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Roberto Ierusalimschy wrote:
What may be worth is to return 'block' when size == oldsize. (Lua
sometimes does this kind of call.)

Good to know. So this should be it (there is no test for (oldsize != 0) before memcpy, but that should be guaranteed when (block != NULL) unless something is broken in the caller):

static void *
l_alloc (void *ud, void *block, size_t oldsize, size_t size)
    /* free */
    if (size == 0) {
        return NULL;

    /* realloc, same size */
    if (size == oldsize) {
        return block;

    /* allocate for alloc | realloc */
    void *newblock = malloc(size);
    if (newblock == NULL) {
        return NULL;

    /* realloc, different size */
    if (block != NULL) {
        memcpy(newblock, block, (oldsize < size) ? oldsize : size);
    return newblock;

In a later phase of development, I'll probably add counters to do some statistics on allocator usage, sizes, etc. with a real program.