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The following table summarizes key rules regarding memory allocation.
| MIDL element | Description |
|---|---|
| Top-level [ ref] pointers | Must be non-null pointers. |
| Function return value | New memory is always allocated for pointer return values. |
| [ unique, out] or [ ptr, out] pointer | Not allowed by MIDL. |
| Non-top-level [unique, in, out] or [ptr, in, out] pointer that changes from null to non-null | Client stubs allocate new memory on client on return. |
| Non-top-level [unique, in, out] pointer that changes from non-null to null | Memory is orphaned on client; client application is responsible for freeing memory and preventing leaks. |
| Non-top-level [ptr, in, out] pointer that changes from non-null to null | Memory will be orphaned on client if not aliased; client application is responsible for freeing and preventing memory leaks in this case. |
| [ref] pointers | Client-application layer usually allocates. |
| Non-null [in, out] pointer | Stubs attempt to write into existing storage on client. If [string] and size increases beyond size allocated on the client, it will cause a GP-fault on return. |
The following table summarizes the effects of key IDL and ACF attributes on memory management.
| MIDL feature | Client issues | Server issues |
|---|---|---|
| [ allocate(single_node)], [allocate(all_nodes)] | Determines whether one or many calls are made to the memory functions. | Same as client, except private memory can often be used for allocate (single_node) [in] and [in,out] data. |
| [allocate(free)] or [allocate(dont_free)] | (None; affects server.) | Determines whether memory on the server is freed after each remote procedure call. |
| array attributes [ max_is] and [ size_is] | (None; affects server.) | Determines size of memory to be allocated. |
| [ byte_count] | Client must allocate buffer; not allocated or freed by client stubs. | ACF parameter attribute determines size of buffer allocated on server. |
| [ enable_allocate] | Usually, none. However, the client may be using a different memory management environment. | Server uses a different memory management environment. RpcSmAllocate should be used for allocations. |
| [ in]attribute | Client application responsible for allocating memory for data. | Allocated on server by stubs. |
| [ out] attribute | Allocated on client by stubs. | [out]-only pointer must be [ref] pointer; allocated on server by stubs. |
| [ ref] attribute | Memory referenced by pointer must be allocated by client application. | Top-level and first-level reference pointers managed by stubs. |
| [ unique] attribute | Non-null to null can result in orphaned memory; null to non-null causes client stub to call midl_user_allocate. | (Affects client.) |
| [ ptr] attribute | (See [ unique].) | (See [ unique].) |