Share via

Argument Passing and Naming Conventions

Microsoft Specific

The Microsoft C++ compilers allow you to specify conventions for passing arguments and return values between functions and callers. Not all conventions are available on all supported platforms, and some conventions use platform-specific implementations. In most cases, keywords or compiler switches that specify an unsupported convention on a particular platform are ignored, and the platform default convention is used.

On x86 platforms, all arguments are widened to 32 bits when they are passed. Return values are also widened to 32 bits and returned in the EAX register, except for 8-byte structures, which are returned in the EDX:EAX register pair. Larger structures are returned in the EAX register as pointers to hidden return structures. Parameters are pushed onto the stack from right to left. Structures that are not PODs will not be returned in registers.

The compiler generates prolog and epilog code to save and restore the ESI, EDI, EBX, and EBP registers, if they are used in the function.


When a struct, union, or class is returned from a function by value, all definitions of the type need to be the same, else the program may fail at runtime.

For information on how to define your own function prolog and epilog code, see Naked Function Calls.

For information about the default calling conventions in code that targets x64 platforms, see x64 Calling Convention. For information about calling convention issues in code that targets ARM platforms, see Common Visual C++ ARM Migration Issues.

The following calling conventions are supported by the Visual C/C++ compiler.

Keyword Stack cleanup Parameter passing
__cdecl Caller Pushes parameters on the stack, in reverse order (right to left)
__clrcall n/a Load parameters onto CLR expression stack in order (left to right).
__stdcall Callee Pushes parameters on the stack, in reverse order (right to left)
__fastcall Callee Stored in registers, then pushed on stack
__thiscall Callee Pushed on stack; this pointer stored in ECX
__vectorcall Callee Stored in registers, then pushed on stack in reverse order (right to left)

For related information, see Obsolete Calling Conventions.

END Microsoft Specific

See also

Calling Conventions