I am not understanding how the pipe works in UNIX, I did this code and I stumbled in a strange fact. The trace of the exercise can be found on the top of the code. Ill'explain what I can't get. In the father process when I want to print the values from the pipe, the value from where the "i" variable begins can be whatever number. I put "4" but it works for every number 2, 3 ,4 excetera. How can it work every time?

/*****************************************************************
    The candidate should complete the program provided, implementing
    the main.
    The program creates a child process; the child process reads
    from the keyboard an integer N >= 0, and transmits the values N, N-1, N-2, N-3, ..., 0
    (inclusive) to the parent process via a pipe.
    The father process reads from the pipe the values transmitted by the child process
    and prints them, until it receives the value 0; then the father process
    process waits for the termination of the child process and terminates.
    
    Example:
    I am the child process. Enter a number >=0: 4
    I am the father process. I have received: 4
    I am the father process. I have received: 3
    I am the father process. I have received: 2
    I am the father process. I have received: 1
    I am the father process. I have received: 0
    I am the father process. The son has finished.
    
******************************************************************/
    
    
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <unistd.h>
#include <sys/wait.h>
    
int main(int argc, char *argv[]) 
{
    
    int fd[2];
    int num, i;
    //Pipe creation
    if(pipe(fd)<0)
    {
        printf("Pipe creation failed\n");
        return 1;
    }
    //Creating a child process
    pid_t pid=fork();
    //Fork check
    if(pid<0)
    {
        printf("Fork failed\n");
        return 1;
    }
    //Entering the child process
    else if(pid==0){
        close (fd[0]);  // Not interested in reading
        printf("I am the child process\n");
        //Acquiring a number from input
        printf("Give me a number: ");
        scanf("%d", &num);
        //Sending the numbers trough a pipe
        for(i=num; i>=0; i--)
        {
            int sent=write(fd[1], &i, sizeof(num));
            //Check on the number of bytes the function wrote
            if(sent<0 || sent<sizeof(num))
            {
                printf("Error when sending\n");
                return 1;
            }
        }
        close (fd[1]);
        return 0;
    }
    //Entering the father process
    if(pid>0)
    {
        //Father process
        wait(NULL);
        close (fd[1]);// Not interested in writing
        for(i=4;i>=0;i--)//4 is a random number and it still works
        {
            int ricevuti=read(fd[0], &i, sizeof(num));
            //Check on the number of bytes the function read
            if(ricevuti<0 || ricevuti<(sizeof(num)))
            {
                printf("Error when receiving\n");
                return 1;
            }
            //Printing the values read by the function
            else
            {
                printf("I am the father process and i received: %d\n", i);
            }
        }
        printf("The child process has terminated\n");
    }
    close (fd[0]);
    return 0;
}

CodePudding user response：

The reason it works for any number is because you're reading from the pipe into the iteration variable used by the for loop. So even though you started it at i = 4, the first read(fd[0], &i, sizeof(num)) will change i to the starting number that the child sent.

You should be reading into num, not i. The father code should be:

    //Entering the father process
    if(pid>0)
    {
        //Father process
        wait(NULL);
        close (fd[1]);// Not interested in writing
        for(i=4;i>=0;i--)//4 is a random number and it still works
        {
            int ricevuti=read(fd[0], &num, sizeof(num));
            //Check on the number of bytes the function read
            if(ricevuti<0 || ricevuti<(sizeof(num)))
            {
                printf("Error when receiving\n");
                return 1;
            }
            //Printing the values read by the function
            else
            {
                printf("I am the father process and i received: %d\n", num);
            }
        }
        printf("The child process has terminated\n");
    }