I don't have either. I have a statically allocated buffer big enough for one frame of data, and I need to guarantee that it never gets used twice. My code does not have a custom allocator. It does not allocate.
void * your_custom_allocator(size_t size)
{
// Handle your locks.
// Sanity checks, assertions, bounds checks, etc...
void * result = g_your_buffer + g_position;
// "Commit Memory" from your buffer.
g_position += size;
// Some more code...
return result;
}
BASE64DECODE_DecodeEx(..., your_custom_allocator, your_custom_deallocator);
A guarantee that this "allocator function" is only ever called once.
Use whatever mechanism is available to you. Use a global condition variable, check it atomically every time you're entering your custom allocator, increment after a successful allocation. I don't know your system's constraints, nor should I...
If I pass in an allocator that returns the statically allocated buffer, then the second call to it must abort loudly.