Doubly linked list - most recently added variable can't be retrieved

chrisname (7395)
I'm working on a shell and I have a pair of linked lists (one to store shell variables (ones that have been defined by or accessible to scripts/users) and one to store internal variables (ones which are accessible only within the shell itself)) to store variables. I'm having a problem, though, where if I add more than one variable, I can't access the most recently added one, but I can add any that were added before it.

shell_varlist.c 
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struct varlist {
	char*	variable;
	char*	value;
	struct varlist*	prev;
	struct varlist* next;
};
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/* Get a pointer to the node containing a variable or NULL if the variable is
 * not stored */
static struct varlist* __get_variable(const char* var)
{
	struct varlist* pvar = NULL;

	if (!vars)
		return NULL;

	__rewind_list();
	if (__list_end()) {
		if (strcmp(vars->variable, var) == 0)
			pvar = vars;
	} else {
		while (!(__list_end())) {
			if (strcmp(vars->variable, var) == 0) {
				pvar = vars;
				break;
			}
			__list_roll_right();
		}
	}
	__fastforward_list();

	return pvar;
}

/**
 * \brief	Add a variable to the list
 * \param var	Used to refer to the variable
 * \param value	Value to set for the variable
 *
 * \note	If the variable exists already, its value will be changed
 */
void add_shell_variable(const char* var, const char* value)
{
	if (shell_variable_exists(var)) {
		/*
		 * Varialbe already exists, modify its value
		 */
		struct varlist* node = __get_variable(var);
		free(node->value);
		node->value = calloc(strlen(value) + 1, 1);
		strcpy(node->value, value);
	} else {
		/*
		 * New variable - add it to the end of the list
		 */
		struct varlist* newvar = malloc(sizeof(struct varlist));
		newvar->variable = calloc(strlen(var) + 1, 1);
		newvar->value = calloc(strlen(value)  + 1, 1);
		strcpy(newvar->variable, var);
		strcpy(newvar->value, value);

		newvar->prev = NULL;
		newvar->next = NULL;

		if (vars) {
			__fastforward_list();
			newvar->prev	= vars;
			vars->next	= newvar;
			__fastforward_list();
		} else {
			vars = newvar;
		}
	}
}
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/**
 * \brief	Check if a variable exists
 * \param var	Name of the variable
 * \return	If the variable exists, 1 is returned. Otherwise, 0 is returned.
 */
int shell_variable_exists(const char* var)
{
	if (__get_variable(var))
		return 1;
	return 0;
}

/**
 * \brief	Retrieve the value associated with a variable
 * \param var	Name of the variable
 * \return	On success, a string containing the value is returned. On error,
 *		a pointer to NULL is returned.
 */
const char* get_shell_value(const char* var)
{
	if (shell_variable_exists(var)) {
		struct varlist* node = NULL;
		if (!(node = __get_variable(var)))
			return NULL;
		return node->value;
	}
	return NULL;
}


At the start of the program, main() calls a function called push_builtins() to add built-in shell variables (like the default prompt string):
builtins.c 
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void push_builtins()
{
	add_shell_variable("PS1", BUILTIN_PS1);
	add_shell_variable("PS2", BUILTIN_PS2);
}


after initialization, my (skeletal) interactive function tries to use the variables:
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/**
 * \brief	Read commands from a file, writing prompts to stdout in between
 * \return	On success, 0 is returned. On error, -1 is returned.
 */
int seash_interactive()
{
	for (;;) {
		char* input = readline(get_shell_value("PS1"));
		add_history(input);
		input = readline(get_shell_value("PS2"));
	}

	return 0;
}

but the output is
seash $ 

seash $ 

seash $ ^C

(the value of PS2 is not printed).

If you need to see anything else, just ask.

Note 1: This code is C, so I can't use C++ constructs or the STL or anything like that.
Note 2: The code for both lists is exactly the same. I'm in the process of unifying it so that both lists will use one set of functions, but I want to work out this problem first.
Last edited on
hamsterman (4538)
lines 81-87 will only work until __list_end() but form lines above it seems that that is still a valid case. I'd have (instead of lines 77-88)
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while(true){
   if (strcmp(vars->variable, var) == 0) {
      pvar = vars;
      break;
   }
   if(__end_list())break;
   else __list_roll_right();
}
I think..
anyway, this looks like a task for a map..

by the way, how do you set the number of first line in [code] ?
chrisname (7395)
hamsterman wrote:
lines 81-87 will only work until __list_end() but form lines above it seems that that is still a valid case. I'd have (instead of lines 77-88)

Of course! __list_end() returns 0 when vars->next is NULL; so I'm always ignoring the last node. That fixed it. Thanks!

hamsterman wrote:
by the way, how do you set the number of first line in [code] ?

Use [code firstline=n].
hamsterman (4538)
glad I could help.

but really, as I said, why would you use a list (in a rather inefficient way, I should add) for a task like this.
don't be lazy, write a map (I'm actually unsure what exactly a map is. I'm thinking of a binary search tree really..)
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