问题描述
我正在尝试用C语言重现此命令:
I'm trying to reproduce this command in c language:
ls | wc > output.txt
为此,我编写了以下程序:
So, to do that, I wrote the following program:
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>
int main()
{
pid_t lsFork, wcFork;
int tube[2];
pipe(tube);
lsFork = fork();
if(lsFork == 0) // ls command
{
close(tube[0]);
dup2(tube[1], STDOUT_FILENO);
close(tube[1]);
if(execl("/usr/bin/ls", "ls", NULL) == -1)
perror("Cannot execute ls");
}
else
{
wcFork = fork();
if(wcFork == 0) // wc command
{
sleep(1);
int file = open("output.txt", O_WRONLY | O_CREAT);
if(file == -1)
perror("Cannot open output.txt");
close(tube[1]);
dup2(tube[0], STDIN_FILENO);
close(tube[0]);
dup2(file, STDOUT_FILENO);
close(file);
/*char buffer[BUFSIZ];
read(STDIN_FILENO, buffer, BUFSIZ);
write(STDOUT_FILENO, buffer, BUFSIZ);*/
if(execl("/usr/bin/wc", "wc", NULL) == -1)
perror("Cannot execute wc");
close(STDOUT_FILENO);
}
else // parent
{
int status;
waitpid(lsFork, &status, 0);
waitpid(wcFork, &status, 0);
}
}
return EXIT_SUCCESS;
}
但是,该程序不会退出.根据htop的说法,wc命令正在阻止该程序.为了理解这种行为,我编写了一段代码(在execl()之前的行注释了),但我不理解它的工作原理,而不是execl().调用此函数时我会忘记一些东西吗?
But, the program does not exit. According to htop, the wc command is blocking the program. To understand this behaviour, I wrote a piece of code (the lines commented before execl()) and I don't understand what this works and not execl(). Am I forgetting something when calling this function?
推荐答案
在您可以轻松完成时不要使事情复杂化.请尝试以下& ;;中的简单代码看看您是否能听懂任何内容.
Don't complicate things when you can do it easily..Try the simpler code below & see if you can understand anything or not.
int main(){
int tube[2];
int fID;
pipe(tube);
if (fork() == 0){
// this is the child process
close(tube[0]); // reading end of the pipe
dup2(tube[1], 1); // stdout ---> pipe writing end
execlp("ls", "ls", NULL);
}else{
if (fork() == 0){
//umask(0022);
fID = open("sample.txt", O_WRONLY | O_CREAT, 0644);
close(tube[1]); // writing end of the pipe
dup2(tube[0], 0); // stdin ----> pipe reading end
dup2(fID, 1);
execlp("wc", "wc", NULL);
}
}
return 0;
}
注意如果代码的目的是仅实现上述管道,那么您无需实现任何等待机制.操作系统将自动杀死所有僵尸孩子(如果有).此外, execlp("wc","wc",NULL);
将自动阻止程序结束.因此它不会提早退出
Note If the purpose of the code is to solely implement the above mentioned piping, then you don't need to implement any waiting mechanisms. The OS will auto-kill all the zombie child, if any. Moreover execlp("wc", "wc", NULL);
will auto block the program to end. Hence it will not exit early
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