Function Pointers, part 3

Alright, here is an overview of how different some Windows calling conventions can be (__pascal, __fortran and __syscall are no longer supported):

__cdecl
Argument Passing: right to left
Stack Maintenance: Calling function pops arguments from the stack
Name Decoration (C only): ‘_’ prefixed to function names (e.g. ‘_foo’)

__stdcall
Argument Passing: right to left
Stack Maintenance: Called function pops its own arguments from the stack
Name Decoration (C only): ‘_’ prefixed to function name, ‘@’ appended followed by the number of decimal bytes in the argument list. (e.g. ‘_foo@10’)

__fastcall (applies to Intel CPUs, this is the default calling convention for Borland)
Argument Passing: First two DWORD arguments are passed in ECX and EDX, the rest is passed right to left
Stack Maintenance: Called function pops its own arguments from the stack
Name Decoration (C only): ‘@’ is prefixed to the name, ‘@’ appended followed by the number decimal bytes in the argument list. (e.g. ‘@foo@10’)

thiscall (used automatically by C++ code)
Argument Passing: ‘this’ pointer put in ECX, arguments passed right to left
Stack Maintenance: Calling function pops arguments from the stack
Name Decoration: None

You see that there are many ways leading to Rome, and this is just on a Windows machine. What is interesting for this article though, is how we declare a function pointer to the following function definition:

void __stdcall Convention(int value, char const *string)
{
  (void)printf(“%d, ‘%s’\n”, value, string);
}

Again, the answer here is simple; we simply integrate the calling convention we need into the function pointer declaration:

void (__stdcall *ConvPtr)(int, char const*) = &Convention;

The principle remains the same whether it is a member function, a function argument, etc. Just bolt what you need into the declaration.

When your linker is giving you errors that it cannot resolve external symbols while you know that you are linking with the correct libraries; be sure to check which calling conventions are being used.