9 lines seem to be sufficient (assuming string keys and int values).

    // Hashtable definition
    #include <string.h>
    #define N 1024
    int* map_ptr(const char **keys, int *values, const char *key){
        size_t h = 0, c = 0, i;
        for (const char *c = key; *c; c++) h = h * 33 + *(unsigned char*)c;
        for (i = h % N; c < N && keys[i] && 0 != strcmp(keys[i], key); i = (i + 1) % N, c++);
        if (!keys[i]) keys[i] = key;
        return 0 == strcmp(keys[i], key) ? &values[i] : NULL;
    }

    // Example usage
    const char *keys[N];
    int values[N];

    #include <stdio.h>

    int main(){
        // Set some values
        *map_ptr(keys, values, "one") = 1;
        *map_ptr(keys, values, "two") = 2;
        *map_ptr(keys, values, "three") = 3;

        // Retrieve values
        printf("one: %i\n", *map_ptr(keys, values, "one"));
        printf("two: %i\n", *map_ptr(keys, values, "two"));
        printf("three: %i\n", *map_ptr(keys, values, "three"));

        return 0;
    }