Thread: Question-please help me

Now that that's sorted, on to the OP's original question.
Here's one way to control the processes.
proc3 below just reports how many words it sees and, to test the STOP command, stops when it sees a list of more than 80 words.

I don't fully understand what proc3 is supposed to do in the question. It's apparently supposed to start up a bunch of threads (not processes), one thread for each pair of words that are the same length. That sounds like it could be quite a few threads.

Code:

// procmain.c  /////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/types.h>

void forkproc(const char *proc, int pin, int pout, const char *arg) {
    pid_t pid;
    if ((pid = fork()) == (pid_t)-1) {
        fprintf(stderr, "Error: fork %s: %s\n", proc, strerror(errno));
        exit(EXIT_FAILURE);
    }
    if (pid == 0) {
        dup2(pin, 0);
        dup2(pout, 1);
        execl(proc, proc, arg, (const char *)NULL);
        fprintf(stderr, "Error: exec %s: %s\n", proc, strerror(errno));
        exit(EXIT_FAILURE);
    }
}

int main(int argc, char **argv) {
    if (argc != 2) {
        fprintf(stderr, "Usage: procmain MAXREPS\n");
        exit(EXIT_FAILURE);
    }

    int p[3][2];
    if (pipe(p[0]) == -1 || pipe(p[1]) == -1 || pipe(p[2]) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    forkproc("proc1", p[0][0], p[1][1], argv[1]);
    forkproc("proc2", p[1][0], p[2][1], NULL);
    forkproc("proc3", p[2][0], p[0][1], NULL);

    for (int i = 0; i < 3; i++) {
        close(p[i][0]); close(p[i][1]);
    }

    wait(NULL); wait(NULL); wait(NULL);
    return 0;
}


// proc1.c  /////////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

int main(int argc, char **argv) {
    if (argc != 2)
        return EXIT_FAILURE;
    int maxreps = atoi(argv[1]);

    char line[1000];
    srand(time(NULL));

    for (int r = 0; r < maxreps; r++) {
        int n = rand() % 90 + 10;
        for (int i = 0; i < n; i++)
            printf("%d ", rand() % 5 + 5);
        putchar('\n');
        fflush(stdout);

        if (fgets(line, sizeof line, stdin) == NULL)
            break;
        // Break if told to STOP by proc3
        if (strcmp(line, "STOP\n") == 0)
            break;
    }

    // Send EXIT command to proc2
    printf("EXIT\n"); fflush(stdout);

    return 0;
}


// proc2.c  /////////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

int main() {
    char line[1000];
    srand(time(NULL));

    while (fgets(line, sizeof line, stdin) != NULL) {
        if (strcmp(line, "EXIT\n") == 0) {
            // Pass EXIT command on to proc3
            printf("EXIT\n"); fflush(stdout);
            break;
        }

        char *pline = line;
        int n = 0, delta = 0, word_cnt = 0;
        while (sscanf(pline += delta, "%d%n", &n, &delta) == 1) {
            for (int i = 0; i < n; i++)
                putchar('a' + rand() % 26);
            putchar('\n');
            fflush(stdout);
            word_cnt++;
        }
        // Indicate end of word list.
        printf("LAST\n"); fflush(stdout);
    }

    return 0;
}


// proc3.c  /////////////////////////////////////////////////////
#include <stdio.h>
#include <string.h>

int main() {
    char line[1000];
    int word_cnt = 0;

    while (fgets(line, sizeof line, stdin) != NULL) {
        if (strcmp(line, "EXIT\n") == 0)
            break;

        if (strcmp(line, "LAST\n") == 0) {

            fprintf(stderr, "proc3 saw %d words\n", word_cnt);

            // Arbitrarily stopping when proc3 sees a word
            // list more than 80 words long
            if (word_cnt > 80) {
                // Send STOP command to proc1
                printf("STOP\n"); fflush(stdout);
                break;
            }
            else {
                // Tell proc1 to continue
                printf("\n"); fflush(stdout);
            }
            word_cnt = 0;
        }
        else
            word_cnt++;
    }

    return 0;
}