UNIX环境高级编程8.5exit 8.6wait waitpid

不管进程如何终止，最后都会执行内核中的同一段代码，为相应的进程关闭所有打开描述符，释放它所使用的存储器。无论进程如何终止，我们都希望该进程能够通知其父它是如何终止的，对于exit,_exit,_Exit（一种情况），将其退出状态作为参数传送给函数（exit(3))，对于异常终止(另一种情况），内核产生一个指示其异常终止原因的终止状态。这两种情况，该进程父进程都能用wait或waitpid函数取得其终止状态。

退出状态与终止状态有所区别。

僵死进程是，一个已经终止，但是其父进程尚未对其进行善后处理（获取终止子进程的有关信息，释放它仍占用的资源）的进程。

当一个进程正常或异常终止时，内核向父进程发送SIGCHLD信号，该信号是内核向父进程发的异步通知，父进程可以忽略它，也可定义一个函数（信号处理程序）来处理这个信号。默认是忽略。父调用wait/waitpid时：

1. 如果其所有子进程都在运行，则阻塞。
2. 如果一个子终止，则父取得子终止状态，父wait或者waitpid立即返回。
3. 如果父没有任何子，立即返回。

如果父由于接到SIGCHLD而调用wait，则可期待wait立即返回。

waitpid可选择等待终止的子进程。

进程终止状态在<sys/wait.h>/usr/include/sys/wait.h

UNIX系统信号

signum.h

如果一个子进程已经终止，并且是一个僵死进程，则wait立即返回并取得该子进程状态（就是说父调用wait的时候，其儿子中已经有一个是僵死进程了，因为子终止之前，父没有调用wait，并进行收尸工作），否则wait阻塞到任一子进程终止。

// proc/wait1.c 8-4
#include "apue.h"
#include <sys/wait.h>

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

    if ((pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0) /* child */
        exit(7); // terminate normally

    if (wait(&status) != pid) /* wait for child */
        err_sys("wait error");
    pr_exit(status); /* and print its status */

    if ((pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0) /* child */
        abort(); /* generates SIGABRT */

    if (wait(&status) != pid) /* wait for child */
        err_sys("wait error");
    pr_exit(status); /* and print its status */

    if ((pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0) /* child */
        status /= 0; /* divide by 0 generates SIGFPE */

    if (wait(&status) != pid) /* wait for child */
        err_sys("wait error");
    pr_exit(status); /* and print its status */

    return 0;
}

// lib/prexit.c
#include "apue.h"
#include <sys/wait.h>

void pr_exit(int status)
{
    // two condition of a process terminate:
    // 1, child invoke exit.
    // 2, child terminate abnormally.
    if (WIFEXITED(status)) // exited
        printf("normal termination, exit status = %d
",
                WEXITSTATUS(status));
    else if (WIFSIGNALED(status)) // signaled, and parent has no function handle this signal
        printf("abnormal termination, signal number = %d%s
",
                WTERMSIG(status),
#ifdef WCOREDUMP
                WCOREDUMP(status) ? " (core file generated)" : "");
#else
    "");
#endif
    else if (WIFSTOPPED(status))
        printf("child stopped, signal number = %d
",
                WSTOPSIG(status));
}

all: shell1 shell2 fork1 vfork1 wait1
shell1: shell1.c
	g++ -g -Wall shell1.c ../lib/libapue.a -I ../include -o shell1
shell2: shell2.c
	g++ -g -Wall shell2.c ../lib/libapue.a -I ../include -o shell2
fork1: fork1.c
	g++ -g -Wall fork1.c ../lib/libapue.a -I ../include -o fork1
vfork1: vfork1.c
	g++ -g -Wall vfork1.c ../lib/libapue.a -I ../include -o vfork1
wait1: wait1.c
	g++ -g -Wall wait1.c ../lib/libapue.a -I ../include -o wait1
clean:
	rm shell1 shell2 fork1 vfork1 wait1

调用fork两次以避免僵死进程

// proc/fork2.c 8-5
#include "apue.h"
#include <sys/wait.h>

int main(void)
{
    printf("file %s, line %d parent, getpid() = %d
", __FILE__, __LINE__, getpid());
    fflush(stdout);
    pid_t pid;

    if ((pid = fork()) < 0)
    {
        err_sys("fork error");
    }
    else if (pid == 0)
    { /* first child */
        printf("file %s, line %d first child, getpid() = %d
", __FILE__, __LINE__, getpid());
        fflush(stdout);
        if ((pid = fork()) < 0)
        {
            err_sys("fork error");
        }
        else if (pid > 0)
        {
            printf("first child, going to sleep(2)
");
            sleep(2);
            printf("first child, after sleep(2), begin exiting
");
            exit(0);   /* parent from second fork == first child */
        }

        /*
         * We're the second child; our parent becomes init as soon
         * as our real parent calls exit() in the statement above.
         * Here's where we'd continue executing, knowing that when
         * we're done, init will reap our status.
         */
        printf("file %s, line %d, child 2, getpid() = %d
", __FILE__, __LINE__, getpid());
        printf("second child, going to sleep(5)
");
        sleep(5);
        printf("second child, after sleep(5)
");
        printf("second child, parent pid = %d, parent id is 1, which means my parent process is init
", getppid());
        exit(0);
    }

    if (waitpid(pid, NULL, 0) != pid)  /* wait for first child */
        err_sys("waitpid error");
    else
        printf("after sleep(2), first child exit caught by parent (waitpid)
");

    printf("file %s, line %d, getpid() = %d
", __FILE__, __LINE__, getpid());

    /*
     * We're the parent (the original process); we continue executing,
     * knowing that we're not the parent of the second child.
     */
    exit(0);
}

all: shell1 shell2 fork1 vfork1 wait1 fork2
shell1: shell1.c
	g++ -g -Wall shell1.c ../lib/libapue.a -I ../include -o shell1
shell2: shell2.c
	g++ -g -Wall shell2.c ../lib/libapue.a -I ../include -o shell2
fork1: fork1.c
	g++ -g -Wall fork1.c ../lib/libapue.a -I ../include -o fork1
fork2: fork2.c
	g++ -g -Wall fork2.c ../lib/libapue.a -I ../include -o fork2
vfork1: vfork1.c
	g++ -g -Wall vfork1.c ../lib/libapue.a -I ../include -o vfork1
wait1: wait1.c
	g++ -g -Wall wait1.c ../lib/libapue.a -I ../include -o wait1
clean:
	rm shell1 shell2 fork1 vfork1 wait1 fork2