Symbol table based off of Sedgewick (http://www.amazon.com/Algorithms-Parts-1-4-Fundamentals-Structures/dp/0201314525). This code can be considered to be BSD.

symbol_table.c

#include <stdlib.h>
#include "symbol_table.h"

typedef struct STNode* stlink;
struct STNode {
    int index, d;
    stlink l, m, r; // left, middle, right
};

static stlink head;
static int val, N;

/* STinit
 *
 * Initialize the symbol table
 */
void STinit() {
    head = NULL;
    N = 0;
}

/* STnew
 *
 * create a new member of the symbol table
 */
static stlink STnew(int d) {
    stlink x = malloc(sizeof(*x));
    x->index = -1;
    x->d = d;
    x->l = NULL;
    x->m = NULL;
    x->r = NULL;
    return x;
}

/* STindexR
 *
 * Recursively determine where to place a new member in the table
 */
static stlink STindexR(stlink h, char* v, int w) {
    int i = v[w];

    if(h == NULL)
        h = STnew(i);

    if(i == 0) {
        if(h->index == -1)
            h->index = N++;

        val = h->index;
        return h;
    }

    if(i < h->d)
        h->l = STindexR(h->l, v, w);

    if(i == h->d)
        h->m = STindexR(h->m, v, w+1);

    if(i > h->d)
        h->r = STindexR(h->r, v, w);

    return h;
}

/* STindex
 *
 * Driver method, place into the symbol table
 */
int STindex(char *key) {
    head = STindexR(head, key, 0);
    return val;
}

symbol_table.h

#ifndef MCDONALD_SYMBOLTABLE_H
    #define MCDONALD_SYMBOLTABLE_H
#endif

int STindex(char *key);
void STinit();