#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>

#define CHUNK 128

int cat(int fd);
void cleanup(void);

int fd = STDIN_FILENO;

int main(int argc, char **argv) {
  atexit(cleanup);

  if(argc >= 2) {
    char *path = argv[1];
    fd = open(path, O_RDONLY);

    if(fd == -1) return 1;
  }

  return cat(fd);
}

void cleanup(void) {
  if(fd != STDIN_FILENO) close(fd);
}

int cat(int fd) {
  uint8_t buf[CHUNK + 1] = {0};
  ssize_t count = 0;

  while((count = read(fd, buf, CHUNK))) {
    if(count == -1) return 1;
    printf("%s", buf);
    memset(buf, 0, CHUNK);
  }

  return 0;
}

I: Reimplementing the coreutils in C