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 )
CodePudding user response:
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.
CodePudding user response:
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 intoretType __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 atypedef
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)(...);