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I'm new to C++ and trying to understand what these two macros are doing in this FileMaker Plugin Example.

#define FMX_PROC(retType)           retType __stdcall
#define FMX_PROCPTR(retType, name)  typedef retType (__stdcall *name)

So far I understand that they are both macros, and that the FMX_PROCPTR is a pointer to a function that takes those two arguments, and that __stdcall is some sort of calling convention (decided to not dig to much into what that means).

What I don't understand are the ends of each lines, the parts that come after FMX_PROC(retType) and FMX_PROCPT(retType, name).

It's possible that it is the spacing that is confusing me, but is retType __stdcall the return type for FMX_PROC(retType) ? Or is it giving the argument a type?

Somewhere else in the code FMX_PROC(retType) is used like this

static FMX_PROC(fmx::errcode) Do_FMmp_ConvertToBase( short /* funcId */, const fmx::ExprEnv& /* environment */, const fmx::DataVect& dataVect, fmx::Data& results )
Nick
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    `__stdcall ` defines the calling convention to be used, that's basically what's going on here. – Paul Sanders Jul 21 '22 at 14:29
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    These marcos are handled by the preprocessor by replacing strings (they are not functions). Wherever you see e.g. `FMX_PROC(fmx::errcode)` it is replaced to `fmx::errcode __stdcall` (whatever you specify as `retType` followed by `__stdcall`). Regarding the meaning of `__stdcall` - the link you have in your question explains it. – wohlstad Jul 21 '22 at 14:31

2 Answers2

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Macros dont have return types. They aren't functions. To express that difference, such macros are called "Function-like macros", ie they look like functions but they aren't functions. The preprocessor replaces FMX_PROC(fmx::errcode) with fmx::errcode __stdcall. That is, this:

static FMX_PROC(fmx::errcode) Do_FMmp_ConvertToBase( short /* funcId */, const fmx::ExprEnv& /* environment */, const fmx::DataVect& dataVect, fmx::Data& results )

Gets expanded to this

static fmx::errcode __stdcall Do_FMmp_ConvertToBase( short /* funcId */, const fmx::ExprEnv& /* environment */, const fmx::DataVect& dataVect, fmx::Data& results )

Its a function declaration and fmx::errcode is its return type.

463035818_is_not_an_ai
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2

These macros are just to provide the calling-convention __stdcall into the function being defined, or a function pointer alias.

__stdcall is a non-standard compiler-intrinsic that ensures that functions documented with this attribute will be called with the stdcall calling-convention. This applies to the function itself.

So the macros respectively:

  • FMX_PROC expands into retType __stdcall, which provides __stdcall to the function. E.g.

    FMX_PROC(fmx::errcode) Do_FMmp_ConvertToBase(...)

    expands into:

    fmx::errcode __stdcall Do_FMmp_ConvertToBase(...)

  • FMX_PROCPTR expands into a typedef of a function pointer, that is also documented with __stdcall. This is necessary because function pointers don't normally carry calling conventions -- so the compiler doesn't implicitly know when a function expects a different calling convention. To bind a function marked __stdcall to a function pointer, the function pointer itself must carry this information (which is why this alias is needed). This expands into part of the function typedef:

    FMX_PROCPTR(fmx::errcode,Do_FMmp_ConvertToBase)(...);

    will expand into

    typedef fmx::errcode(__stdcall *Do_FMmp_ConvertToBase)(...);
Human-Compiler
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