CODEWITHSUNDEEP
cpipe
user2949883
user2949883

Reputation: 13

Piping between processes in C

I am trying to understand piping in C. I have constructed two test programs that should be working but while running ,the second one prints uninitialized chars rather than the expected string. Is there anything obvious I am missing?

int main() 
{
    int parentToChild[2]; //1 Write
    int childToParent[2]; //0 Read
    pipe(parentToChild);
    pipe(childToParent);
    int parent = fork();    
    if(!parent)
    {
        close(parentToChild[1]);
        close(childToParent[0]);
        dup2(1,childToParent[1]); //sets child's stdout to send to parent
        dup2(0,parentToChild[0]); //sets child's stdin to come from parent
        execl("./program","./program",NULL);
    }    
    close(childToParent[1]);
    close(parentToChild[0]);
    FILE* readin = fdopen(childToParent[0],"r");
    FILE* writeto = fdopen(parentToChild[1],"w");
    fprintf(writeto,"msg\n");
    fflush(writeto);
    return 0;
}      
int main()
{        //The child process
    fprintf(stderr,"Program started\n");
    char buff[1024];
    fgets(buff,1024,stdin);
    fprintf(stderr,"%s",buff);
}

Upvotes: 0

Views: 294

Answers (1)

rkachach
rkachach

Reputation: 17345

The issue is in the lines:

            dup2(1,childToParent[1]); // Error!
            dup2(0,parentToChild[0]); // Error!

The order of the file descriptors in the dup2 call is switched. To fix you code you have to change these lines by:

            dup2(childToParent[1],1); //sets child's stdout to send to parent
            dup2(parentToChild[0],0); //sets child's stdin to come from parent

The dup2(new_fd, old_fd) makes the newe fd to be a copy of the old fd argument. For more info just check the man page of the dup2.

Note: I tested your code by introducing the changes above and now it's working correctly.

Upvotes: 1

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