Piping two commands together on POSIX

main.c

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

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

#define ERR(expr)                                  \
    do {                                           \
        int err = expr;                            \
        if (err < 0) {                             \
            fprintf(stderr, "%s: %s\n", #expr, strerror(errno));    \
            exit(1);                                                 \
        }                                          \
    } while (0)

int main(void)
{
    // cat ./main.c | sort

    int pipefd[2] = {0};

    ERR(pipe(pipefd));

    pid_t cpid = fork();
    assert(cpid >= 0);

    if (cpid == 0) {
        int input = open(__FILE__, O_RDONLY);
        assert(input >= 0);

        ERR(close(pipefd[0]));
        ERR(dup2(input, STDIN_FILENO));
        ERR(dup2(pipefd[1], STDOUT_FILENO));
        ERR(execlp("cat", "cat", NULL));
    }

    cpid = fork();
    assert(cpid >= 0);

    if (cpid == 0) {
        int output = open("./output.txt", O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
        assert(output >= 0);

        ERR(close(pipefd[1]));
        ERR(dup2(pipefd[0], STDIN_FILENO));
        ERR(dup2(output, STDOUT_FILENO));
        ERR(execlp("sort", "sort", NULL));
    }

    ERR(close(pipefd[0]));
    ERR(close(pipefd[1]));

    ERR(wait(NULL));
    ERR(wait(NULL));

    return 0;
}