Read from a file handle until you get all the bytes or an error happens.

read_exactly.c

/*
 * read_exactly
 *
 *    Automatically re-reads a file descriptor until an error occurs or the
 *    expected number of bytes has been read.
 *
 *    fd - the file descriptor to read from.
 *    buf - the buffer to read into.
 *    nbyte - the number of bytes to read into the buffer.
 *    rbyte - the actual number of bytes read into the buffer. Will always
 *    equal nbyte if all reads were successful.
 *
 *    Returns true when no errors occurred and the proper number of bytes was
 *    read. Returns false otherwise.
 */
bool read_exactly(int fd, void * buf, size_t nbyte, size_t * rbyte) {
  size_t r = 0;

  while(r < nbyte) {
    int l = read(fd, buf, nbyte - r);
    if (l <= 0) {
      break;
    } else {
      r += l;
    }
  }

  if (rbyte) {
    *rbyte = r;
  }

  return (r == nbyte);
}