A simple (dumbed down) cat clone

catclone.c

#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv)
{
  FILE *read_this;
  if (isatty(fileno(stdin))) {
    if(argc > 1) {
      printf("filename: %s\n", argv[1]);
      read_this = fopen(argv[1], "r");
    } else {
      puts("give me a file! Either stdin, a pipe or a filename after programname");
      return 1;
    }
  }
  else {
    read_this = stdin;
  }

  if (read_this == NULL) {
    fprintf(stderr, "Can't open input file\n");
    return 1;
  }

  char buf[BUFSIZ];
  while(NULL != fgets(buf, sizeof buf, read_this)) {
      printf("%s", buf);
  }
  return 0;
}