thread.c

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include <unistd.h>
#include<math.h>
#include<stdbool.h>
#include<pthread.h>
#include<sys/types.h>

int value = 1;

static void *runner (void *param);

int main(int argc, char ** argv)
{
    puts("--------\n\n");
    pid_t pid = fork();
    if (pid > 0) {
        printf("A=%d\n", value);
    }
    else if (pid==0) {
        pid_t pid = fork();
        if (pid > 0) {
            printf("B=%d\n", value);
        }
        else if (pid==0) {
            pid_t pid = fork();
            pthread_t tid;
            pthread_attr_t attr;
            pthread_attr_init(&attr);
            pthread_create(&tid, &attr, runner, NULL);
            pthread_join(tid, NULL);
            
            if (pid > 0) 
                printf("C=%d\n", value);
            else
                printf("D=%d\n", value);
        }
        else {
            exit(1);
        }
    }
    else {
        exit(1);
    }
    return 0;

}

static void *runner (void *param)
{
    value += 1;
    pthread_exit(0);
}