2 ms·

If you want to "shim" the API, then it's easier to have the whole previous object, rather than just the cookie.

cataphract 3y ago
You don't need the cookie then. You can just allocate a larger struct (sort of subclassing it). You save a pointer indirection.
- thechao 3y ago
  There's a limit to what can be crammed into a HN comment!
  - pixelpoet 3y ago
    I have discovered a truly marvelous allocator pattern which this HN comment is too small to contain.
    - jjgreen 3y ago
      [350 years later] I have confirmed the optimality of the allocator pattern as a special case in my proof of the Inter-universal Teichmüller theorem (Springer, 879pp).
  - inopinatus 3y ago
    The solution is obviously to realloc the comment.
  - cataphract 3y ago
    Fair enough: struct hamt_allocator { void *(*realloc)(struct hamt_allocator *h, void *chk, size_t oldsize, size_t newsize); }; struct my_allocator { struct hamt_allocator parent; size_t used; char buffer[8192]; }; static void *alloc_func(struct hamt_allocator *h, void *chk, const size_t oldsize, const size_t newsize) { if (!newsize) { return NULL; } if (h && newsize <= oldsize) { return h; } // this cast is allowed by the standard for this purpose __auto_type *alloc = (struct my_allocator*) h; if (newsize > sizeof(alloc->buffer) - alloc->used) { return NULL; } void *ret = alloc->buffer + alloc->used; alloc->used += newsize; if (h && oldsize) { memcpy(ret, h, oldsize); } return ret; } void func_taking_allocator(struct hamt_allocator *h); void user_code() { struct my_allocator alloc = { .parent.realloc = alloc_func }; func_taking_allocator(&alloc.parent); }
    - mkirchner 3y ago
      Beautiful, thanks!
    - cataphract 3y ago
      While this was not the point of the comment, one thing missing is that oldsize and newsize should be rounded for alignment purposes: struct my_allocator { struct hamt_allocator parent; size_t used; union { char buffer[8192]; max_align_t _align; }; }; static inline size_t align_size(size_t value) { size_t alignment = _Alignof(max_align_t); // if (value % _Alignof(max_align_t) == 0) { // return value; // } else { // return ((value / alignment) + 1) * alignment; // } return value + (alignment - 1) & ~(alignment - 1); } Even better, the allocator function should get the alignment so don't waste bytes unnecessarily.

You don't need the cookie then. You can just allocate a larger struct (sort of subclassing it). You save a pointer indirection.

thechao 3y ago
There's a limit to what can be crammed into a HN comment!
- pixelpoet 3y ago
  I have discovered a truly marvelous allocator pattern which this HN comment is too small to contain.
  - jjgreen 3y ago
    [350 years later] I have confirmed the optimality of the allocator pattern as a special case in my proof of the Inter-universal Teichmüller theorem (Springer, 879pp).
- inopinatus 3y ago
  The solution is obviously to realloc the comment.
- cataphract 3y ago
  Fair enough: struct hamt_allocator { void *(*realloc)(struct hamt_allocator *h, void *chk, size_t oldsize, size_t newsize); }; struct my_allocator { struct hamt_allocator parent; size_t used; char buffer[8192]; }; static void *alloc_func(struct hamt_allocator *h, void *chk, const size_t oldsize, const size_t newsize) { if (!newsize) { return NULL; } if (h && newsize <= oldsize) { return h; } // this cast is allowed by the standard for this purpose __auto_type *alloc = (struct my_allocator*) h; if (newsize > sizeof(alloc->buffer) - alloc->used) { return NULL; } void *ret = alloc->buffer + alloc->used; alloc->used += newsize; if (h && oldsize) { memcpy(ret, h, oldsize); } return ret; } void func_taking_allocator(struct hamt_allocator *h); void user_code() { struct my_allocator alloc = { .parent.realloc = alloc_func }; func_taking_allocator(&alloc.parent); }
  - mkirchner 3y ago
    Beautiful, thanks!
  - cataphract 3y ago
    While this was not the point of the comment, one thing missing is that oldsize and newsize should be rounded for alignment purposes: struct my_allocator { struct hamt_allocator parent; size_t used; union { char buffer[8192]; max_align_t _align; }; }; static inline size_t align_size(size_t value) { size_t alignment = _Alignof(max_align_t); // if (value % _Alignof(max_align_t) == 0) { // return value; // } else { // return ((value / alignment) + 1) * alignment; // } return value + (alignment - 1) & ~(alignment - 1); } Even better, the allocator function should get the alignment so don't waste bytes unnecessarily.