scan.c

scanner(FILE *fd) {
beginning:
  c = fgetc(fd);
  switch (c) {
    case ' ': goto beginning;
    case '\n': return TOKEN_NEWLINE;
    default: fseek(fd, -1, SEEK_CUR);
  }

  ...
    
  fscanf(fd, "%[a-z...]", buffer);
  
  ...
    
  fscanf(fd, "%2c", c);
  
  ...
}