主 function 中的 printf 在 C 中返回两次

Question

I am running such a weird problem in C. Here is my code:

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdbool.h>
                                                                        
#define NUM_OF_CORES 1
#define MAX_PRIME 100000
                                                                        
unsigned long do_primes()
{
    unsigned long i, num, primes = 0;
    for (num = 1; num <= MAX_PRIME; ++num) {
        for (i = 2; (i <= num) && (num % i != 0); ++i);
        if (i == num)
            ++primes;
    }
    //printf("Calculated %ld primes.\n", primes);
    return primes;
}
                                                                        
int main(int argc, char ** argv)
{
    //printf ("TEST <CPUBURN_PRIME TEST>\n");
    time_t start, end;
    time_t run_time;
    int i;
    static unsigned long prime_count = 0;
    pid_t pids[NUM_OF_CORES];
                                                                        
    /* start of test */
    start = time(NULL);
    for (i = 0; i < NUM_OF_CORES; i++) {
        if (!(pids[i] = fork())) {
            prime_count = do_primes();
            printf("Calculated %ld primes.\n", prime_count);
            exit(0);
        }
        if (pids[i] < 0) {
            perror("Fork");
            exit(1);
        }
    }
    for (i = 0; i < NUM_OF_CORES; ++i) {
        waitpid(pids[i], NULL, 0);
    }
    end = time(NULL);
    run_time = (end - start);
    
    printf("<TEST:CPUBURN_PRIME TEST><%d--%ld--%ld>\n", MAX_PRIME, prime_count, run_time);
    
    return 0;
}

The output is:

Calculated 9592 primes. <TEST:CPUBURN_PRIME TEST><100000--0--3>

The problem is the last printf is returing twice. Therefore I am loosing the value of the variable prime_count. What wrong that I am doing?

Answer 1

prime_count would not be updated since it's not changed in the parent process. Only the fork -d child process updates its own copy of prime_count . You would need to establish some form of Inter-Process communication between the two processes so that the parent can read back the value of prime_count from the child. One of the (many) ways could be to use pipes as follows (this is not production level code )

if (!(pids[i] = fork())) {
    close(pipes[i][0]);
    prime_count = do_primes();
    printf("%d Calculated %ld primes.\n",getpid(), prime_count);
    char buff[100] = {0};
    sprintf(buff, "%ld", prime_count);
    write(pipes[i][1], buff, strlen(buff));
    close(pipes[i][1]);
    exit(0);
} else if(pids[i] > 0) {
    close(pipes[i][1]);
    char buff[100];
    read(pipes[i][0], buff, sizeof(buff));
    printf("Read num primes %s\n", buff);
    prime_count = strtol(buff, NULL, 10);
    close(pipes[i][0]);
} else {
    perror("Fork");
    exit(1);
}

This is just an example and might suffice when NUM_CORES is 1. You'll need to adapt it to read asynchronously from the pipe since read is a blocking call by default and the parent process won't get to proceed with the subsequent loop iterations in case the read is blocked.