再次在Unix中递归管道

我知道这个话题已经好几次了,但我仍然坚持一点.我需要编写一个模拟cmd1 | cmd2 | cmd3 ...管道的程序.

我的代码在这里:http://ideone.com/fedrB8

#include 
#include 
#include 
#include 

void pipeline( char * ar[], int pos, int in_fd);
void error_exit(const char*);
static int child = 0; /* whether it is a child process relative to main() */

int main(int argc, char * argv[]) {
    if(argc < 2){
        printf("Usage: %s option (option) ...\n", argv[0]);
        exit(1);
    }
    pipeline(argv, 1, STDIN_FILENO);
    return 0;
}

void error_exit(const char *kom){
    perror(kom);
    (child ? _exit : exit)(EXIT_FAILURE);
}

void pipeline(char *ar[], int pos, int in_fd){
    if(ar[pos+1] == NULL){ /*last command */
        if(in_fd != STDIN_FILENO){
            if(dup2(in_fd, STDIN_FILENO) != -1)
                close(in_fd); /*successfully redirected*/
            else error_exit("dup2");
        }
        execlp(ar[pos], ar[pos], NULL);
        error_exit("execlp last");
    }
    else{
        int fd[2];
        pid_t childpid;

        if ((pipe(fd) == -1) || ((childpid = fork()) == -1)) {
            error_exit("Failed to setup pipeline");
        }
        if (childpid == 0){ /* child executes current command */
            child = 1;
            close(fd[0]);
            if (dup2(in_fd, STDIN_FILENO) == -1) /*read from in_fd */
                perror("Failed to redirect stdin");
            if (dup2(fd[1], STDOUT_FILENO) == -1)   /*write to fd[1]*/
                perror("Failed to redirect stdout");
            else if ((close(fd[1]) == -1) || (close(in_fd) == - 1))
                perror("Failed to close extra pipe descriptors");
            else {
                execlp(ar[pos], ar[pos], NULL);
                error_exit("Failed to execlp");
            }
        }
        close(fd[1]);   /* parent executes the rest of commands */
        close(in_fd);
        pipeline(ar, pos+1, fd[0]);
    }
}

对于最多3个命令,它完全正常,但是当它达到4个或更多时它不再需要和经过几个小时的分析后,我仍然无法解决问题所在.

Example: 
./prog ls uniq sort head 

gives: 
sort: stat failed: -: Bad file descriptor

Dipto.. 5

不是专家,但似乎以下问题是:

((close(fd[1]) == -1) || (close(in_fd) == - 1))

尽量不要在in_fd那里关闭.

我认为,父母试图关闭由孩子关闭的同一个fd.
使用时,dup2()您不需要关闭文件,因为dup2()已经关闭文件.