Forking and piping processes in c

Question

So I have a project to do but I am completely stumped. I have spent ten hours and have gotten nowhere. I don't specifically want the code to the answer, but some pseudocode and good hints in the right direction would help a heap!!

It forks a number of processes, k - a command-line argument, connected by pipes - each process is connected to the next, and the last process is connected to the first. Process number k sends its message on to process number (k+1)%n.

Process 0 reads a line from stdin. It then transmits it to process 1. Each other process reads the line, increments the first byte of the string by 1, and then relays the line to the next process. As it relays, it prints a status message (shown below).

When the message gets back to process 0, it is output to the standard output as well. When a process receives EOF (either from pipe, if its a process other than 0, or from stdin, for process 0), it prints the final string. This will close all pipes.

The expected output is:

$ ./ring 4
hello
process #0 (32768) sending message: hello
process #1 (32769) relaying message: iello
process #2 (32770) relaying message: jello
process #3 (32767) relaying message: kello
I hear kello
^C
$

What I have written so far:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define BUFFER_SIZE 80
#define READ_END 0
#define WRITE_END 1

int main(int argc, char *argv[])
{
    char readmsg[BUFFER_SIZE], readmsg2[BUFFER_SIZE], final[BUFFER_SIZE];
    int pid, process;
    int parent_child[2], child_parent[2];
    process = 0;

    if (pipe(child_parent) == -1) {
        fprintf(stderr, "Pipe failed");
        return 1;
    }
    if (pipe(parent_child) == -1) {
        fprintf(stderr, "Pipe failed");
        return 1;   
    }

    pid = fork();
    if (pid < 0) {
        fprintf(stderr, "Fork failed");
        return 1;
    } else if (pid > 0) {
        /* PARENT */
        read(0, &readmsg, BUFFER_SIZE);
        printf("process #%d (%d) sending message: %s", 
            0, getpid(), readmsg);
        write(parent_child[WRITE_END], &readmsg, BUFFER_SIZE);
        close(parent_child[WRITE_END]);
    } else {
        /* CHILD */
        read(parent_child[READ_END], &readmsg2, BUFFER_SIZE);
        readmsg2[0] += 1;
        printf("process #%d (%d) relaying message: %s", 
            1, getpid(), readmsg2);
        process += 1;
        write(child_parent[WRITE_END], &readmsg2, BUFFER_SIZE);
    }

    read(child_parent[READ_END], &final, BUFFER_SIZE);
    printf("I hear %d %s", pid - getpid(), final);

    return 0;
}

What it does currently is read in a string from stdin, pass it to the first process and print process 0 (can't actually get the 0 though, simply printing 0), it then pipes the string to process 1 which distorts byte 1 and then writes to a pipe again and then outside of the pipes, the string is read and outputs the distorted string.

$ ./ring
hello
process #0 (6677) sending message: hello
process #1 (6678) relaying message: iello
I hear -6678 iello
^C
$ 

I don't know where to go from here. Thank you in advance, anything will help!!

Given some help this is what I have now:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define BUFFER_SIZE 80
#define READ_END 0
#define WRITE_END 1

int main(int argc, char **argv) {
    char buf[BUFFER_SIZE];
    int process, rings, pid, pid_n, pid_n1, pid_1, i;
    int Pn[2];   //Pipe for process n   -> 0
    int Pn_1[2]; //Pipe for process n-1 -> n
    int Pn_2[2]; //Pipe for process n-2 -> n-1
    int P_0[2];  //Pipe for process 0   -> 1
    process = 0;

    if (argc == 2) {
        rings = atoi(argv[1]);
    } else {
        fprintf(stderr, "Usage: %s n, where n is number of rings\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if ((pid = fork()) < 0) {
        fprintf(stderr, "Fork failed");
        return 1;
    } else if (pid == 0) {
        if ((pid_n = fork()) < 0) {
            fprintf(stderr, "Fork failed");
            return 1;
        } else if (pid_n == 0) {
            /* CHILD */
            close(Pn[WRITE_END]);
            close(Pn_1[READ_END]);
        } else {
            /* PARENT */
            close(Pn[READ_END]);
            close(Pn_1[WRITE_END]);
        }
        for (i = 0; i < rings; i++) {
            if ((pid_n1 = fork()) < 0) {
                fprintf(stderr, "Fork failed");
                return 1;
            } else if (pid_n1 == 0) {
                /* CHILD */
                close(Pn_1[WRITE_END]);
                close(Pn_2[READ_END]);
            } else {
                /* PARENT */
                close(Pn_1[READ_END]);
                close(Pn_2[WRITE_END]);
            }
        }

