Calling convention
From Wikipedia, the free encyclopedia
A calling convention is a method for a programming language to send data to a function, and receive data back from functions. When writing a piece of software in multiple languages and modules, it is necessary for all modules to use compatible calling conventions.
Calling conventions differ in the order passed parameters are placed on the stack, methods for sending data to a function, receiving return data from a function, and methods of Name mangling.
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cdecl
The cdecl calling convention is used by many C and C++ systems for the x86 architecture. In cdecl, function parameters are passed on the stack in a right-to-left order. Function return values are returned in the EAX register. Registers EAX, ECX, and EDX are available for use in the function.
For instance, the following C code function prototype and function call:
int function(int, int, int); int a, b, c, x; ... x = function(a, b, c);
will produce the following x86 Assembly code (written in MASM syntax):
push c push b push a call function add esp, 12 ;Stack clearing mov x, eax
The calling function cleans the stack after the function call returns.
The cdecl calling convention is usually the default calling convention for x86 C compilers, although many compilers provide options to automatically change the calling conventions used. To manually define a function to be cdecl, some support the following syntax:
void _cdecl function(params);
The _cdecl modifier must be included in the function prototype, and in the function declaration to override any other settings that might be in place.
Pascal
The Pascal calling convention is the reverse of the C calling convention. The parameters are pushed on the stack in left-to-right order and the callee is responsible for balancing the stack before return.
The callee balances the stack by the assembly code: "ret freestack", where freestack is a constant integer.
Register (FastCall)
The Register or FastCall calling convention is compiler-specific; for specific information, consult the compiler's documentation. In general, however, the register calling convention states that the first 2 or 3 function arguments with a size of 32 bits or lower will be passed in the EAX, EDX, and possibly ECX registers instead of on the stack. The remaining arguments are passed right-to-left on the stack similar to cdecl. Return values are passed in the AL, AX, or EAX register.
stdcall
The stdcall[1] calling convention is the de facto standard calling convention for Microsoft Windows NT programming API. Function parameters are passed Right-to-Left. Registers EAX, ECX, and EDX are preserved for use within the function. Return values are stored in the EAX register. Unlike cdecl, the called function cleans the stack, instead of the calling function. Because of this fact, stdcall functions cannot support variable-length argument lists.
On a Microsoft Windows system, a function may be declared to be stdcall using the following syntax in the function prototype, and in the function declaration:
void __stdcall function(params);
Stdcall functions are easy to recognize in ASM code because those functions will all unwind the stack prior to returning. the x86 ret instruction allows an optional byte parameter that specifies the number of stack locations to unwind before returning to the caller. such code looks like this:
ret 14
safecall
In Borland Delphi on Microsoft Windows, the safecall calling convention encapsulates COM (Component Object Model) error handling, so that exceptions aren't leaked out to the caller, but are reported in the HRESULT return value, as required by COM/OLE. When calling a safecall function from Delphi code, Delphi also automatically checks the returned HRESULT and raises an exception if necessary. Together with language-level support for COM interfaces and automatic IUnknown handling (implicit AddRef/Release/QueryInterface calls), the safecall calling convention makes COM/OLE programming in Delphi very nice and elegant.
thiscall
This calling convention is used for calling C++ member functions. There are two primary versions of thiscall used depending on the compiler and whether or not the function uses variable arguments.
For the GCC compiler, thiscall is almost identical to cdecl: the calling function cleans the stack, and the parameters are passed in right-to-left order. The difference is the addition of the this pointer, which is pushed onto the stack last, after all the parameters. This is the same method used by Windows thiscall functions that use variable arguments.
Windows functions that do not use variable arguments also have their arguments passed in right-to-left order, but the called function cleans the stack and the this pointer is passed in the ECX register.
The thiscall calling convention cannot be explicitly specified as thiscall is not a keyword.
Intel ABI
The Intel Application Binary Interface is a computer programming standard that most compilers and languages follow. According to the Intel ABI, the EAX, EDX, and ECX are to be free for use within a procedure or function, and need not be preserved.
Microsoft x64 calling convention
The x64 calling convention takes advantage of additional register space in the x86-64 / Intel EM64T platform. The registers RCX, RDX, R8, R9 are used for integer and pointer arguments, and XMM0, XMM1, XMM2, XMM3 are used for floating point arguments. Additional arguments are pushed onto the stack. The return value is stored in RAX.
Standard Exit and Entry Sequences
The standard entry sequence to a function is as follows:
_function:
push ebp ;store the old base pointer
mov ebp, esp ;make the base pointer point to the current stack location
;which is where the parameters are
sub esp, x ;x is the size, in bytes, of all "automatic variables"
;in the function
This sequence preserves the original base pointer ebp, points ebp to the location of the function parameters on the stack, and creates space for automatic variables on the stack. Local variables are created on the stack with each call to the function, and are cleaned up at the end of each function. This behavior allows for functions to be called recursively. In C and C++, variables declared "automatic" are created in this way.
The Standard Exit Sequence goes as follows:
mov esp, ebp ;reset the stack to "clean" away the local variables pop ebp ;restore the original base pointer ret ;return from the function
The following C function:
int _cdecl MyFunction(int i){
int k;
return i + k;
}
would produce the equivalent asm code:
;entry sequence
push ebp
mov ebp, esp
sub esp, 4 ;create space for "int k"
;function code
mov eax, [ebp + 8]
;move parameter i to accumulator
add eax, [ebp - 4]
;add k to i
;answer is returned in eax
;exit sequence
mov esp, ebp
pop ebp
ret
See also
External links
- The Code Project—Calling Conventions Demystified
- About Calling conventions
- S. C. Johnson, D. M. Ritchie, Computing Science Technical Report No. 102: The C Language Calling Sequence, Bell Laboratories, September, 1981
- Intel x86 Function-call Conventions - Assembly View
- Microsoft x64 Calling Convention
- Calling Conventions on x86 by Agner Fog (pdf)
