Processes not executing in expected order; fork() and waitpid()-CodePudding

I wrote a program that calls fork() two times, generating three children and a parent.

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

int main() {
    int child1 = fork();
    int child2 = fork();

    // Check for error
    if (child1 < 0 || child2 < 0) {
        fprintf(stderr, "A fork failed :(\n");
        exit(1);
    }

    // Parent Process
    if (child1 > 0 && child2 > 0) {
        waitpid(child1, NULL, 0);
        waitpid(child2, NULL, 0);
        printf("I'm the parent\n");
    }
    // Child B -- CONFUSION AREA --
    else if (child1 > 0 && child2 == 0) {
        waitpid(child1, NULL, 0); // Not waiting for process to finish
        printf("I'm child B\n");
    }
    // Child A
    else if (child1 == 0 && child2 > 0) {
        waitpid(child2, NULL, 0);
        printf("I'm child A\n");
    }
    // Child C
    else if (child1 == 0 && child2 == 0) {            
        printf("I'm child C\n");
    }

    return 0;
}

I'm trying to print out

I'm child C
I'm child A
I'm child B
I'm the parent

but instead, the program prints out

I'm child B
I'm child C
I'm child A
I'm the parent

I'm confused why child B isn't waiting for child1process to finish?

I drew a process tree to try and understand what is going on, and my understanding is that it looks like:

/*

         Parent (15543)
         child1 = 15544;
         child2 = 15545
           /       \
          /         \
         /           \
        A (15544)     B (15545)
     child1 = 0;      child1 = 15544;
     child2 = 15546;  child2 = 0;
        /
       /
      /
     C (15546)
     child1 = 0;
     child2 = 0;


*/

CodePudding user response：

When child B calls waitpid() the call fails and returns -1 because the parent is already waiting on child A, and you can't reap a child process twice.

For the behavior you want, you may be better off using inter-process semaphores to synchronize your process execution. Try including semaphore.h and take a look at the documentation for sem_init().