        /* Not sure about these last ones */
        if ((pid_1 = fork()) < 0) {
            fprintf(stderr, "Fork failed");
            return 1;
        } else if (pid_1 == 0) {
            /* CHILD */
            close(P_n2[WRITE_END]);
            close(P_0[READ_END]);

        } else {
            /* PARENT */
            close(P_n2[READ_END]);
            close(P_0[WRITE_END]);
        }
    } else {
        /* PARENT */
        read(0, &buf, BUFFER_SIZE);
        buf[BUFFER_SIZE - 1] = '\0';
        printf("process first # (%d) sending message: %s", getpid(), buf);
        write(P_0[WRITE_END], &buf, BUFFER_SIZE);
        read(Pn[READ_END], &buf, BUFFER_SIZE);
        buf[BUFFER_SIZE - 1] = '\0';
        printf("I hear %s", buf);
    }

    return 0;
}

Answer 1

This is a diagram I drew for myself showing how the processes are to be interconnected:

                  p4
           C5 <--------- C4
          /               \
     p5  /              p3 \
        /                   \
o----> C0 ---->o            C3
        \                   /
     p0  \              p2 /
          \               /
           C1 ---------> C2
                  p1

The Cn represent the processes; C0 is the parent process. The pn represent the pipes; the other two lines are standard input and standard output. Each child has a simple task, as befits children. The parent has a more complex task, mainly ensuring that exactly the right number of file descriptors are closed. In fact, the close() is so important that I created a debugging function, fd_close(), to conditionally report on file descriptors being closed. I used that too when I had silly mistakes in the code.

The err_*() functions are simplified versions of code I use in most of my programs. They make error reporting less onerous by converting most error reports into a one-line statement, rather than requiring multiple lines. (These functions are normally in 'stderr.c' and 'stderr.h', but those files are 750 lines of code and comment and are more comprehensive. The production code has an option to support prefixing each message with a PID, which is also important with multi-process systems like this.)

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

enum { BUFFER_SIZE = 1024 };

typedef int Pipe[2];

static int debug = 0;
static void fd_close(int fd);

/* These functions normally declared in stderr.h */
static void err_setarg0(const char *argv0);
static void err_sysexit(char const *fmt, ...);
static void err_usage(char const *usestr);
static void err_remark(char const *fmt, ...);

static void be_childish(Pipe in, Pipe out)
{
    /* Close irrelevant ends of relevant pipes */
    fd_close(in[1]);
    fd_close(out[0]);
    char buffer[BUFFER_SIZE];
    ssize_t nbytes;
    while ((nbytes = read(in[0], buffer, sizeof(buffer))) > 0)
    {
        buffer[0]++;
        if (write(out[1], buffer, nbytes) != nbytes)
            err_sysexit("%d: failed to write to pipe", (int)getpid());
    }
    fd_close(in[0]);
    fd_close(out[1]);
    exit(0);
}

int main(int argc, char **argv)
{
    err_setarg0(argv[0]);

    int nkids;
    if (argc != 2 || (nkids = atoi(argv[1])) <= 1 || nkids >= 10)
        err_usage("n   # for n in 2..9");

    err_remark("Parent  has PID %d\n", (int)getpid());

    Pipe pipelist[nkids];
    if (pipe(pipelist[0]) != 0)
        err_sysexit("Failed to create pipe #%d", 0);
    if (debug)
        err_remark("p[0][0] = %d; p[0][1] = %d\n", pipelist[0][0], pipelist[0][1]);

    for (int i = 1; i < nkids; i++)
    {
        pid_t pid;
        if (pipe(pipelist[i]) != 0)
            err_sysexit("Failed to create pipe #%d", i);
        if (debug)
            err_remark("p[%d][0] = %d; p[%d][1] = %d\n", i, pipelist[i][0], i, pipelist[i][1]);
        if ((pid = fork()) < 0)
            err_sysexit("Failed to create child #%d", i);
        if (pid == 0)
        {
            /* Close irrelevant pipes */
            for (int j = 0; j < i-1; j++)
            {
                fd_close(pipelist[j][0]);
                fd_close(pipelist[j][1]);
            }
            be_childish(pipelist[i-1], pipelist[i]);
            /* NOTREACHED */
        }
        err_remark("Child %d has PID %d\n", i, (int)pid);
    }

