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01.26.05
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 By Steve Friedl
When writing code for the Win32 platform, most developers don't pay attention to selecting a "calling convention", and in most cases it doesn't really matter much. But as systems get larger and split into more modules (especially when third-party modules are to be included), this becomes something that cannot really be ignored.
In this Tech Tip, we discuss what the MSVC calling conventions are, why to choose one over the other, and "big system" considerations that may not be obvious.
Calling Conventions
Traditionally, C function calls are made with the caller pushing some parameters onto the stack, calling the function, and then popping the stack to clean up those pushed arguments.
/* example of __cdecl */
push arg1
push arg2
push arg3
call function
add sp,12 // effectively "pop; pop; pop"
It turns out that Microsoft compilers on Windows (and probably most others) support not just this convention, but two others as well. The technical details are found at Microsoft's MSDN site, but we'll touch on them here as well.
The default convention - shown above - is known as __cdecl, and the other most popular one is __stdcall. In this convention, the parameters are pushed by the caller, but the stack is cleaned up by the callee.
This is the standard convention for Win32 API functions (as defined by the WINAPI macro in <windows.h>), and it's also sometimes called the "Pascal" calling convention.
/* example of __stdcall */
push arg1
push arg2
push arg3
call function
// no stack cleanup - callee does this
This looks like a minor technical detail, but if there is a disagreement on how the stack is managed between the caller and the callee, the stack will be destroyed in a way that is unlikely to be recovered.
A mismatch in calling convention is catastrophic for a running program.
At first this seems like a not-that-interesting distinction (to many it is in fact not-that-interesting), but there are several implications that arise when considering one or the other.
 We'll note that there is also a __fastcall convention that uses registers, but we don't believe it's really that useful in the general case - the save and restore of the registers often removes any speed benefit of using the register for arg passing. We'll only touch on it in passing.
Since __stdcall does stack cleanup, the (very tiny) code to perform this task is found in only one place, rather than being duplicated in every caller as it is in __cdecl. This makes the code very slightly smaller, though the size impact is only visible in large programs.
Variadic functions like printf() are almost impossible to get right with __stdcall, because only the caller really knows how many arguments were passed in order to clean them up. The callee can make some good guesses (say, by looking at a format string), but the stack cleanup would have to be determined by the actual logic of the function, not the calling-convention mechanism itself. Hence only __cdecl supports variadic functions so that the caller can do the cleanup.
There isn't really a "right or wrong" with respect to which one is best, but it is positively fatal to "mix and match". The general principle is "the stack-cleanup must match the arg-pushing", and this only happens when caller and callee know what the other is doing. Calling a function with the "wrong" convention will destroy the stack.
Linker symbol name decorations
As mentioned in a bullet point above, calling a function with the "wrong" convention can be disastrous, so Microsoft has a mechanism to avoid this from happening. It works well, though it can be maddening if one does not know what the reasons are.
They have chosen to resolve this by encoding the calling convention into the low-level function names with extra characters (which are often called "decorations"), and these are treated as unrelated names by the linker. The default calling convention is __cdecl, but each one can be requested explicitly with the /G? parameter to the compiler.
__cdecl (cl /Gd ...)
All function names of this type are prefixed with an underscore, and the number of parameters does not really matter because the caller is responsible for stack setup and stack cleanup. It is possible for a caller and callee to be confused over the number of parameters actually passed, but at least the stack discipline is maintained properly.
__stdcall (cl /Gz ...)
These function names are prefixed with an underscore and appended with the number of bytes of parameters passed. By this mechanism, it's not possible to call a function with the "wrong" type, or even with the wrong number of parameters.
__fastcall (cl /Gr ...)
These function names start with an @ sign and are suffixed with the @parameter count, much like __stdcall.
Read the Rest of the Article.
About the Author:
Steve Friedl is a software and network security consultant in Southern California. He runs Unixwiz.net which features a collection of tools, tech tips, and other information in the scope of his consulting practice. |
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