B.1 Option Parser

The BST test program contains an option parser for handling command-line options. See User Interaction, for an introduction to its public interface. This section describes the option parser's implementation.

The option parsing state is kept in struct option_state:

588. <Option parser 588> =
/* Option parsing state. */
struct option_state 
  {
    const struct option *options; /* List of options. */
    char **arg_next;            /* Remaining arguments. */
    char *short_next;           /* When non-null, unparsed short options. */
  };

See also 589 and 590.

The initialization function just creates and returns one of these structures:

589. <Option parser 588> +=
/* Creates and returns a command-line options parser.  
   args is a null-terminated array of command-line arguments, not
   including program name. */
static struct option_state *
option_init (const struct option *options, char **args) 
{
  struct option_state *state;

  assert (options != NULL && args != NULL);

  state = xmalloc (sizeof *state);
  state->options = options;
  state->arg_next = args;
  state->short_next = NULL;

  return state;
}

The option retrieval function uses a couple of helper functions. The code is lengthy, but not complicated:

590. <Option parser 588> +=
/* Parses a short option whose single-character name is pointed to by
   state->short_next.  Advances past the option so that the next one
   will be parsed in the next call to option_get().  Sets *argp to
   the option's argument, if any.  Returns the option's short name. */
static int 
handle_short_option (struct option_state *state, char **argp) 
{
  const struct option *o;

  assert (state != NULL 
          && state->short_next != NULL && *state->short_next != '\0'
          && state->options != NULL);

  /* Find option in o. */
  for (o = state->options; ; o++)
    if (o->long_name == NULL)
      fail ("unknown option `%c'; use -help for help", *state->short_next);
    else if (o->short_name == *state->short_next)
      break;
  state->short_next++;

  /* Handle argument. */
  if (o->has_arg) 
    {
      if (*state->arg_next == NULL || **state->arg_next == ”)
        fail ("`%c' requires an argument; use -help for help");

      *argp = *state->arg_next++;
    }

  return o->short_name;
}

/* Parses a long option whose command-line argument is pointed to by
   *state->arg_next.  Advances past the option so that the next one
   will be parsed in the next call to option_get().  Sets *argp to
   the option's argument, if any.  Returns the option's identifier. */
static int 
handle_long_option (struct option_state *state, char **argp) 
{
  const struct option *o;	/* Iterator on options. */
  char name[16];		/* Option name. */
  const char *arg;		/* Option argument. */

  assert (state != NULL 
          && state->arg_next != NULL && *state->arg_next != NULL
          && state->options != NULL 
          && argp != NULL);

  /* Copy the option name into name
     and put a pointer to its argument, or NULL if none, into arg. */
  {
    const char *p = *state->arg_next + 2;
    const char *q = p + strcspn (p, "=");
    size_t name_len = q - p;

    if (name_len > (sizeof name) - 1)
      name_len = (sizeof name) - 1;
    memcpy (name, p, name_len);
    name[name_len] = '\0';

    arg = (*q == '=') ? q + 1 : NULL;
  }

  /* Find option in o. */
  for (o = state->options; ; o++)
    if (o->long_name == NULL)
      fail ("unknown option -%s; use -help for help", name);
    else if (!strcmp (name, o->long_name))
      break;

  /* Make sure option has an argument if it should. */
  if ((arg != NULL) != (o->has_arg != 0)) 
    {
      if (arg != NULL)
        fail ("-%s can't take an argument; use -help for help", name);
      else 
        fail ("-%s requires an argument; use -help for help", name);
    }

  /* Advance and return. */
  state->arg_next++;
  *argp = (char *) arg;
  return o->short_name;
}

/* Retrieves the next option in the state vector state.
   Returns the option's identifier, or -1 if out of options.
   Stores the option's argument, or NULL if none, into *argp. */
static int 
option_get (struct option_state *state, char **argp) 
{
  assert (state != NULL && argp != NULL);

  /* No argument by default. */
  *argp = NULL;

  /* Deal with left-over short options. */
  if (state->short_next != NULL) 
    {
      if (*state->short_next != '\0')
        return handle_short_option (state, argp);
      else 
        state->short_next = NULL;
    }

  /* Out of options? */
  if (*state->arg_next == NULL) 
    {
      free (state);
      return -1;
    }

  /* Non-option arguments not supported. */
  if ((*state->arg_next)[0] != ”)
    fail ("nonoption arguments encountered; use -help for help");
  if ((*state->arg_next)[1] == '\0')
    fail ("unknown option `'; use -help for help");

  /* Handle the option. */
  if ((*state->arg_next)[1] == ”)
    return handle_long_option (state, argp);
  else 
    {
      state->short_next = *state->arg_next + 1;
      state->arg_next++;
      return handle_short_option (state, argp);
    }
}