Linux进程实践(2) --僵尸进程与文件共享

孤儿进程与僵尸进程

孤儿进程:

如果父进程先退出，子进程还没退出那么子进程的父进程将变为init进程。（注：任何一个进程都必须有父进程）

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//生成孤儿进程

int main(int argc, char *argv[])

{

pid_t pid = fork();

if (pid < 0)

err_exit("fork error");

else if (pid > 0)

exit(0);

else

{

sleep(10);

cout << "Child, ppid = " << getppid() << endl;

}

exit(0);

}

僵尸进程:

如果子进程先退出，父进程还没退出，那么子进程必须等到父进程捕获到了子进程的退出状态才真正结束，否则这个时候子进程就成为僵尸进程。

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//生成僵尸进程

int main(int argc, char *argv[])

{

pid_t pid = fork();

if (pid < 0)

err_exit("fork error");

else if (pid == 0)

exit(0);

else

{

sleep(50);

}

exit(0);

}

附-查询父子进程状态

ps -le | grep main

避免僵尸进程

signal(SIGCHLD, SIG_IGN);

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//示例: 避免僵尸进程

int main(int argc, char *argv[])

{

signal(SIGCHLD, SIG_IGN);

pid_t pid = fork();

if (pid < 0)

err_exit("fork error");

else if (pid == 0)

exit(0);

else

{

sleep(50);

}

exit(0);

}

文件共享

父进程的所有文件描述符都被复制到子进程中, 就好像调用了dup函数, 父进程和子进程每个相同的打开文件描述符共享一个文件表项(因此, 父子进程共享同一个文件偏移量);



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//根据上图: 理解下面这段程序和下图的演示

int main(int argc, char *argv[])

{

signal(SIGCHLD, SIG_IGN);

int fd = open("test.txt", O_WRONLY|O_CREAT|O_TRUNC, 0666);

if (fd == -1)

err_exit("file open error");

cout << "We Don`t flash memory\n";

char buf[BUFSIZ];

bzero(buf, sizeof(buf));

pid_t pid = fork();

if (pid < 0)

err_exit("fork error");

else if (pid > 0)

{

strcpy(buf, "Parent...");

write(fd, buf, strlen(buf));

close(fd);

cout << "fd = " << fd << endl;

exit(0);

}

else if (pid == 0)

{

strcpy(buf, "Child...");

write(fd, buf, strlen(buf));

close(fd);

cout << "fd = " << fd << endl;

exit(0);

}

}

fork VS vfork

在UNIX/Linux中的fork还没实现copy on write(写时复制)技术之前。Unix设计者很关心fork之后立刻执行exec所造成的地址空间浪费，所以引入了vfork系统调用。其中,vfork子进程与父进程共享数据段,并不真正复制父进程内存,因此在vfork之后执行exec系列函数,并不会导致地址空间浪费以及无用的空间复制时间.而且,即使fork实现了copy on write，效率也没有vfork高.

但是,vfork有个限制，子进程必须立刻执行_exit或者exec系列函数。因此我们不推荐使用vfork，因为几乎每一个vfork的实现，都或多或少存在一定的问题(可以尝试在vfork之后的子进程中既不执行_exit,也不执行exec函数)。

fork与vfork的区别

1. fork子进程拷贝父进程的数据段(但是现在提供了写时复制技术,只有当子进程真正需要写内存时,才复制出该内存的一段副本),因此,在父进程/子进程中对全局变量所做的修改并不会影响子进程/父进程的数据内容.

vfork子进程与父进程共享数据段,因此父子进程对数据的更新是同步的；

2. fork父、子进程的执行次序是未知的,取决于操作系统的调度算法

vfork：子进程先运行，父进程后运行；

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//示例1:vfork出错情况

//在Linux 2.6内核上会持续执行,不会退出

//而在Linux 3.13内核上, 则会引发core dump

int main()

{

int iNumber = 0;

pid_t pid = vfork();

if (pid == -1)

{

perror("fork");

return -1;

}

else if (pid > 0)

{

cout << "In Parent Program..." << endl;

cout << "iNumber = " << iNumber << endl;

cout << "pid = " << static_cast<int>(getpid());

cout << "\t ppid = " << static_cast<int>(getppid()) << endl;

//_exit(0);

}

else if (pid == 0)

{

iNumber ++;

cout << "In Child Program..." << endl;

cout << "iNumber = " << iNumber << endl;

cout << "pid = " << static_cast<int>(getpid());

cout << "\t ppid = " << static_cast<int>(getppid()) << endl;

//_exit(0);

}

return 0;

}



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//示例2: 父进程/子进程修改全局数据的情况

int main()

{

int iNumber = 10;

cout << "Before vfork, pid = " << getpid() << endl;

//对比fork()

pid_t pid = vfork();

if (pid == -1)

err_exit("fork");

else if (pid > 0)

{

sleep(4);

cout << "Parent, iNumber: " << iNumber << endl;

}

else if (pid == 0)

{

++ iNumber;

cout << "Child, iNumber = " << iNumber << endl;

_exit(0);

}

return 0;

}

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//示例3:用vfork执行当前目录下的hello程序

int main()

{

int iNumber = 0;

pid_t pid = vfork();

if (pid == -1)

{

perror("fork");

return -1;

}

else if (pid > 0)

{

cout << "In Parent Program..." << endl;

cout << "iNumber = " << iNumber << endl;

cout << "pid = " << static_cast<int>(getpid());

cout << "\t ppid = " << static_cast<int>(getppid()) << endl;

}

else if (pid == 0)

{

iNumber ++;

cout << "In Child Program..." << endl;

cout << "iNumber = " << iNumber << endl;

cout << "pid = " << static_cast<int>(getpid());

cout << "\t ppid = " << static_cast<int>(getppid()) << endl;

//将自己写的程序启动起来

execve("./hello",NULL,NULL);

_exit(0);

}

return 0;

}



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//测试4,用vfork执行系统命令

int main()

{

int iNumber = 0;

pid_t pid = vfork();

if (pid == -1)

{

perror("fork");

return -1;

}

else if (pid > 0)

{

cout << "In Parent Program..." << endl;

cout << "iNumber = " << iNumber << endl;

cout << "pid = " << static_cast<int>(getpid());

cout << "\t ppid = " << static_cast<int>(getppid()) << endl;

}

else if (pid == 0)

{

iNumber ++;

cout << "In Child Program..." << endl;

cout << "iNumber = " << iNumber << endl;

cout << "pid = " << static_cast<int>(getpid());

cout << "\t ppid = " << static_cast<int>(getppid()) << endl;

//将ls命令启动起来,注意:由于C++严格的类型转换机制,需要在字符串前加(char*)

char *const args[] = {(char *)"/bin/ls", (char *)"-l", NULL};

int res = execve("/bin/ls",args,NULL);

if (res == -1)

{

perror("execve");

_exit(1);

}

_exit(0);

}

return 0;

}