Need free the old memory if realloc failed.

[loggerhead]

The old memory should freed in tgc_realloc because:

For realloc(), the input pointer is still valid if reallocation failed.

  void *qtr = realloc(ptr, size);

  if (qtr == NULL) {
    // free(ptr);
    tgc_rem(gc, ptr);
    return qtr;
  }

[orangeduck]

I think it should be up to the caller of tgc_realloc to free the old pointer. Essentially tgc_realloc should match the semantics of realloc.

[loggerhead]

Oh, I don't consider this point, you are right 😄