[pjmlp]

Something like this, without any kind of error checking being done.

    #include <stdlib.h>

    // assume some hash table library, exercise for the reader
    typedef struct{} *hashtable_t;
    extern hashtable_t hashtable_init(void);
    extern void hashtable_put(hashtable_t hashtable, const char* key, const char* value);
    extern void hashtable_dump(hashtable_t hashtable);

    typedef struct {
      hashtable_t table;
    } data_t;

    void foo(data_t* a)
    {
        hashtable_put(a->table, "otherKey", "b");
    }

    int main(void)
    {
        // let a = {key: 'a'};
        data_t* a = (data_t*) malloc(sizeof(data_t));
        a->table = hashtable_init();
        hashtable_put(a->table, "key", "a");

        foo(a);

        // console.log( a );
        hashtable_dump(a->table);
        return 0;
    }

The function foo() gets the numeric value of the a pointer, thus a parameter inside foo() points to the same memory location.

If C had pass-by-reference, it would be possible to give (implicitly) the memory location of the local variable a in main() instead. For example, like in Pascal (var) or C++ (& in function declaration).