• The implementation of copy (designed to mimic the behavior of cp) illustrates how to use open, read, write, close, stat. It also introduces the notion of a file descriptor.
• man pages exist for all of these functions (e.g. man 2 open, man 2 read, etc.)
• Full implementation of our own copy executable is right here.

• The file descriptor abstraction provides direct, low level access to a stream of data without the fuss of data structures or objects. It certainly can't be slower, and depending on what you're doing, it may even be faster.
• FILE pointers and C++ iostreams work well when you know you're layering over standard output, standard input, and local files. They are less useful when the stream of bytes is associated with a network connection. (FILE pointers and C++ iostreams assume they can rewind and move the file pointer back and forth freely, but that's not the case with file descriptors associated with network connections).
• File descriptors, however, work with read and write and nothing else. C FILE pointers and C++ streams provide automatic buffering and more elaborate formatting options.

Implementation of copy

int main(int argc, char *argv[]) {
  if (argc != 3) {
    fprintf(stderr, "%s <source-file> <destination-file>.\n", argv[0]);
    return kWrongArgumentCount;
  }

  int fdin = open(argv[1], /* flags = */ O_RDONLY);
  if (fdin == -1) {
    fprintf(stderr, "%s: source file could not be opened.\n", argv[1]);
    return kSourceFileNonExistent;
  }

  int fdout = open(argv[2], /* flags = */ O_WRONLY | O_CREAT | O_EXCL, 0644);
  if (fdout == -1) {
    switch (errno) {
      case EEXIST:
        fprintf(stderr, "%s: destination file already exists.\n", argv[2]);
        break;
      default:
        fprintf(stderr, "%s: destination file could not be created.\n", argv[2]);
        break;
    }   
    return kDestinationFileOpenFailure;
  }

Implementation of copy, continued

  char buffer[1024];
  while (true) {
    ssize_t bytesRead = read(fdin, buffer, sizeof(buffer));
    if (bytesRead == 0) break;
    if (bytesRead == -1) {
      fprintf(stderr, "%s: lost access to file while reading.\n", argv[1]);
      return kReadFailure;
    }

    size_t bytesWritten = 0;
    while (bytesWritten < bytesRead) {
      ssize_t count = write(fdout, buffer + bytesWritten, bytesRead - bytesWritten);
      if (count == -1) {
        fprintf(stderr, "%s: lost access to file while writing.\n", argv[2]);
        return kWriteFailure;
      }
      bytesWritten += count;
    }
  }

  if (close(fdin) == -1) fprintf(stderr, "%s: had trouble closing file.\n", argv[1]);
  if (close(fdout) == -1) fprintf(stderr, "%s: had trouble closing file.\n", argv[2]);
  return 0;
}