fork的问题,为什么父进程ID消失了

linux_c_py_php

1. 
   
2. DESCRIPTION
   
3.    Standard Description
   
4.        (From  POSIX.1)  The  vfork()  function  has the same effect as fork(2), except that the behavior is undefined if the process created by vfork()
   
5.        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
   
6.        vfork() was called, or calls any other function before successfully calling _exit(2) or one of the exec(3) family of functions.
   
7. 
   
8.    Linux Description
   
9.        vfork(), just like fork(2), creates a child process of the calling process.  For details and return value and errors, see fork(2).
   
10. 
    
11.        vfork()  is  a  special  case of clone(2).  It is used to create new processes without copying the page tables of the parent process.  It may be
    
12.        useful in performance-sensitive applications where a child is created which then immediately issues an execve(2).
    
13. 
    
14.        vfork() differs from fork(2) in that the calling thread is suspended until the child terminates (either normally, by calling _exit(2), or abnor-
    
15.        mally,  after  delivery  of  a  fatal  signal), or it makes a call to execve(2).  Until that point, the child shares all memory with its parent,
    
16.        including the stack.  The child must not return from the current function or call exit(3), but may call _exit(2).
    
17. 
    
18.        As with fork(2), the child process created by vfork() inherits copies of various of the caller’s process  attributes  (e.g.,  file  descriptors,
    
19.        signal  dispositions,  and current working directory); the vfork() call differs only in the treatment of the virtual address space, as described
    
20.        above.
    
21. 
    
22.        Signals sent to the parent arrive after the child releases the parent’s memory (i.e., after the child terminates or calls execve(2)).
    
23. 
    
24.    Historic Description
    
25.        Under Linux, fork(2) is implemented using copy-on-write pages, so the only penalty incurred by fork(2) is the time and memory required to dupli-
    
26.        cate the parent’s page tables, and to create a unique task structure for the child.  However, in the bad old days a fork(2) would require making
    
27.        a complete copy of the caller’s data space, often needlessly, since usually immediately afterward an exec(3) is done.  Thus, for  greater  effi-
    
28.        ciency, BSD introduced the vfork() system call, which did not fully copy the address space of the parent process, but borrowed the parent’s mem-
    
29.        ory and thread of control until a call to execve(2) or an exit occurred.  The parent process  was  suspended  while  the  child  was  using  its
    
30.        resources.   The  use of vfork() was tricky: for example, not modifying data in the parent process depended on knowing which variables were held
    
31.        in a register.

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对vfork调return 0不对吧?

folklore

对vfork调return 0不对吧?

@linux_c_py_php

???
can you wirte it in chinese?

folklore

回复 1# nola_r

楼主坑爹，特意试了一下，没有发现这个问题，很正常

parent:

1. #include <unistd.h>
   
2. #include <stdio.h>
   
3. int main( void )
   
4. {
   
5.   if(fork()==0)
   
6.   {
   
7.      printf("pid-%d, ppid-%d.\n",getpid(), getppid());
   
8.      execlp("./child.exe","child.exe",NULL);
   
9.      *((int *)"no reach code")=1;
   
10.   }
    
11.   printf("the parent  process  is %d.\n",getpid());
    
12.   while(1); /*or wait();*/
    
13.   return 0; /*it is not impotant, but it is needed*/
    
14. }

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child:

1. #include <unistd.h>
   
2. #include <stdio.h>
   
3. int main( void )
   
4. {
   
5.   printf("the child process  is %d.\n",getpid());
   
6.   while(1); /*or wait();*/
   
7.   return 0; /*it is not impotant, but it is needed*/
   
8. }

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$cat /etc/issue
Ubuntu 12.04.1 LTS \n \l

nola_r

回复 13# folklore

没有坑谁呢，呵呵，哪有时间干这活呀，我还不如抽口烟
复制你的代码跑了一下，还是一样
orcl :~ # ./e3.exe
pid-22618, ppid-22617.
the parent  process  is 22617.  ---这个还是先于子进程跑完前出现了，而且你用的是fork 不是vfork()
the child process  is 22618.

folklore

@nola_r

---这个还是先于子进程跑完前出现了

人家没保证这个要后出现啊，现出现和后出现都符合Fork语义