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Question

我正在尝试在C中实现一个shell。 参考https://stackoverflow.com/questions/12128185/implementing-multiple-pipes-for-linux-unix-shell， 我设计了我的代码：

void piped(char *** tokens_basic, int nPipes)
{
int pipefds[2 * nPipes];

for(int i=0; i<nPipes; i++)
    {
    if(pipe(pipefds + i*2) < 0) 
    {
        perror("Error in piping");
                exit(EXIT_FAILURE);
    }
    }

    pid_t pid;
int status;
int i=0, j=0, k;

while(tokens_basic[i] != NULL)
{
    pid = fork();

    if(pid == 0)
    {
        if(tokens_basic[i+1] != NULL)
        {
            if(dup2(pipefds[j + 1], 1) < 0)
                {
                    perror("dup2 in j+1");
                    exit(EXIT_FAILURE);
                    }
        }
        if(j != 0)
        {
            if(dup2(pipefds[j-2], 0) < 0)
            {
                perror("dup2");
                exit(EXIT_FAILURE);
            }
        }

        for(k = 0; k < 2*nPipes; k++)
        {
            close(pipefds[k]);
        }

        if(execvp(tokens_basic[i][0], tokens_basic[i]) < 0)
                {
                    perror("Error in child execvp");
                    exit(EXIT_FAILURE);
                }
    }
    else if(pid < 0)
    {
        perror("Error in pipe fork.");
        exit(EXIT_FAILURE);
    }
    else
    {
        i++;
        j += 2;
    }
}

for(i = 0; i < 2 * nPipes; i++)
    {
        close(pipefds[i]);
}

for(i = 0; i < nPipes + 1; i++)
    wait(&status);
}

在此我得到以下输出：

dup2 in j+1: Bad file descriptor
dup2: Bad file descriptor
dup2: Bad file descriptor
dup2 in j+1: Bad file descriptor
dup2: Bad file descriptor
dup2 in j+1: Bad file descriptor
dup2 in j+1: Bad file descriptor
dup2: Bad file descriptor

Error in child execvp: No such file or directory
Error in child execvp: No such file or directory
Error in child execvp: No such file or directory
Error in child execvp: No such file or directory