为什么方案没有在这​​个C编程或UNIX编程（execvp（）系统调用）执行一些句子？ [英] Why the program didn't execute some sentences in this C programming or unix programming(execvp() System calls)?

问题描述

我有以下的程序，当我运行该程序，我觉得真的很困惑就是为什么我的程序没有EXCUTE

I have the following program, when I run the program, I feel really confused that why my program didn't excute

   int num=i;
       printf("it is No.%d !",num);
       printf("hello , I will excute execvp!");

我的程序基本上是创建6子进程EXCUTE executionbode（）函数，然后使用execvp超载原来的程序。不过，每次当我运行该程序，字符串你好，我将执行execvp一直没有出现！此外，我认为那些三句以上也是在正在运行的程序没有执行？有人可以告诉我为什么？下面是我的程序

My program basically create 6 child processes to excute executionbode() function, and then use execvp to overload original program. However, everytime when I run the program, the string "hello, I will execute execvp" never shows up! Also I think those three sentences above also didn't execute in the running program? can someone tell me why? Here is my program

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include "makeargv.h"
#include "redirection.h"
#include <sys/wait.h>



int executionnode(int i);

int main(){
pid_t childpid;
     int i;
     int row=6;
     for(i=0;i<row;i++)
     {   childpid=fork();
         if(childpid==0)
            continue;
         else if (childpid>0)
            executionnode(i);

         else {
           perror("something wrong");
            exit(1);
          }
      }


}


int executionnode(int i){
   sleep(i);
   printf("hello, I am process:%ld\n",(long)getpid());
   wait(NULL);
   char *execArgs[] = { "echo", "Hello, World!", NULL };
   int num=i;
   printf("it is No.%d !",num);
   printf("hello , I will excute execvp!");
   execvp("echo", execArgs);

}

有人能告诉我为什么吗？以及如何解决它？我觉得这是真的很奇怪？是因为execvp（）函数呢？我刚开始学习的操作系统，所以我真搞不清楚了！谢谢你对我的帮助！

Can someone tell me why? and how to fix it? I feel it is really strange? Is it because of execvp() functions? I just began to learn operating system,so I am really confused about it! Thank you for helping me!

推荐答案

您所面临的问题是2个不同的东西组合的结果是：

The issue you are facing is the result of the combination of 2 different things:

1 - 随着的 tonysdg 的注释中，execvp overwites当前的进程映像，并引用的这个：

1 - As tonysdg mentioned in the comment, execvp overwites the current process image, and to quote this:

的execve（）不会成功返回，文本，数据，BSS和
  调用进程的堆栈是由程序的改写
  加载。

execve() does not return on success, and the text, data, bss, and stack of the calling process are overwritten by that of the program loaded.

（ execvp（）是一个前端的execve（） 的）

(execvp() is a front-end for execve())

和

2 - 该stdout是一个缓冲流，这意味着当一个换行符发现或缓冲区将刷新实际打印发生。

2 - The stdout is a buffered stream, which means that the actual printing takes place when a newline character is found or the buffer is flushed.

（看到更多关于这个在的陆克文Zwolinski的的答案这里）

(see more on this at Rudd Zwolinski's answer here )

所以，现在，让我们来看看如何将这些东西互动，并产生您的问题：
你叫就在 execvp（）您的输出缓冲已经包含了2 previous内容的的printf的的，但是因为没有换行符，没有什么是印在屏幕上。

So, now, let's see how these things interact and produce your issue: Just before you call execvp() your output buffer already contains the contents of the 2 previous printf's, however since there is no newline character, nothing is printed on the screen.

然后， execvp（）被执行，在当前的过程中，当然这意味着你的'previous输出缓冲区的内容会丢失所有信息被覆盖

Then, execvp() is executed and overwrites everything on your current process, which of course means that the contents of your 'previous' output buffer are lost.

<一个href=\"http://stackoverflow.com/questions/1716296/why-does-printf-not-flush-after-the-call-unless-a-newline-is-in-the-format-strin\">Here你可以找到很多方法来解决这个问题，并且只是为了完整性，我建议你添加 \\ n 最终的printf ：

Here you can find many ways to address this issue and, just for completeness, I suggest you add \n to the final printf:

...
printf("hello , I will excute execvp!\n");
...

，然后你去好：

$ ./soc
hello, I am process:4701
hello, I am process:4702
hello, I am process:4703
hello, I am process:4704
hello, I am process:4705
hello, I am process:4706
it is No.5 !hello , I will excute execvp!
Hello, World!
it is No.4 !hello , I will excute execvp!
Hello, World!
it is No.3 !hello , I will excute execvp!
Hello, World!
it is No.2 !hello , I will excute execvp!
Hello, World!
it is No.1 !hello , I will excute execvp!
Hello, World!
it is No.0 !hello , I will excute execvp!
Hello, World!

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