Weird memory issue (undefined behavior) on macOS compiled with clang

[dmitry-kabanov]

Hi,

I use hashmap in my project. In Linux everything works.
I try to make my code work on macOS and I run into some strange behavior: code basically segfaults immediately.
After running with -fsanitize=undefined, I have managed to find a potential reason for this:

/Users/dima/Developer/tmp/open-interfaces-update-2025-09-nr-3/build.debug_verbose_info_and_sanitize/_deps/hashmap-src/src/hashmap.c:188:47: runtime error: applying zero offset to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/dima/Developer/tmp/open-interfaces-update-2025-09-nr-3/build.debug_verbose_info_and_sanitize/_deps/hashmap-src/src/hashmap.c:188:47

which is about this code:

static int hashmap_rehash(struct hashmap_base *hb, size_t table_size)
{
    size_t old_size;
    struct hashmap_entry *old_table;
    struct hashmap_entry *new_table;
    struct hashmap_entry *entry;
    struct hashmap_entry *new_entry;

    assert((table_size & (table_size - 1)) == 0);
    assert(table_size >= hb->size);

    new_table = (struct hashmap_entry *)calloc(table_size, sizeof(struct hashmap_entry));
    if (!new_table) {
        return -ENOMEM;
    }
    old_size = hb->table_size;
    old_table = hb->table;
    assert(old_table != NULL);
    hb->table_size = table_size;
    hb->table = new_table;

    /* Rehash */
    for (entry = old_table; entry < old_table + old_size; ++entry) {
        if (!entry->key) {
            continue;
        }
        new_entry = hashmap_entry_find(hb, entry->key, true);
        /* Failure indicates an algorithm bug */
        assert(new_entry != NULL);

        /* Shallow copy */
        *new_entry = *entry;
    }
    free(old_table);
    return 0;
}

The assertin the middle is added by me; the problem (line 188, column 47) is in the end condition of the for loop (old_table + old_size). And actually the assert assert(old_table != NULL) fails, that is, for some reason old_table is NULL.

Could you please help me find out what can be the problem? Thanks!