apientry

APIENTRY is a macro used in Windows API headers to indicate the calling convention of functions in the Windows API and related interfaces. It ensures that function declarations across DLL boundaries use a calling convention compatible with the Windows runtime and with applications that import or export those functions. The macro is defined in Windows SDK header files, such as Windef.h, and is commonly used in conjunction with other Windows-specific macros.

On 32-bit Windows, APIENTRY expands to __stdcall, the stdcall calling convention in which the callee cleans the stack. This convention affects how arguments are passed and how the stack is cleaned after a function call. On 64-bit Windows, APIENTRY expands to nothing, because the 64-bit Windows Application Binary Interface uses a single default calling convention and does not require __stdcall annotations. This dual behavior maintains binary compatibility across architectures.

APIENTRY is closely related to the WINAPI macro, which is typically defined as APIENTRY. This means most Windows API function prototypes use WINAPI to denote the standard entry-point convention. Another related macro is CALLBACK, which is defined as __stdcall to standardize callbacks, such as window procedures, in the Windows API.

Using APIENTRY is important when declaring functions that are exported from a DLL or that must be called by Windows components expected to use the standard Windows calling convention. Mismatches in calling convention can lead to stack corruption or crashes, especially when integrated with binary interfaces across compilers.

Example: int APIENTRY MyFunction(int a, int b);

LRESULT APIENTRY WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);