race.c

#include "types.h"
#include "stat.h"
#include "user.h"
#include "condvar.h"
#include "fcntl.h"

// We want Child 1 to execute first, then Child 2, and finally Parent.
int main() {
    struct condvar cv;
    int fd = open("flag", O_RDWR | O_CREATE);
    init_lock(&cv.lk);
    
    int pid = fork(); // fork the first child
    if (pid < 0) {
        printf(1, "Error forking first child.\n");
    } else if (pid == 0) {
        sleep(5);
        printf(1, "Child 1 Executing\n");
        lock(&cv.lk);
        write(fd, "done", 4);
        cv_signal(&cv);
        unlock(&cv.lk);
    } else {
        pid = fork(); // fork the second child
        if (pid < 0) {
            printf(1, "Error forking second child.\n");
        } else if (pid == 0) {
            lock(&cv.lk);
            struct stat stats;
	    fstat(fd, &stats);
	    printf(1, "file size = %d\n", stats.size);
	    while(stats.size <= 0){
	        cv_wait(&cv);
		fstat(fd, &stats);
		printf(1, "file size = %d\n", stats.size);
	    }
            unlock(&cv.lk);
            printf(1, "Child 2 Executing\n");
        } else {
            printf(1, "Parent Waiting\n");
            int i;
            for (i = 0; i < 2; i++)
                wait();
            printf(1, "Children completed\n");
            printf(1, "Parent Executing\n");
            printf(1, "Parent exiting.\n");
        }
    }
    close(fd);
    unlink("flag");
    exit();
}