关于fork()与vfork()的两个实例的疑问

李某人

fork():

int glob = 6;
char buf[] = "a write to stdout\n";

int main()
{
        int var;
        pid_t pid;

        var = 88;
        if( write(STDOUT_FILENO, buf, sizeof(buf))-1 != sizeof(buf)-1)
                err_sys("write error");
        printf("before fork\n");

        if( (pid = fork()) < 0 ) {
                err_sys("fork error");
        } else if(pid == 0) {
                glob++;
                var++;
        } else {
                sleep(2);
        }

        printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var);
        exit(0);
}
输出如下:
[root@www apue2]# ./a.out
a write to stdout
before fork
pid = 26062, glob = 7, var = 89
pid = 26060, glob = 6, var = 88


vfork():

int glob = 6;

int main()
{
        int var;
        pid_t pid;

        var = 88;
        printf("before fork\n");

        if( (pid = vfork()) < 0 ) {
                err_sys("fork error");
        } else if(pid == 0) {
                glob++;
                var++;
                _exit(0);                       //不是exit(0)
        }
       
        printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var);
        exit(0);       
}
输出如下:
[root@www apue2]# ./a.out
before fork
pid = 27042, glob = 7, var = 89


我的疑问是: vfork()的输出应该是:
before fork
pid = 27043, glob = 7, var = 89         //vfork()保证子进程先运行
pid = 27042, glob = 7, var = 89         //在子进程调用exec或exit之前,它在父进程的地址空间运行

但,为什么vfork()输出却只有一行,而不是像fork()那样是两行?

[ 本帖最后由 李某人 于 2009-4-19 15:17 编辑 ]

astezheng

你的vfor子进程里面的exit，让它走不到最下面的输出了！

雨过白鹭洲

The vfork() function shall be equivalent to fork(), except that the behavior is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec family of functions.

Conforming applications are recommended not to depend on vfork(), but to use fork() instead. The vfork() function may be withdrawn in a future version.

On some implementations, vfork() is equivalent to fork().

引自The Open Group Base Specifications Issue 6


然后是关于你的问题，子进程在输出之前已经调用_exit退出了，所以只有父进程执行了输出语句

[ 本帖最后由 雨过白鹭洲 于 2009-4-20 09:25 编辑 ]