linux_wait_and_threads.cpp

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <assert.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>

void*thread1(void*) {
    auto p1 = fork();
    assert(p1!=-1);
    if (!p1) {
        sleep(4);
        exit(0);
    }
    
    auto p2 = fork();
    assert(p2!=-1);
    if (!p2) {
        sleep(4);
        exit(0);
    }
    
    printf("p1 %d\n", p1);
    printf("p2 %d\n", p2);
    
    
    
    
    // wait for either p1 or p2
    while (p1 != -1 || p2 != -1) {
        
#if 1
        siginfo_t w1;
        auto r1 = waitid(P_ALL, -1, &w1, WEXITED|WNOWAIT);
        
        printf("W1 %d %d %d %s\n", r1, w1.si_pid, w1.si_status, strerror(errno));
        
        
        if (r1 != -1 && w1.si_pid == p1) {
            int w2;
            auto r2 = waitpid(p1, &w2, 0);
            printf("W2 %d %d %s\n", r2, w2, strerror(errno));
            assert(r2 == p1);
            p1 = -1;  
            
        };
        if (r1 != -1 && w1.si_pid == p2) { 
            int w2;
            auto r2 = waitpid(p2, &w2, 0);
            printf("W2 %d %d %s\n", r2, w2, strerror(errno));
            assert(r2 == p2);
            p2 = -1;  
            
        };
        
        
#else
        int w1;
        
        auto r1 = wait(&w1);
        printf("W %d %d %s\n", r1, w1, strerror(errno));
        
        if (r1 != -1 && r1 == p1) p1 = -1;
        if (r1 != -1 && r1 == p2) p2 = -1;
        
#endif
        
    }
    printf("W\n");
    return 0;
}


void thread2() {
    sleep(1);
    auto r1 = system("sleep 1");
    printf("S1 %d\n", r1);
    auto r2 = system("sleep 1");
    printf("S2 %d\n", r2);
    
}

int main () {
    
    pthread_t thread1_h;
    pthread_create(&thread1_h, 0, thread1, 0);
    
    thread2();
    pthread_join(thread1_h, 0);
}