fork and vfork

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

int global=5;

int main(void)
{
    pid_t pid;
    int var=1,i;
    printf("The difference between fork and vfork:\n");
    pid=vfork();
    switch(pid)
    {
    case 0:
    i=3;
    while(i-->0)
    {
    printf("Child process is running\n");
    global++;
    var++;
    }
    printf("Child End:global=%d,var=%d\n",global,var);
    break;
    case -1:
    perror("fork error:");
    exit(1);
    default:
    i=5;
    printf("%10d\n",var);
    while(i-->0)
    {
    printf("Parent process is running\n");
    global++;
    var++;
    }
    printf("Parent End:global=%d,var=%d\n",global,var);
    exit(0);
    }
}

The program running result as follows:

The difference between fork and vfork:
Child process is running
Child process is running
Child process is running
Child End:global=8,var=4
4359003
Parent process is running
Parent process is running
Parent process is running
Parent process is running
Parent process is running
Parent End:global=13,var=4359008


My question is: The value of 'var' is 4359003, why not 4 ?

[prajna]

vfork最早起源于2.9BSD，它与fork的不同就在于它并不将父进程的地址空间完全复制到子进程中，因为子进程会立即调用exec.vfork出来的子进程是在父进程的空间中运行的，它的存在就是为了exec调用，所以它不需要复制这些东西。如果这时子进程修改了某个变量，这将影响到父进程. vfork与fork的另一区别是：vfork保证子进程先运行，在它调用exec或exit后父进程才可能调度运行。如果在调用这两个函数之前子进程依赖于父进程的进一步动作，则会导致死锁。 而fork的父子进程运行顺序是不定的，它取决于内核的调度算法. http://blog.csdn.net/buaalei/article/details/5348382

[prajna]

改爲下面的就可以。

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
 
int global=5;
 
int main(void)
{
    pid_t pid;
    int var=1,i;
    printf("The difference between fork and vfork:\n");
    pid=vfork();
    switch(pid)
    {
    case 0:
    i=3;
    while(i-->0)
    {
    printf("Child process is running\n");
    global++;
    var++;
    }
    printf("Child End:global=%d,var=%d\n",global,var);
    break;

    case -1:
    perror("fork error:");
    exit(1);

    default:break;

    }
    i=5;
    printf("%10d\n",var);
    while(i-->0)
    {
    printf("Parent process is running\n");
    global++;
    var++;
    }
    printf("Parent End:global=%d,var=%d\n",global,var);
    exit(0);
}

The difference between fork and vfork: Child process is running Child process is running Child process is running Child End:global=8,var=4 4 Parent process is running Parent process is running Parent process is running Parent process is running Parent process is running Parent End:global=13,var=9

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引用 4 楼 gumh 的回复:

改爲下面的就可以。 C/C++ code?1234567891011121314151617181920212223242526272829303132333435363738394041424344#include <stdio.h>#include <sys/types.h>#include <unistd.h>#include <stdlib.h> int ……

为什么这样就可以了呢? 真心求解

[子善旬]

首先，运行在哪个环境？cygwin？linux？ 对vfork实现有差异 其次，如果在linux下 vfork的MAN手册中说其是未定义的. The vfork() function has the same effect as fork(), except that the behaviour is undefined if the process cre- ated 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. 因为在退出前调用了printf函数，去掉printf，lz再试试呢

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引用 6 楼 enlinux 的回复:

首先，运行在哪个环境？cygwin？linux？ 对vfork实现有差异 其次，如果在linux下 vfork的MAN手册中说其是未定义的. The vfork() function has the same effect as fork(), except that the behaviour is undefined if th……

calls any other function before successfully calling _exit() 非常感谢！

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Linux下，没有中文输入法……vfork创建的子进程，不是和父进程共享地址空间吗？那怎么全局变量可以，局部变量var没了呢？？？（我的调试环境是：Red hat Linux 2.6.32;GCC 4.4.4）

[nadleeh]

看看vfork 和fork的区别。。。。

[samuel417]

所以调用_exit(0)就OK了。