VMA provides some features that help with interoperability with OpenGL.

Exporting memory

If you want to attach VkExportMemoryAllocateInfoKHR structure to pNext chain of memory allocations made by the library:

It is recommended to create a Custom memory pools for such allocations. Define and fill in your VkExportMemoryAllocateInfoKHR structure and attach it to VmaPoolCreateInfo::pMemoryAllocateNext while creating the custom pool. Please note that the structure must remain alive and unchanged for the whole lifetime of the VmaPool, not only while creating it, as no copy of the structure is made, but its original pointer is used for each allocation instead.

If you want to export all memory allocated by the library from certain memory types, also dedicated allocations or other allocations made from default pools, an alternative solution is to fill in VmaAllocatorCreateInfo::pTypeExternalMemoryHandleTypes. It should point to an array with VkExternalMemoryHandleTypeFlagsKHR to be automatically passed by the library through VkExportMemoryAllocateInfoKHR on each allocation made from a specific memory type. This is currently the only method to use if you need exported dedicated allocations, as they cannot be created out of custom pools. This will change in future versions of the library though.

You should not mix these two methods in a way that allows to apply both to the same memory type. Otherwise, VkExportMemoryAllocateInfoKHR structure would be attached twice to the pNext chain of VkMemoryAllocateInfo.

Custom alignment

Buffers or images exported to a different API like OpenGL may require a different alignment, higher than the one used by the library automatically, queried from functions like vkGetBufferMemoryRequirements. To impose such alignment:

It is recommended to create a Custom memory pools for such allocations. Set VmaPoolCreateInfo::minAllocationAlignment member to the minimum alignment required for each allocation to be made out of this pool. The alignment actually used will be the maximum of this member and the alignment returned for the specific buffer or image from a function like vkGetBufferMemoryRequirements, which is called by VMA automatically.

If you want to create a buffer with a specific minimum alignment out of default pools, use special function vmaCreateBufferWithAlignment(), which takes additional parameter minAlignment. This is currently the only method to use if you need exported dedicated allocations, as they cannot be created out of custom pools. This will change in future versions of the library though.

Note the problem of alignment affects only resources placed inside bigger VkDeviceMemory blocks and not dedicated allocations, as these, by definition, always have alignment = 0 because the resource is bound to the beginning of its dedicated block. Contrary to Direct3D 12, Vulkan doesn't have a concept of alignment of the entire memory block passed on its allocation.