Help using pipes

I'm trying to use pipes to write some data on a program stdin and get the stdout.

I found this one on the internet and changed it, (the original didn't seems to work to):

Code:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <iostream>
#define WRITE 1
#define READ 0

using namespace std;

int main()
{
        char    buf[5];
        int     fd_in[2], fd_out[2],
                c;
        pipe(fd_in);pipe(fd_out);
        if(fork() == 0)
        {
                close(fd_in[WRITE]);dup2(fd_in[READ], READ);
        close(fd_out[READ]); dup2(fd_out[WRITE], WRITE);

        dup2(fd_out[WRITE], WRITE); //stdout -> pipe's write (everything what would go to stdout will go to the pipe's write...
                execlp("sort", "sort", NULL);
        }

        write(fd_out[1], "c\n", 2);
        write(fd_out[1], "a\n", 2);
        write(fd_out[1], "b\n", 2);

        while((c = read(fd_in[0], buf, 5)) > 0)
                write(1, buf, c);

        return  0;
}

I need to write some data on the sort's stdin and get the stdout, someone can help me?

Are you closing the wrong end of the pipes in the child?

now i'm using this code

Code:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sstream>
#include <iostream>
#define READ 0
#define WRITE 1

using namespace std;

int main()
{
    int ficheiro_fd;
    int pipe_fd[2];
    char buffer[20], buffer2[20];
    int num_bytes;
    int infp;
    pipe(pipe_fd);

    switch ( fork() ) {
    case -1:
        exit(1);
    case 0:
        close(pipe_fd[1]);
        //close(pipe_fd[0]);
        dup2(pipe_fd[0], 0);
        
        execlp("/usr/bin/base64"," ", NULL);
        break;
    default:
        close(pipe_fd[0]);
        //dup2(pipe_fd[1], 1);
        //ficheiro_fd = open("output.txt", O_RDONLY);
    while ((num_bytes = read(fileno(stdin), buffer, 1)) > 0){
            write(pipe_fd[1], buffer, num_bytes);            
            }
            
            while ((num_bytes = read(pipe_fd[1], buffer, 1)) > 0){
                //    write(fileno(stdout), buffer, num_bytes);
                //    break;
                    }
        close(pipe_fd[1]);
        wait((int*)getpid());
    }
    
    

    return 0;
}

but i dont know how to get the data from the command, just how to send

FYI, in the future, please put these questions in the Linux forum.

but its not a c++ question?

While that's true (though it mostly looks like C right now), it's still more of a Linux question. If your question involves how to use Linux APIs correctly, then it is unlikely that C++ gurus will be able to help you. It is far more likely that those skilled with Linux API can help you better.