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Microsoft Specific

The __fastcall calling convention specifies that arguments to functions are to be passed in registers, when possible. This calling convention only applies to the x86 architecture. The following list shows the implementation of this calling convention.

Element Implementation
Argument-passing order The first two DWORD or smaller arguments that are found in the argument list from left to right are passed in ECX and EDX registers; all other arguments are passed on the stack from right to left.
Stack-maintenance responsibility Called function pops the arguments from the stack.
Name-decoration convention At sign (@) is prefixed to names; an at sign followed by the number of bytes (in decimal) in the parameter list is suffixed to names.
Case-translation convention No case translation performed.
Classes, structs, and unions Treated as "multibyte" types (regardless of size) and passed on the stack.
Enums and enum classes Passed by register if their underlying type is passed by register. For example, if the underlying type is int or unsigned int of size 8, 16, or 32 bits.


Future compiler versions may use different registers to store parameters.

Using the /Gr compiler option causes each function in the module to compile as __fastcall unless the function is declared by using a conflicting attribute, or the name of the function is main.

The __fastcall keyword is accepted and ignored by the compilers that target ARM and x64 architectures; on an x64 chip, by convention, the first four arguments are passed in registers when possible, and additional arguments are passed on the stack. For more information, see x64 Calling Convention. On an ARM chip, up to four integer arguments and eight floating-point arguments may be passed in registers, and additional arguments are passed on the stack.

For nonstatic class functions, if the function is defined out-of-line, the calling convention modifier doesn't have to be specified on the out-of-line definition. That is, for class non-static member methods, the calling convention specified during declaration is assumed at the point of definition. Given this class definition:

struct CMyClass {
   void __fastcall mymethod();


void CMyClass::mymethod() { return; }

is equivalent to this:

void __fastcall CMyClass::mymethod() { return; }

For compatibility with previous versions, _fastcall is a synonym for __fastcall unless compiler option /Za (Disable language extensions) is specified.


In the following example, the function DeleteAggrWrapper is passed arguments in registers:

// Example of the __fastcall keyword
#define FASTCALL    __fastcall

void FASTCALL DeleteAggrWrapper(void* pWrapper);
// Example of the __ fastcall keyword on function pointer
typedef BOOL (__fastcall *funcname_ptr)(void * arg1, const char * arg2, DWORD flags, ...);

END Microsoft Specific

See also

Argument Passing and Naming Conventions