用叉子和孩子创造的管道

Question

This is my code for a problem that I am trying to solve. If anyone one have any idea how to solve this please help. I am trying to create infinity children with as many pipe as the child's that we will create, but I am losing some children with their pipe and I don't know why.

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <sys/wait.h>
void main() {
    int n;
    pid_t p;
    int pfd[2];
    int val=0;
    int i;

    printf("Enter the number of child you want:");
    scanf("%d",&n);

    pipe(pfd);

    while(1) {
        for(i=1;i<=n;i++) {

            p=fork();
            if(p>0){
                close(pfd[0]);
                sleep(5);
                val++;
                printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getppid(),getpid());
                write(pfd[0],&val,sizeof(val));
                close(pfd[1]);
                //wait(NULL);
                //exit(EXIT_SUCCESS);
            }
            else {
                close(pfd[1]);
                val++;
                //printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getpid(),getppid());
                //while(read(pfd[0],&val,sizeof(val))>0)
                read(pfd[0],&val,sizeof(val));
                //printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getpid(),getppid());
                write(pfd[0],&val,sizeof(val));
                close(pfd[0]);
                _exit(EXIT_SUCCESS);
            }
        }
    }
    printf("Message %d: last child's pid is %d and my father is : %d \n", n,getpid(),getppid());    
} 

OUTPUT IS: Enter the number of child you want: 3

Message 1: Process 1 has process id 678 and his child's id is : 15041

Message 2: Process 2 has process id 678 and his child's id is : 15041

Message 3: Process 3 has process id 678 and his child's id is : 15041

Message 4: Process 1 has process id 678 and his child's id is : 15041

Message 5: Process 2 has process id 678 and his child's id is : 15041

Message 6: Process 3 has process id 678 and his child's id is : 15041

Message 7: Process 1 has process id 678 and his child's id is : 15041

Message 8: Process 2 has process id 678 and his child's id is : 15041

Message 9: Process 3 has process id 678 and his child's id is : 15041

Message 10: Process 1 has process id 678 and his child's id is : 15041

Message 11: Process 2 has process id 678 and his child's id is : 15041

Message 12: Process 3 has process id 678 and his child's id is : 15041

So my problem is that the process id's are not changing and I can't find why

Answer 1

So my problem is that the process id's are not changing and I can't find why

because

printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getppid(),getpid());

is executed by the parent, so getppid (the pid of the parent of the parent) is always the same, getpid also remains the same. The only thing that changes are val and `i'.

You only create a pipe once and then use it for all forks, meanwhile you close the only created pipe and then fork again, this is not going to end well.

You have to create the pipes before you do the fork. Also you are closing the reading end of the pipe on the parent process, and the writing to the close reading end. You have to write on the writing end.

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

int main(void)
{
    pid_t p;
    int pfd[2];
    int i;
    int val = 0;
    int n = 3;

    for(i=1;i<=n;i++) {
        if(pipe(pfd) < 0)
        {
            perror("pipe");
            continue;
        }

        p = fork();

        if(p < 0)
        {
            perror("fork");
            continue;
        }

        if(p)
        {
            // PARENT PROC
            close(pfd[0]);
            val++;
            printf("Message %d: Parent process has process id %d and its child has process id %d\n", val, getpid(), p);
            write(pfd[1], &val, sizeof val);
            close(pfd[1]);
            int status;
            waitpid(p, &status, 0);

            if(WIFEXITED(status))
                printf("Child %d exit status: %d\n", i, WEXITSTATUS(status));
            else
                printf("Child %d did not exit normally\n", i);
        } else {
            // CHILD PROC

            close(pfd[1]);
            if(read(pfd[0], &val, sizeof val) < 0)
            {
                printf("Child %d: could not read from pipe\n", i);
                _exit(EXIT_FAILURE);
            }

            printf("Child %d, message: %d. Child pid: %d, parent pid: %d\n", i, val, getpid(), getppid());

            close(pfd[0]);
            _exit(EXIT_SUCCESS);
        }
    }

    return 0;
}

This prints

Message 1: Parent process has process id 9829 and its child has process id 9830
Child 1, message: 1. Child pid: 9830, parent pid: 9829
Child 1 exit status: 0
Message 2: Parent process has process id 9829 and its child has process id 9831
Child 2, message: 2. Child pid: 9831, parent pid: 9829
Child 2 exit status: 0
Message 3: Parent process has process id 9829 and its child has process id 9832
Child 3, message: 3. Child pid: 9832, parent pid: 9829
Child 3 exit status: 0

One last thing: the correct prototypes for main are:

• int main(void);
• int main(int argc, char *argv[]);
• int main(int argc, char **argv);

Your void main() is incorrect.

Answer 2

When p>0 , that means you are in the parent process, not the child, and p is the pid of the child.

this

printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getppid(),getpid());

should be

printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getpid(),p)

and if you want the child to display a similar message in the else block your first statement works, but just change the word child to parent.