uniq | wc -l in C using pipe, dup2, fork, execl

#include <stddef.h>
#include <unistd.h>

static void readwrite_pipe(int, int);

int main(void) {
  int fd1[2];
  int fd2[2];
  pipe(fd1);
  pipe(fd2);
  if (fork() == 0) {
    dup2(fd1[0], STDIN_FILENO);
    close(fd1[0]);
    close(fd1[1]);
    dup2(fd2[1], STDOUT_FILENO);
    close(fd2[0]);
    close(fd2[1]);
    execl("/usr/bin/uniq", "uniq", NULL);
  } else {
    close(fd1[0]);
    readwrite_pipe(STDIN_FILENO, fd1[1]);
    close(fd1[1]);
    dup2(fd2[0],STDIN_FILENO);
    close(fd2[0]);
    close(fd2[1]);
    execl("/usr/bin/wc", "wc", "-l", NULL);
  }
}

static void readwrite_pipe(int in, int out) {
  size_t line_max = sysconf(_SC_LINE_MAX);
  char *buf[line_max];
  for (ssize_t size = read(in, buf, line_max); size > 0; size = read(in, buf, line_max)) {
    write(out, buf, size);
  }
}