C子进程运行其代码两次

Question

Total C noob，寻求fork()的帮助。我不确定为什么它打印子进程2代码两次。我认为它与sleep调用有关，因为它之前工作正常。

输出

Child Process 2
Process ID: 31973
Parent Process ID: 1

Child Process 1
Process ID: 31972
Parent Process ID: 1

Child Process 2
Process ID: 31974
Parent Process ID: 1

C代码

#define _XOPEN_SOURCE // required for cuserid to work

// includes
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/resource.h>
#include <time.h>


int main(void) {
    getP0Info();
    putenv("WHALE=7");
    forkProcesses();
    return 0;
}
// Prints an error message and exits if one occurs. Else, returns the system call value.
int print_if_err(int syscall_val, const char* syscall_name) {
    if (syscall_val < 0) {
        perror(syscall_name);
        exit(errno);
    } else {
        //No syscall error; we can return
        return syscall_val;
    }
}

void forkProcesses() {

    pid_t child1_pid = print_if_err(fork(), "fork");
    if (child1_pid == 0) {
        printf("\nChild Process 1\n");
        printf("Process ID: %d\n", getpid());
        printf("Parent Process ID: %d\n", getppid());

        sleep(1);
        int whale = atoi(getenv("WHALE"));
        printf("C1: %d\n", whale);

    }

    pid_t child2_pid = print_if_err(fork(), "fork");
    if (child2_pid == 0) {
        printf("\nChild Process 2\n");
        printf("Process ID: %d\n", getpid());
        printf("Parent Process ID: %d\n", getppid());

    }
}

Answer 1

这是因为即使在if语句结束后，子进程1的执行仍在继续。您需要明确地执行return或致电_exit()等，以防止这种情况发生：

void forkProcesses() {

    pid_t child1_pid = print_if_err(fork(), "fork");
    if (child1_pid == 0) {
        printf("\nChild Process 1\n");
        printf("Process ID: %d\n", getpid());
        printf("Parent Process ID: %d\n", getppid());

        sleep(1);
        int whale = atoi(getenv("WHALE"));
        printf("C1: %d\n", whale);
        _exit(0);
    }

    pid_t child2_pid = print_if_err(fork(), "fork");
    if (child2_pid == 0) {
        printf("\nChild Process 2\n");
        printf("Process ID: %d\n", getpid());
        printf("Parent Process ID: %d\n", getppid());

    }
}