stack.h

#include <stdlib.h>
#include <string.h>

/**
 * Type for individual stack entry
 */
struct stack_entry
{
    char *data;
    struct stack_entry *next;
};

/**
 * Type for stack instance
 */
struct stack_t
{
    struct stack_entry *head;
    size_t stackSize; // not strictly necessary, but
                      // useful for logging
};

/**
 * Create a new stack instance
 */
struct stack_t *
newStack(void)
{
    struct stack_t *stack = malloc(sizeof *stack);
    if (stack)
    {
        stack->head = NULL;
        stack->stackSize = 0;
    }
    return stack;
}

/**
 * Make a copy of the string to be stored (assumes
 * strdup() or similar functionality is not
 * available
 */
char *copyString(char *str)
{
    char *tmp = malloc(strlen(str) + 1);
    if (tmp)
        strcpy(tmp, str);
    return tmp;
}

/**
 * Push a value onto the stack
 */
void push(struct stack_t *theStack, char *value)
{
    struct stack_entry *entry = malloc(sizeof *entry);
    if (entry)
    {
        entry->data = copyString(value);
        entry->next = theStack->head;
        theStack->head = entry;
        theStack->stackSize++;
    }
    else
    {
        // handle error here
    }
}

/**
 * Get the value at the top of the stack
 */
char *top(struct stack_t *theStack)
{
    if (theStack && theStack->head)
        return theStack->head->data;
    else
        return NULL;
}

/**
 * Pop the top element from the stack; this deletes both
 * the stack entry and the string it points to
 */
void pop(struct stack_t *theStack)
{
    if (theStack->head != NULL)
    {
        struct stack_entry *tmp = theStack->head;
        theStack->head = theStack->head->next;
        free(tmp->data);
        free(tmp);
        theStack->stackSize--;
    }
}

/**
 * Clear all elements from the stack
 */
void clear(struct stack_t *theStack)
{
    while (theStack->head != NULL)
        pop(theStack);
}

/**
 * Destroy a stack instance
 */
void destroyStack(struct stack_t **theStack)
{
    clear(*theStack);
    free(*theStack);
    *theStack = NULL;
}