C program fork() results repeating

Question

I am learning about fork() and pipes by writing a simple program. I have it returning results successfully but one of the results repeats itself. I have been experimenting with the code to figure it out but can't manage to get it right.

The sleep times are just there to simulate system adjustment. I have been trying to get the navigation sleep time to be random number of seconds between 0-6 inclusive but I can't manage to get it to do that and print out the random number generated to the report. That is an issue I am willing to live with but would like if it worked.

Current code:

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

#define MAX 150
#define PipeStdIn 0             //UNIX StdIn
#define PipeStdOut 1            //UNIX StdOut

void lifeSupport();
void navigation();

int main()
{
    const char *message = {"Calibrate Systems\n"};
    int pipes[2], ret;
    char buf[MAX + 1];


    if(pipe(pipes) == 0)
    {
        if(fork() == 0)
        {
            ret = read(pipes[PipeStdIn], buf, MAX);
            printf("Life Support receives instruction: %s\n", buf);

            sleep(5);

            sleep(4);

            const char *breathGL = {"Breathing gas levels have been adjusted\nAdjustment time: 5 seconds\nLighting and temperature levels have been adjusted\nAdjustment time: 4 seconds\n"};

            ret = write(pipes[PipeStdOut], breathGL, strlen(breathGL) + 1);
        }
        else
        {       
            ret = write(pipes[PipeStdOut], message, strlen(message) + 1);                   //write

            ret = wait(NULL);

            ret = read(pipes[PipeStdIn], buf, MAX);

            time_t now;
            time(&now);
            printf("Report received: %s\n", buf);
            printf("Report time: %s\n", ctime(&now));
        }
    }

    if(pipe(pipes) == 0)
    {
        if(fork() == 0)
        {
            ret = read(pipes[PipeStdIn], buf, MAX);
            printf("Navigation receives instruction: %s\n", buf);

            sleep(3);

            const char *nav = {"Navigation system has been adjusted\nAdjustment time: 3 seconds\n"};

            ret = write(pipes[PipeStdOut], nav, strlen(nav) + 1);
        }
        else
        {       
            ret = write(pipes[PipeStdOut], message, strlen(message) + 1);                   //write

            ret = wait(NULL);

            ret = read(pipes[PipeStdIn], buf, MAX);

            time_t now;
            time(&now);
            printf("Report received: %s\n", buf);
            printf("Report time: %s\n", ctime(&now));
        }       
    }

    close(pipes[PipeStdIn]);
    close(pipes[PipeStdOut]);

    return 0;
}

Expected results:

Life Support receives instruction: Calibrate Systems

Report received: Breathing gas levels have been adjusted
Adjustment time: 5 seconds
Lighting and temperature levels have been adjusted
Adjustment time: 4 seconds

Report time: Tue Apr 28 08:48:49 2020

Navigation receives instruction: Calibrate Systems

Report received: Navigation system has been adjusted
Adjustment time: 3 seconds

Report time: Tue Apr 28 08:48:52 2020

Current results:

Life Support receives instruction: Calibrate Systems

Navigation receives instruction: Calibrate Systems

Report received: Navigation system has been adjusted
Adjustment time: 3 seconds

Report time: Tue Apr 28 08:48:49 2020

Report received: Breathing gas levels have been adjusted
Adjustment time: 5 seconds
Lighting and temperture levels have been adjusted
Adjustment time: 4 seconds

Report time: Tue Apr 28 08:48:49 2020

Navigation receives instruction: Calibrate Systems

Report received: Navigation system has been adjusted
Adjustment time: 3 seconds

Report time: Tue Apr 28 08:48:52 2020

Answer 1

If the explanation wasn't clear from the comment, you need to move the complete second if(fork() == 0) {...} else {...} inside the first else to prevent both parent and child from executing that block. After you move the if(fork() == 0) {...} else {...} up, your logic would look like:

    if(pipe(pipes) == 0)
    {
        if(fork() == 0)
        {
            ret = read(pipes[PipeStdIn], buf, MAX);
            ...
            ret = write(pipes[PipeStdOut], breathGL, strlen(breathGL) + 1);
        }
        else
        {       
            ret = write(pipes[PipeStdOut], message, strlen(message) + 1);
            ...

            if(fork() == 0)
            {
                ret = read(pipes[PipeStdIn], buf, MAX);
                ...
                const char *nav = "Navigation system has been adjusted\n"
                                    "Adjustment time: 3 seconds\n";

                ret = write(pipes[PipeStdOut], nav, strlen(nav) + 1);
            }
            else
            {       
                ret = write(pipes[PipeStdOut], message, strlen(message) + 1);
                ...
                // time_t now;
                time(&now);
                printf("Report received: %s\n", buf);
                printf("Report time: %s\n", ctime(&now));
            }       
        }
    }

    close(pipes[PipeStdIn]);
    close(pipes[PipeStdOut]);

Also note, the C compile will concatenate adjacent strings into a single during compile allowing you to break up long lines into a series of double-quoted strings that will be joined at compile time, e.g.

        const char *breathGL = "Breathing gas levels have been adjusted\n"
                        "Adjustment time: 5 seconds\n"
                        "Lighting and temperature levels have been adjusted\n"
                        "Adjustment time: 4 seconds\n";

and

            const char *nav = "Navigation system has been adjusted\n"
                                "Adjustment time: 3 seconds\n";

(also note enclosing {...} are not required)

Look things over and let me know if you have further questions.