使用 fork 后，可能需要获取 fork 的进程的运行状况，比如有没有异常、崩溃。
wait 在 man 中关键的描述如下：
All of these system calls are used to wait for state changes in a child of the calling process, and obtain information about the child whose state has changed. A state change is considered to be: the child terminated; the child was stopped by a signal; or the child was resumed by a signal.

示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(void)
{
    pid_t pid;
    int status;

    printf("before fork\n");
    fflush(stdout);

    if ( (pid = fork()) < 0)
    {
        printf("fork error\n");
    }
    else if (pid == 0)
    {
        printf("after fork, child\n");

        // 4种测试情况
        exit(7); // -> normal termination, exitstatus = 7
        // abort(); // -> abnormal termination, signalstatus = 6 (SIGABRT)
        // int i = 1 / 0; // -> abnormal termination, signalstatus = 8 (SIGFPE)
        // char *p = NULL; *p = 'a'; // -> abnormal termination, signalstatus = 11 (SIGSEGV)
    }

    wait(&status);

    if (WIFEXITED(status))
    {
        printf("normal termination, exitstatus = %d\n", WEXITSTATUS(status));
    }
    else if (WIFSIGNALED(status))
    {
        printf("abnormal termination, signalstatus = %d\n", WTERMSIG(status), 
#ifdef WCOREDUMP
        WCOREDUMP(status)?"(core file generated)":""); 
#else 
        "");
#endif
    }
    else if (WIFSTOPPED(status))
    {
        printf("child stopped, signal number = %d\n", WSTOPSIG(status));
    }

    printf("after fork, parent\n");

    return 0;
}

运行效果：