Question

I am trying to create a child that calls sort. The parent sends data to the child through a pipe. My code compiles and runs, but there is no output. What am I doing wrong? Am I not closing the pipes correctly, writing the pipes or outputting the data correctly?

[eddit] On my system I need to call /bin/sort NOT /usr/bin/sort!

#include <sys/wait.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <ctype.h>

int main(void){
int pipes[2];
pid_t pid;
FILE *stream;

if(pipe(pipes) == -1)
    printf("could not create pipe\n");

switch(fork()){
    case -1:
        fprintf(stderr, "error forking\n");
        break;
    case 0:
        dup2(pipes[0], STDIN_FILENO);

        pid = getpid();
        printf("in child, pid=%d\n");

        if(close(pipes[1]) == -1)
            fprintf(stderr,"err closing write end pid=%d\n", pid);

        execl("/usr/bin/sort", "sort",  (char*) NULL);
        break;
    default:
        stream = fdopen(pipes[1], "w");
        pid = getpid();
        printf("in parent, pid=%d\n", pid);

        if (stream == NULL)
            fprintf(stderr, "could not create file streami\n");

        if(close(pipes[0]) == -1)
            printf("err closing read end pid=%d\n");

                   fputs("bob\n",stream);
        fputs("cat\n",stream);
        fputs("ace\n",stream);
        fputs("dog\n",stream);

        if(fclose(stream) == EOF)
            fprintf(stderr, "error while closing stream\n");
        break;
    }
    return 0;
}

[edit] Here is my working code. Thank you everyone

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

int main(void){
int pipes[2];
pid_t pid;
FILE *stream;

int stat;
if(pipe(pipes) == -1)
    printf("could not create pipe\n");

switch(fork()){
    case -1:
        fprintf(stderr, "error forking\n");
        break;
    case 0:
        dup2(pipes[0], STDIN_FILENO);

        pid = getpid();
        printf("in child, pid=%d\n", pid);

        if(close(pipes[1]) == -1)
            fprintf(stderr,"err closing write end pid=%d\n", pid);

        if(close(pipes[0]) == -1)
            fprintf(stderr,"err closing write end pid=%d\n", pid);

        execl("/bin/sort", "sort",  (char*) NULL);
        exit(EXIT_FAILURE);
        break;
    default:
        stream = fdopen(pipes[1], "w");
        pid = getpid();
        printf("in parent, pid=%d\n", pid);

        if (stream == NULL)
            fprintf(stderr, "could not create file streami\n");

        if(close(pipes[0]) == -1)
            printf("err closing read end pid=%d\n");

        fputs("bob\n",stream);
        fputs("cat\n",stream);
        fputs("ace\n",stream);
        fputs("dog\n",stream);

        if(fclose(stream) == EOF)
            fprintf(stderr, "error while closing stream\n");
        break;
}

wait(&stat);
return 0;
}   

Answer 1

You most certainly do not have enough close() calls in the code, which will lock the processes up.

Pseudo code:

Create pipe
Fork
In parent:
    Close read end of pipe
    Write data to be sorted down write end of pipe
    Close write end of pipe
    Wait for child to die
In child
    Close write end of pipe
    Duplicate read end of pipe to stdin
    Close read end of pipe
    Exec the sort program
    Exit with an error if the exec returns

Note that the pseudo code ends up closing all four ends of the pipe – the two in the parent and the two in the child. If you don’t do that, you will run into deadlock.