CODEWITHSUNDEEP
cforkpipeparent-child
R0drom
R0drom

Reputation: 51

Child reading from std input and writes to std output

I have a program where the child runs a program but the parent process passes the child a number and the child writes back to the parent a response. However whenever I run the code, it does not give me anything back, so I must be passing or receiving to the child wrong, but I'm not sure how. Any help is appreciated. Here is my code:

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


int main() {

int pid;
int n;
char buf[101];
int pfdA[2];
int pfdB[2];

// CREATES FIRST PIPE
if (pipe(pfdA) == -1) {
    perror("pipe failed");
    exit(-1);
}

// CREATES SECOND PIPE
if (pipe(pfdB) == -1) {
    perror("pipe failed");
    exit(-1);
}   

// FORK()
if ((pid == fork()) < 0) {
    perror("fork failed");
    exit(-2);
}


    if (pid == 0 ) {
    // duplicate file descriptor 0 to point to FIRST pipe
    dup(pfdA[0]);

    // CLOSES ends of FIRST pipe you don't need anymore
    close(pfdA[0]);
    close(pfdA[1]);

    // duplicates file descriptor 1 to point to SECOND pipe
    dup(pfdA[1]);


    // CLOSES ends of SECOND pipe you don't need anymore
    close(pfdB[0]);
    close(pfdB[1]);

            execlp("./A5_CHILD", "./A5_CHILD", (char *) 0);
            perror("execlp");
            exit(-3);
    } 

else {

    while( 1 ) {
            char NUM[100];
            close(pfdA[0]);
            close(pfdB[1]);

            int r=0;

            printf("Enter a Number: ");
            fflush(stdout);
            scanf("%s", NUM);

    // SENDS   NUM   to Child process
    write(pfdA[1], NUM, strlen(NUM));


    // READS FROM CHILD THE RESPONSE into the variable buf and
    //      store the return value from read() into the variable r
    r= read(pfdB[0], buf, 100);

    if( r > 0 ) {
                    buf[r] = '\0';
                    printf("%s\n", buf);
                    fflush(stdout);
            }
            else {
                    printf("[PARENT] Reading from child: read() returned %d\n", r);
                    break;
            }
    }
}

    return(0);

}

Upvotes: 0

Views: 62

Answers (1)

William Pursell
William Pursell

Reputation: 212238

Unless you explicitly close(0), dup(pfdA[0]) almost certainly does not return 0. Try dup2 to specify which descriptor you want as the new one. That is (error checking omitted for brevity):

dup2( pfdA[0], STDIN_FILENO );
close( pfdA[0])

Similarly for stdout.

Upvotes: 1

Related Questions