如何解决在C中使用两个管道读取后无法退出循环进程
我无法理解关闭管道写端的过程。在下面的代码中，我有3个进程，一个父进程，一个父进程的子进程和一个子进程的子进程。我正在尝试模拟命令-cat xxx | grep 28 | sort的管道。我为此编写了一些代码，它实质上创建了排序，“抓取” /过滤器并打印了我的输入，但最终挂起了。我必须按Ctrl + C退出。我的代码有点混乱，但是如果您能帮助我找出问题的话，那会很好。
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
/**
* Executes the command "cat scores | grep Lakers". In this quick-and-dirty
* implementation the parent doesn't wait for the child to finish and
* so the command prompt may reappear before the child terminates.
*
*/
int main(int argc,char **argv)
{
int pipefd[2];
int pipefd2[2];
int pid;
char *cat_args[] = {"cat","scores",NULL};
char *grep_args[] = {"grep","28",NULL};
char *sort_args[] = {"sort",NULL};
// make a pipe (fds go in pipefd[0] and pipefd[1])
if (pipe(pipefd) != 0){
return 1;
}
if (pipe(pipefd2) != 0){
return 1;
}
pid = fork();
if (pid < 0)
{
fprintf(stderr,"fork Failed" );
exit(EXIT_FAILURE);
}
else if (pid == 0)
{
int pid2;
pid2 = fork();
if (pid2 < 0){
fprintf(stderr,"fork Failed" );
return 1;
}
else if (pid2 == 0){
// replace standard input with input part of pipe
// close(0);
// close(pipefd[1]);
// close(pipefd2[1]);
dup2(pipefd2[0],0);
// close unused hald of pipe
close(pipefd2[0]);
close(pipefd[1]);
close(pipefd2[1]);
close(pipefd[0]);
// execute grep
execvp("sort",sort_args);
close(pipefd[1]);
close(pipefd2[1]);
exit(0);
}else{
// replace standard input with input part of pipe
// close(pipefd[1]);
// close(pipefd2[1]);
dup2(pipefd[0],0);
dup2(pipefd2[1],1);
// close unused hald of pipe
close(pipefd[0]);
close(pipefd2[1]);
close(pipefd[1]);
close(pipefd2[0]);
// execute grep
execvp("grep",grep_args);
waitpid(pid2,NULL,0);
close(pipefd[1]);
close(pipefd[0]);
close(pipefd2[1]);
close(pipefd2[0]);
exit(0);
waitpid(pid2,0);
}
}
else
{
// close(pipefd[1]);
// close(pipefd2[1]);
dup2(pipefd[1],1);
// close unused unput half of pipe
close(pipefd[1]);
close(pipefd[0]);
close(pipefd2[1]);
close(pipefd2[0]);
// execute cat
execvp("cat",cat_args);
exit(0);
waitpid(pid,0);
}
close(pipefd[1]);
close(pipefd[0]);
close(pipefd2[1]);
close(pipefd2[0]);
}
这是我得到的输出。不确定是否相关，但是如您所见，结果按团队名称排序。它只是没有终止。
Houston 44 28 .611
Indiana 45 28 .616
Oklahoma City 44 28 .611
Utah 44 28 .611
^C
解决方法
调用execvp会将当前过程映像替换为新的过程映像。如果没有错误发生，那么代码将永远不会到达那一行，因此您的 close（）和 waitpid（）函数调用是无用的。
编辑
这是解决您问题的完整功能代码。评论应具有自我解释性。请注意，命令的执行顺序是不同的，我正在等待过程完成。
从空管道读取将阻塞，直到有一些数据要读取或发生错误为止，因此这是 不是 的唯一解决方案。
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
static void die (const char *msg) {
perror (msg);
exit (EXIT_FAILURE);
}
int main (int argc,char** argv) {
int pipefd[2];
int pid;
char *cat_args[] = {"cat","scores",NULL};
char *grep_args[] = {"grep","28",NULL};
char *sort_args[] = {"sort",NULL};
//make a pipe (file descriptor to read is pipefd[0],fd to write is pipefd[1])
if (pipe (pipefd) < 0)
die ("creating a pipe failed");
pid = fork();
if (pid < 0)
die ("fork");
else if (pid == 0) {
//child process
int pipefd2[2]; //only visible to the affected processes
if (pipe (pipefd2) < 0)
die ("pipe");
int pid2;
pid2 = fork();
if (pid2 < 0)
die ("fork");
else if (pid2 == 0) {
//child of child will execute cat command
close (pipefd2[0]); //we only need to write to the second pipe. close its reading end
//first pipe is for communication between parent and grandparent only
close (pipefd[0]);
close (pipefd[1]);
dup2 (pipefd2[1],STDOUT_FILENO); //write the output to the second pipe instead of the standard output
close (pipefd2[1]); //close writing end of second pipe
execvp("cat",cat_args); //execute cat command
die ("execvp should never return");
}
else {
//child process will execute the grep command
close (pipefd2[1]); // we only need to read from the second pipe. close its writing end
close(pipefd[0]); //we won't read from the first pipe
waitpid (pid2,NULL,0); //wait for cat command to finish
dup2 (pipefd2[0],STDIN_FILENO); //read from the second pipe instead of the stdin
close (pipefd2[0]); //child finished. close reading end of second pipe
dup2 (pipefd[1],STDOUT_FILENO); //write the results of grep command to first pipe instead of standard output
close (pipefd[1]); //we dup2 the output,close the writing end of first pipe
execvp ("grep",grep_args);
die ("execvp should never return");
}
} else {
//parent process will execute the sort command
close (pipefd[1]); //we won't write to the first pipe
waitpid (pid,0); //wait for child to write grep output to the first pipe
dup2 (pipefd[0],STDIN_FILENO); //read from the first pipe instead of stdin
close (pipefd[0]); //child finished. close reading end of first pipe
execvp ("sort",sort_args); //execute command
die ("execvp should never return");
}
//exit (EXIT_SUCCESS); we don't need this. the programm will never reach this line
}
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