    /* Close irrelevant pipes */
    for (int j = 1; j < nkids-1; j++)
    {
        fd_close(pipelist[j][0]);
        fd_close(pipelist[j][1]);
    }

    /* Close irrelevant ends of relevant pipes */
    fd_close(pipelist[0][0]);
    fd_close(pipelist[nkids-1][1]);

    int w_fd = pipelist[0][1];
    int r_fd = pipelist[nkids-1][0];

    /* Main loop */
    char buffer[BUFFER_SIZE];

    while (printf("Input:  ") > 0 && fgets(buffer, sizeof(buffer), stdin) != 0)
    {
        int len = strlen(buffer);
        if (write(w_fd, buffer, len) != len)
            err_sysexit("Failed to write to children");
        if (read(r_fd, buffer, len) != len)
            err_sysexit("Failed to read from children");
        printf("Output: %.*s", len, buffer);
    }
    fd_close(w_fd);
    fd_close(r_fd);
    putchar('\n');

    int status;
    int corpse;
    while ((corpse = wait(&status)) > 0)
        err_remark("%d exited with status 0x%.4X\n", corpse, status);

    return 0;
}

static void fd_close(int fd)
{
    if (debug)
        err_remark("%d: close(%d)\n", (int)getpid(), fd);
    if (close(fd) != 0)
        err_sysexit("%d: Failed to close %d\n", (int)getpid(), fd);
}

/* Normally in stderr.c */
static const char *arg0 = "<undefined>";

static void err_setarg0(const char *argv0)
{
    arg0 = argv0;
}

static void err_usage(char const *usestr)
{
    fprintf(stderr, "Usage: %s %s\n", arg0, usestr);
    exit(1);
}

static void err_vsyswarn(char const *fmt, va_list args)
{
    int errnum = errno;
    fprintf(stderr, "%s:%d: ", arg0, (int)getpid());
    vfprintf(stderr, fmt, args);
    if (errnum != 0)
        fprintf(stderr, " (%d: %s)", errnum, strerror(errnum));
    putc('\n', stderr);
}

static void err_sysexit(char const *fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    err_vsyswarn(fmt, args);
    va_end(args);
    exit(1);
}

static void err_remark(char const *fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    vfprintf(stderr, fmt, args);
    va_end(args);
}

Example output:

$  ./pipecircle 9
Parent  has PID 34473
Child 1 has PID 34474
Child 2 has PID 34475
Child 3 has PID 34476
Child 4 has PID 34477
Child 5 has PID 34478
Child 6 has PID 34479
Child 7 has PID 34480
Child 8 has PID 34481
Input:  Hello
Output: Pello
Input:  Bye
Output: Jye
Input:  ^D
34474 exited with status 0x0000
34477 exited with status 0x0000
34479 exited with status 0x0000
34476 exited with status 0x0000
34475 exited with status 0x0000
34478 exited with status 0x0000
34480 exited with status 0x0000
34481 exited with status 0x0000
$

Answer 2

It seems to me that you are pretty close, as this works for two processes. What you need to do now is loop to create more processes from the parent.

(k=N+1 processes: proc0 = parent, proc1, ..., procN)
Create a pipe Pn, that will be for procN->proc0
Create a pipe Pn-1, that will be for procN-1->procN
Create relaying fork procN
    fork closes Pn output and Pn-1 input
    parent closes Pn input and Pn-1 output
(loop here)
Create a pipe Pi-2, that will be for procI-2->procI-1
Create relaying fork procI-1
    fork closes Pi-1 output and Pi-2 input
    parent closes Pi-1 input and Pi-2 output
...
Create a pipe P0 that will be for proc0->proc1
Create relaying fork proc1
    fork closes P1 output and P0 input
    parent closes P1 input and P0 output
(end loop)

(parent final code:)
Read from stdin
Write on P0
Read on Pn
Write on stdout

Once created with fork(), the child processes (i.e. apart from proc0) close the input of the pipe (the output of the other is already closed!), read the message on one, write on the other and exit.

Some remarks on your current code:

• The child shouldn't execute that list bit, when you read on child_parent.
• You don't need that many buffers (you only need one, that will turn into one per process after the fork).
• Put some terminating null bytes before printing :)
• It's good practice to close the ends that you're not going to need