fork(),vfork() 和 clone() 的区别和联系

深入理解内核级进程创建方式：fork(), vfork() 和 clone()

最新推荐文章于 2024-03-12 20:39:42 发布

原创最新推荐文章于 2024-03-12 20:39:42 发布 · 553 阅读

0 ·

CC 4.0 BY-SA版权

操作系统专栏收录该内容

16 篇文章

订阅专栏

本文详细解析 Linux 内核中用于创建子进程的 fork(), vfork() 和 clone() 函数的底层工作原理，包括它们的区别、共享资源的方式以及在不同场景下的应用。特别关注 fork() 的 copy-on-write 特性，vfork() 的内存共享机制，以及 clone() 的灵活性与多选项设置。

以前英文写的，懒得翻译了。。。以后再慢慢扩充吧！

The fork(),vfork() and clone() all call the do_fork() to do the real work, but with different parameters.

asmlinkage int sys_fork(struct pt_regs regs)
{
return do_fork(SIGCHLD, regs.esp, &regs, 0);
}

asmlinkage int sys_clone(struct pt_regs regs)
{
unsigned long clone_flags;
unsigned long newsp;

clone_flags = regs.ebx;
newsp = regs.ecx;
if (!newsp)
newsp = regs.esp;
return do_fork(clone_flags, newsp, &regs, 0);
}
asmlinkage int sys_vfork(struct pt_regs regs)
{
return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs.esp, &regs, 0);
}
#define CLONE_VFORK 0x00004000 /* set if the parent wants the child to wake it up on mm_release */
#define CLONE_VM 0x00000100 /* set if VM shared between processes */

SIGCHLD means the child should send this signal to its father when exit.
For fork, the child and father has the independent VM page table, but since the efficiency, fork will not really copy any pages, it just set all the writeable pages to readonly for child process. So when child process want to write something on that page, an page exception happen and kernel will alloc a new page cloned from the old page with write permission. That's called "copy on write".

For vfork, the virtual memory is exactly by child and father---just because of that, father and child can't be awake concurrently since they will influence each other. So the father will sleep at the end of "do_fork()" and awake when child call exit() or execve() since then it will own new page table. Here is the code(in do_fork()) that the father sleep.

if ((clone_flags & CLONE_VFORK) && (retval > 0))
down(&sem);
return retval;
Here is the code(in mm_release() called by exit() and execve()) which awake the father.

up(tsk->p_opptr->vfork_sem);
For sys_clone(), it is more flexible since you can input any clone_flags to it. So pthread_create() call this system call with many clone_flags:

int clone_flags = (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGNAL | CLONE_SETTLS | CLONE_PARENT_SETTID | CLONE_CHILD_CLEARTID | CLONE_SYSVSEM);

Summary: the fork(),vfork() and clone() will create child processes with different mount of sharing resource with the father process. We also can say the vfork() and clone() can create threads(actually they are processes since they have independent task_struct) since they share the VM page table with father process.

阅读(6) | 评论(0) | 转发(0) |

上一篇：Android线程的生命周期

下一篇：真的要做单元测试吗？