the return code is an 8-bit integer, this will only work for values in up to 255 seconds.

As for pipes, if you already know how to communicate with a single child and want to communicate with multiple children, simply use an array of pipes.

A modified version of your program follows:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <time.h>

int main(int argc, char **argv) {
    int run_time[3];    // Variable to save the running time of the children process
    time_t start[3];        // Variable to help measure the actual time
    pid_t children[3];
    int status;
    int i;

    for ( i = 0; i < 3; i++ ) {
        children[i] = fork();
        if( children[i] == 0 )  {
            start[i] = time(NULL);  // Start time of child process
            usleep(1000000);
            run_time[i] = time(NULL) - start[i];        // Calculate run time
            printf("Running time: %d from child\n",run_time[i]);
            // return the runtime as an 8-bit integer
            exit(run_time[i] & 0xff);
        }
    }
    for ( i = 0; i < 3; i++ ) {   // Waiting all 3 children process finish
        waitpid(children[i], &status, 0);
        if (WIFEXITED(status)) {
            run_time[i] = WEXITSTATUS(status); // use the low-order 8 bits from the exit code
        } else {
            run_time[i] = -1; // unknown run time
        }
    }

    for ( i = 0; i < 3; i++ )    // Printing out run time of children from parent process
        printf("Running time: %d from parent\n", run_time[i]);
}