On 4 May 2012 18:37, Eliot Miranda <[email protected]> wrote: > > > On Fri, May 4, 2012 at 6:13 AM, Igor Stasenko <[email protected]> wrote: >> >> On 4 May 2012 10:38, Luc Fabresse <[email protected]> wrote: >> > Hi Igor, >> > >> >> >> >> there is a workaround. >> >> all structs in C is returned via pointer. >> >> so, actually a true return type of a function is CvSize* >> > >> > >> > I agree that usually structs are returned through pointers. >> > But here it is not the case. >> > >> > I want to wrap this function: >> > CvSize cvGetSize( const CvArr* arr ); >> > >> > and the CvSize struct is returned by value. Am I wrong? >> > >> >> i don't know.. this is a bit shady part of C ABI. >> Some compilers use return-by-value, some return by pointer >> by forcing a caller to pass a hidden implicit argument as a pointer to >> the struct, which then filled with values by >> the function you call. > > > IIRC x86 ABIs (all the ones I encountered at least, windows, mac, linux, > solaris) support structure return (by-value structure return) by passing a > hidden first argument which is a pointer to a structure into which to copy > the result. The callee copies the result back through this pointer. > > Where these ABIs differ is in returning small structures. On win32 > structures a power-of-two in size of 8 bytes or less are returned in > EAX:EDX. IIRC on linux and mac all structures are returned through the > hidden first argument pointer. See > ThreadedIA32FFIPlugin>>returnStructInRegisters: > on macs (at least what gcc generates) it also returns structs <=8 bytes size in registers:
so , currently i implemented it like that: if sizeof(struct) <= 8 then i assume function returns it in EAX: EDX otherwise via hidden pointer. >> >> >> It is easy to add support for that, but to make it work correctly can >> be tricky. >> See this, for example: >> >> http://stackoverflow.com/questions/161788/are-there-any-downsides-to-passing-structs-by-value-in-c-rather-than-passing-a >> >> >> >> >> >> >> >> so, what you do is to change the return value to simple void* >> >> and then copy the contents of struct into instance of corresponding >> >> NBExternalStructure subclass >> >> using #fromPointer: method.. to conveniently access its fields values. >> > >> > >> > yes I saw that in NB tests. >> > >> >> >> >> >> >> >> >> but this is in general, in your case this function is actually inlined >> >> by compiler (as CV_INLINE suggests), so there could be even no >> >> exported symbol for it in dynamic library. >> >> and the function is actually so simple, that you don't need to wrap it >> >> at >> >> all. >> >> you can just create a subclass of NBExternalStructure , named CvSize, >> >> implement fieldDesc method as: >> >> >> >> fieldsDesc >> >> ^ #( >> >> int width; >> >> int height >> >> ) >> >> >> >> >> >> and if you invoke: >> >> >> >> CvSize new width: 1 height:2 >> >> >> >> it will be equivalent as if you will call cvSize(1,2) function. >> > >> > >> > yes it is exactly what I did. >> > >> > So my problem is: >> > >> > NBCvSize implemented as a subclass of NBExternalStruct with fieldDesc >> > correctly set up. >> > Then wrap the function cvGetSize: >> > >> > getSize >> > self nbCall: #(NBCvSize cvGetSize(IplImage self)) module: LIB_CV. >> > >> > But I get a debugger because: >> > >> > NBExternalStruct>>coerceReturn: gen >> > " ... should we support handling return of external structures? " >> > self error: 'returning pointer to structure?' >> > >> > And I have no idea how to implement this (black magic EAX pop push ;-)) >> > to >> > coerce result of struct. >> > >> >> >> you don't. if function takes an implicit struct pointer, you can just >> rewrite the function signature from >> NBCvSize cvGetSize(IplImage self) >> to: >> >> void cvGetSize(NBCvSize hidden, IplImage self) >> >> and so, you pass a fresh instance of NBCvSize as argument, which >> function then fill. >> But if C compiler using registers to return struct.. then you're screwed >> :) >> >> Anyways, Luc i will try to read docs about C ABI and see if we can >> easily add support for that. >> >> -- >> Best regards, >> Igor Stasenko. >> > > > > -- > best, > Eliot > -- Best regards, Igor Stasenko.
typedef struct { int x; int y; } foos;
foos foo ()
{
foos bar = {20 , 100 };
return bar;
}
typedef struct { char x; char y; } foo2;
foo2 b ()
{
foo2 bar = {20 , 100 };
return bar;
}
typedef struct { int a1; int a2; char a3; } foo3;
foo3 c () __attribute__ ((noinline));
foo3 c (int ss, int fff, int yyy)
{
foo3 bar = {20 , 100, 40 };
return bar;
}
foo3 d ()
{
foo3 rr = c(1,2,3);
return rr;
}
test.s
Description: Binary data
