Issue with dllexport/dllimport inline function
Hi,
I'm trying to dllexport/dllimport an inline function, but I get this
warning:
```
inline function ‘g_strcmp0’ declared as dllimport: attribute ignored [-
Wattributes]
```
This is when cross compiling my code on Linux for Windows using mingw.
The relevant code is
```
GLIB_API inline
int g_strcmp0 (const char *str1,
const char *str2)
{
if (!str1)
return -(str1 != str2);
if (!str2)
return str1 != str2;
return strcmp (str1, str2);
}
```
Where GLIB_API is defined to `__declspec(dllexport)` when compiling the
DLL and `__declspec(dllimport)` when including the header from
application.
The goal is to get that function inlined when doing `if (g_strcmp0(a,
b))`, but get address from instance in the DLL when doing
`do_sort(array, g_strcmp0)`.
I believe dllimport of inline function should be supported, it compiles
fine with MSVC. It is even documented there:
https://learn.microsoft.com/en-us/cpp/cpp/defining-inline-cpp-functions-with-dllexport-and-dllimport?view=msvc-170
I believe (but not tested myself) that clang had the same issue but got
fixed years ago: https://reviews.llvm.org/D3772
For context, I'm hitting this issue when working on that GLib patch:
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2926
Is there a trick to get that working with GCC? Or should that issue be
reported somewhere?
Regards,
Xavier Claessens
Issue with dllexport/dllimport inline function
Hi,
I'm trying to dllexport/dllimport an inline function, but I get this
warning:
```
inline function ‘g_strcmp0’ declared as dllimport: attribute ignored [-
Wattributes]
```
This is when cross compiling my code on Linux for Windows using mingw.
The relevant code is
```
GLIB_API inline
int g_strcmp0 (const char *str1,
const char *str2)
{
if (!str1)
return -(str1 != str2);
if (!str2)
return str1 != str2;
return strcmp (str1, str2);
}
```
Where GLIB_API is defined to `__declspec(dllexport)` when compiling the
DLL and `__declspec(dllimport)` when including the header from
application.
The goal is to get that function inlined when doing `if (g_strcmp0(a,
b))`, but get address from instance in the DLL when doing
`do_sort(array, g_strcmp0)`.
I believe dllimport of inline function should be supported, it compiles
fine with MSVC. It is even documented there:
https://learn.microsoft.com/en-us/cpp/cpp/defining-inline-cpp-functions-with-dllexport-and-dllimport?view=msvc-170
I believe (but not tested myself) that clang had the same issue but got
fixed years ago: https://reviews.llvm.org/D3772
For context, I'm hitting this issue when working on that GLib patch:
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2926
Is there a trick to get that working with GCC? Or should that issue be
reported somewhere?
Regards,
Xavier Claessens
Re: Issue with dllexport/dllimport inline function
Le mardi 11 octobre 2022 à 13:00 +0800, LIU Hao a écrit :
> you may try
>
> ```
> __attribute__((__gnu_inline__))
> extern inline
> int g_strcmp0(const char*, const char*)
> { ...
> ```
Thanks, I gave that a try but I get many: multiple definition of
`g_strcmp0'. But at least the warning about dllimport is now gone.
> In contrast to C, you don't need the `extern` in C++.
>
> When this function is being defined in a DLL, `__dllexport__` should
> be used in place of
> `__gnu_inline__`. This may require another macro, similar to
> `GLIB_API`.
Re: Issue with dllexport/dllimport inline function
Le mardi 11 octobre 2022 à 21:35 +0800, LIU Hao a écrit :
> 在 2022/10/11 21:28, LIU Hao via Gcc 写道:
> >
> > Did you have any declarations that had no `__gnu_inline__`? GNU
> > inline functions are supposed to be
> > emitted out of line, even when its address is taken.
> >
>
> This should be 'to be never emitted out of line'. Sorry for that.
I have one .c file, part of libglib-2.0-0.dll, that has that line:
```
extern int g_strcmp0 (const char *str1, const char *str2);
```
The header file has that:
```
GLIB_INLINE
int g_strcmp0 (const char *str1,
const char *str2)
{
if (!str1)
return -(str1 != str2);
if (!str2)
return str1 != str2;
return strcmp (str1, str2);
}
```
I now define GLIB_INLINE that way:
```
#if (defined(_WIN32) || defined(__CYGWIN__)) &&
!defined(GLIB_STATIC_COMPILATION)
# define _GLIB_EXPORT __declspec(dllexport)
# define _GLIB_IMPORT __declspec(dllimport)
#elif __GNUC__ >= 4
# define _GLIB_EXPORT __attribute__((visibility("default")))
# define _GLIB_IMPORT
#else
# define _GLIB_EXPORT
# define _GLIB_IMPORT
#endif
#ifdef GLIB_COMPILATION
# define _GLIB_API _GLIB_EXPORT
# define GLIB_INLINE __attribute__((__dllexport__))
__attribute__((__gnu_inline__)) extern inline
#else
# define _GLIB_API _GLIB_IMPORT
# if defined(__has_attribute) && __has_attribute(__gnu_inline__)
// https://gcc.gnu.org/pipermail/gcc/2022-October/239592.html
#define GLIB_INLINE __attribute__((__gnu_inline__)) extern inline
# else
#define GLIB_INLINE _GLIB_EXPORT extern inline
# endif
#endif
```
`GLIB_COMPILATION` is defined when compiling libglib-2.0-0.dll but not
when compiling other dlls or applications.
