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cinteluefi

Confusing documentation for UEFI's BootServices->GetMemoryMap()

I am trying to call the BootServices->GetMemoryMap() function, but the function definition and the documentation does not make sense to me. The UEFI spec (v2.6) says that the definition of the function is (page: 207):

typedef EFI_STATUS(EFIAPI *EFI_GET_MEMORY_MAP)(IN OUT UINTN *MemoryMapSize, IN OUT EFI_MEMORY_DESCRIPTOR *MemoryMap, OUT UINTN *MapKey, OUT UINTN *DescriptorSize, OUT UINT32 *DescriptorVersion);

But the parameters section says:

MemoryMap: A pointer to the buffer in which firmware places the current memory map. The map is an array of EFI_MEMORY_DESCRIPTORs.

If the memory map is an array of EFI_MEMORY_DESCRIPTORs and the parameter is supposed to be a pointer to a buffer, how can the parameter's type be IN OUT EFI_MEMORY_DESCRIPTOR *MemoryMap, shouldn't it be IN OUT EFI_MEMORY_DESCRIPTOR **MemoryMap?

Solution

  • The map is an array of EFI_MEMORY_DESCRIPTORs.

    If the map is an array of EFI_MEMORY_DESCRIPTORs then the name of the array decays to a pointer to the first element in that array, i.e to the

    EFI_MEMORY_DESCRIPTOR *MemoryMap

    So given the function 

    EFI_STATUS efi_get_memorymap(IN OUT UINTN *MemoryMapSize,
                             IN OUT EFI_MEMORY_DESCRIPTOR *MemoryMap, ...);

    and a map of descriptors

    EFI_MEMORY_DESCRIPTOR memory_map[2];

    you can pass the array this way:

    status = efi_get_memorymap(size, memory_map, ...);