Question

I am currently developing a kind of static analysis tool that performs pattern matching. I am using Flex to generate lexical analyzer, and I wrote code to manage the symbol table. I am not very experienced with C, so I decided to implement the symbol table as a linear linked list.

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

struct symtab {
   int id;
   char *name;
   int type;
   struct symtab *next;
};

enum types {
   KEYWORD = 1,
   CONSTANT,
   IDENTIFIER,
   OPERATOR,
   DELIMITER,
   WHITESPACE
};

struct symtab *last_entry(struct symtab *start)
{
   struct symtab *p;
   p = start;
   while(p -> next != NULL) {
      p = p -> next;
   }
   return p;
}

void add_entry(char* name, int type, struct symtab *start)
{
   struct symtab *new;
   new = last_entry(start);
   int id;
   if(new == start) {
      new = start;
      id = 0;
   }
   else {
      new = malloc(sizeof(struct symtab));
      id = last_entry(start) -> id;
      last_entry(start) -> next = new;
   }
   new -> id = id + 1;
   new -> name = name;
       new -> type = type;
   new -> next = NULL;
}

struct symtab *find_entry(char* name, struct symtab *start)
{
   struct symtab *p;
   p = start;
   while(p -> next != NULL) {
      if(strcmp(p -> name, name) == 0) {
         return p;
      }
   }
}

However, when I use add_entry() to add symbols, and then try to find them with find_entry(), find_entry() returns null. Can someone please assist?

Answer 1

It looks like you’re trying to represent the list as a header object (start), followed by the actual elements of the list. This is a good idea since it simplifies the empty-list case, but you’ve not got the implementation right.

When you add, you need to remove the special case code you’ve got for last_entry being start. The start node will never contain symbol data.

When you lookup, you’ve got to make sure you skip the head (start) since it doesn’t contain symbol data. A second bug in your lookup code is that you stop searching when p->next is NULL (which means you can never return the final element in your list.) You should stop when p is NULL.

Of course, you shouldn’t be using a linked list at all: a hash table would be a better choice since it’s got better performance and memory efficiency.