Question

我正在为课程作业编写一个基本shell，它将在给定的路径列表中找到命令，然后执行命令。它也意味着处理管道。但是，当我分叉子进程时，我在gdb中收到“Write error：Broken Pipe”消息，程序突然终止。

我似乎无法理解为什么会发生这种情况，因为我一直对打开和关闭正确的管道持谨慎态度，并且工艺分叉似乎也能按预期工作。有更多C和unix编程经验的人可以帮我诊断问题吗？我的fork实现/管道实现在逻辑上有什么不对吗？

//commands is of the format {"ls -al", "more", NULL}
//it represents commands connected by pipes, ex. ls -al | more
char **commands = parseArgv(consoleinput, SPECIAL_CHARS[4]);

int numcommands = 0;

while( commands[numcommands]!=NULL )
{
    numcommands++;
}

const int  numpipes = 2*(numcommands-1);

int pipefds[numpipes];

int i=0;
for(i=0; i<numpipes;i=i+2)
{
    pipe(pipefds+i);
}

int pipe_w = 1;
int pipe_r = pipe_w - 3;
int curcommand = 0;

while(curcommand < numcommands)
{
    if(pipe_w < numpipes)
    {
        //open write end
        dup2(pipefds[pipe_w], 1);
    }

    if(pipe_r > 0)
    {
        //open read end
        dup2(pipefds[pipe_r], 0);
    }

    for(i=0;i<numpipes;i++) //close off all pipes
    {
        close(pipefds[i]);
    }

    //Parse current command and Arguments into format needed by execv

    char **argv = parseArgv(commands[curcommand], SPECIAL_CHARS[0]);

    //findpath() replaces argv[0], i.e. command name by its full path ex. ls by /bin/ls
    if(findPath(argv) == 0)
    {
        int child_pid = fork();

        //Program crashes after this point
        //Reason: /bin/ls: write error, broken pipe

        if(child_pid < 0)
        {
            perror("fork error:");
        }
        else if(child_pid == 0)     //fork success
        {
            if(execv(argv[0], argv) == -1)
            {
                perror("Bad command or filename:");
            }

        }
        else
        {
            int child_status;
            child_pid = waitpid(child_pid, &child_status, 0);
            if(child_pid < 0)
            {
                perror("waitpid error:");
            }
        }
    }
    else
    {
        printf("Bad command or filename");
    }
    free(argv);

    curcommand++;
    pipe_w = pipe_w + 2;
    pipe_r = pipe_r + 2;
}

//int i=0;
for(i=0;i<numpipes;i++) //close off all pipes
{
    close(pipefds[i]);
}

free(commands);

Answer 1

在fork（）调用之后（即在子进程中）复制文件描述符是正确的方法。此外，waitpid（）调用使一个子进程等待另一个子进程，并且shell挂起。 wait（）调用应该在循环之后移动，即父级应该等待所有子级。

程序在使用管道进行分叉过程中崩溃

1 个答案: