Learning Pipes and Processes

Question

I'm trying to get a better understanding of pipes and processes. I want to implement multiple chained pipes like cat test.txt | sort | uniq -c. I started my code with the cat test.txt, but it isn't working. It compiles, but when I provide a file name in the command line, for example, ./hwk ./test.txt. Nothing returns. Can someone take a look and give me some hints? I want to use loops because I want to be able to add more pipes. I know there's a lot of issues in my code, so I hope someone can give me some guidance on this topic. Thanks.

#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>

#define SIZE 1024

int main (int argc, char **argv)
{
    int num_pipe = 1;
    int commands = num_pipe + 1; //number of commands is one more than the number of pipes
    int fds[num_pipe * 2];

    int status;
    pid_t pid;
    char *str_ptr;

    //Pass Command
    char *arrayOfCommands[] = {"cat", NULL};


    //Setting up pipes
    int i;
    for (i = 0; i < num_pipe; i++){
        if(pipe(fds + i * 2) == -1) {
            perror("Error creating pipes");
            exit(1);
        }
    }

    int j = 0;
    for (i = 0; i < commands - 1; ++i) {
        pid = fork();

        if (pid == 0) {
            if (i < commands) {
                if (dup2(fds[j+1], 1) < 0) {
                    perror("dup2 error");
                    exit(EXIT_FAILURE);
                }
            }

            if (j != 0) {
                if(dup2(fds[j-2], 0) < 0) {
                    perror("dup2 error");
                    exit(EXIT_FAILURE);
                }
            }

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


            if (execvp(arrayOfCommands[0], arrayOfCommands) < 0) {
                perror("Array error");
                exit(EXIT_FAILURE);
            }


        }
        else if (pid < 0){
            perror("Error");
            exit(EXIT_FAILURE);
        }

        j += 2;
    }

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

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


    return 0;
}

Answer 1

I called this mainly minor adaptation of your program p3.c, compiling it to produce p3. Since there's only one command (cat) being invoked, I juggled things so that it will work correctly. When run as ./p3 p3.c, it prints out the content of the source code.

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>

static void err_exit(const char *str);

int main (int argc, char **argv)
{
    int num_pipe = 0;            // Just cat - no pipes
    int commands = num_pipe + 1; // Number of commands is one more than the number of pipes
    int fds[num_pipe * 2 + 1];   // Avoid size 0 array
    char *arrayOfCommands[3] = { "cat", NULL, NULL};

    if (argc != 2)
        err_exit("Missing filename argument");
    arrayOfCommands[1] = argv[1];

    for (int i = 0; i < num_pipe; i++)
    {
        if (pipe(fds + i * 2) == -1)
            err_exit("Error creating pipes");
    }

    int j = 0;
    for (int i = 0; i < commands; ++i)
    {
        pid_t pid = fork();

        if (pid == 0)
        {
            printf("%d: %s %s\n", (int)getpid(), arrayOfCommands[0], arrayOfCommands[1]);
            fflush(stdout);
            if (i < commands-1 && dup2(fds[j+1], 1) < 0)
                err_exit("dup2 error");
            if (j != 0 && dup2(fds[j-2], 0) < 0)
                err_exit("dup2 error");
            for (i = 0; i < 2*num_pipe; i++)
                close(fds[i]);

            execvp(arrayOfCommands[0], arrayOfCommands);
            err_exit("Array error");
        }
        else if (pid < 0)
            err_exit("Error");

        j += 2;
    }

    for (int i = 0; i < 2 * num_pipe; i++)
        close(fds[i]);

    for (int i = 0; i < num_pipe + 1; i++)
    {
        int status;
        pid_t pid = wait(&status);
        printf("PID %d exited 0x%.4X\n", (int)pid, status);
    }

    return 0;
}

static void err_exit(const char *str)
{
    perror(str);
    exit(EXIT_FAILURE);
}

Check that works for you. Then you'll need to work out how you're going to create a second command. Your arrayOfCommands isn't going to help directly. You'll need another array of strings in some shape or form.

---

An extension to run cat file | rev. The changes are really quite minor. I created a_cat to handle the cat command, a_rev for the rev command, and a_cmds as the array of commands. It was also necessary to fix a loop on i to a loop on k.

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>

static void err_exit(const char *str);

int main (int argc, char **argv)
{
    int num_pipe = 1;
    int commands = num_pipe + 1; //number of commands is one more than the number of pipes
    int fds[num_pipe * 2 + 1];   // Avoid size 0 array
    char *a_cat[3] = { "cat", NULL, NULL};
    char *a_rev[2] = { "rev", NULL};
    char **a_cmds[] = { a_cat, a_rev };

    if (argc != 2)
        err_exit("Missing filename argument");
    a_cat[1] = argv[1];

    for (int i = 0; i < num_pipe; i++)
    {
        if (pipe(fds + i * 2) == -1)
            err_exit("Error creating pipes");
    }

    int j = 0;
    for (int i = 0; i < commands; ++i)
    {
        pid_t pid = fork();

        if (pid == 0)
        {
            printf("%d: %s\n", (int)getpid(), a_cmds[i][0]);
            fflush(stdout);
            if (i < commands-1 && dup2(fds[j+1], 1) < 0)
                err_exit("dup2 error");
            if (j != 0 && dup2(fds[j-2], 0) < 0)
                err_exit("dup2 error");
            for (int k = 0; k < 2*num_pipe; k++)
                close(fds[k]);

            execvp(a_cmds[i][0], a_cmds[i]);
            err_exit("Array error");
        }
        else if (pid < 0)
            err_exit("Error");

        j += 2;
    }

    for (int i = 0; i < 2 * num_pipe; i++)
        close(fds[i]);

    for (int i = 0; i < num_pipe + 1; i++)
    {
        int status;
        pid_t pid = wait(&status);
        printf("PID %d exited 0x%.4X\n", (int)pid, status);
    }

    return 0;
}

static void err_exit(const char *str)
{
    perror(str);
    exit(EXIT_FAILURE);
}