The linker issue actually happens when linking libglib-2.0-0.dll,
multiple definition happens for all .c files that actually uses
g_strcmp0:
```
/usr/bin/x86_64-w64-mingw32-ld: glib/libglib-2.0-
0.dll.p/deprecated_grel.c.obj: in function `g_strcmp0':
/home/xclaesse/programmation/glib/builddir/../glib/gtestutils.h:245:
multiple definition of `g_strcmp0'; glib/libglib-2.0-
0.dll.p/deprecated_gcache.c.obj:/home/xclaesse/programmation/glib/build
dir/../glib/gtestutils.h:245: first defined here
```
Re: Issue with dllexport/dllimport inline function
Le mercredi 12 octobre 2022 à 01:42 +0800, LIU Hao a écrit : > 在 2022-10-11 22:26, xclae...@gmail.com 写道: > > #ifdef GLIB_COMPILATION > > # define _GLIB_API _GLIB_EXPORT > > # define GLIB_INLINE __attribute__((__dllexport__)) > > __attribute__((__gnu_inline__)) extern inline > > This is not correct. Typically, `dllexport` indicates that 'I want an > external definition' but > `gnu_inline` indicates that 'I do not want an external definition', > so it's a contradiction: you get > either multiple definitions, or none (which causes undefined > references). Sorry, that was a wrong copy/paste when writing the email (leftover of things I tried previously). I can confirm the multiple definition error I have is when that line is `# define GLIB_INLINE _GLIB_EXPORT extern inline`, which expands to `__declspec(dllexport) extern inline`. > To user code which imports such a function, it should be seen as > > ``` > __attribute__((__gnu_inline__)) > extern inline > int g_strcmp0(const char*, const char*) > ``` > > This has the desired effect: The function can be inlined where > appropriate. And if not, a call to > the external thunk is emitted; no weak definition is emitted in > either case. > > > To the library itself which exports it, it should be seen as > > ``` > __attribute__((__dllexport__)) > extern inline > int g_strcmp0(const char*, const char*) > ``` > > There is no `gnu_inline` in this case. GCC always generates a > COMDAT/weak/linkonce definition, which > will not cause multiple definition errors. Right, that's what I do and it fails. I made a simplified example: https://gitlab.gnome.org/xclaesse/inline-example Build it with: meson setup builddir --cross-file cross_file_mingw64.txt ninja -C builddir/ And it fails with: ``` x86_64-w64-mingw32-gcc -o libfoo.dll libfoo.dll.p/foo.c.obj libfoo.dll.p/bar.c.obj -Wl,--allow-shlib-undefined -shared -Wl,--start- group -Wl,--out-implib=libfoo.dll.a -fstack-protector -lkernel32 - luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid - lcomdlg32 -ladvapi32 -Wl,--end-group /usr/bin/x86_64-w64-mingw32-ld: libfoo.dll.p/bar.c.obj: in function `g_strcmp0': /home/xclaesse/programmation/inline-example/builddir/../foo.h:5: multiple definition of `g_strcmp0'; libfoo.dll.p/foo.c.obj:/home/xclaesse/programmation/inline- example/builddir/../foo.h:5: first defined here ``` Thanks for your help!
Re: Issue with dllexport/dllimport inline function
Thanks a lot for your help. Sorry for late reply, but I gave your trick a try and it did not work: ``` /home/xclaesse/programmation/inline-example/builddir/../foo.h:7: multiple definition of `g_strcmp0'; libfoo.dll.p/foo.c.obj:/home/xclaesse/programmation/inline- example/builddir/../foo.h:7: first defined here ``` Updated the test case there: https://gitlab.gnome.org/xclaesse/inline-example Regards, Xavier Claessens. Le mercredi 12 octobre 2022 à 11:03 +0800, LIU Hao a écrit : > 在 2022/10/12 02:37, xclae...@gmail.com 写道: > > > > Build it with: > > meson setup builddir --cross-file cross_file_mingw64.txt > > ninja -C builddir/ > > > > And it fails with: > > That's because you compiled it as C. I used g++ to compile these, and > there was no such error. > > > For C, this inline issue is going to be much more messy due to lack > of COMDAT support about > `inline`. Your will need a dedicated macro for each translation unit. > A dirty solution looks like this: > > ``` > #ifndef INSIDE_FOO_C > __attribute__((__dllexport__)) inline > #endif > extern > int > g_strcmp0(const char*str1, const char*str2) > ... > ``` > > and in 'foo.c': > > ``` > #define INSIDE_FOO_C 1 > #include "foo.h" > > extern int g_strcmp0 (const char *str1, const char *str2); > ``` > > > Further reading: > > https://github.com/lhmouse/mcfgthread/wiki/Differences-between-GNU,-C99-and-C---%60inline%60 > > > -- > Best regards, > LIU Hao
Re: Issue with dllexport/dllimport inline function
Oh, that seems to be working now.
Still got a small issue, glib compiles with -Werror=missing-
prototypes:
../foo.h:7:5: error: no previous prototype for ‘g_strcmp0’ [-
Werror=missing-prototypes]
I'm a bit lost with what the prototype would look like in this case.
I think `__attribute__((__dllexport__))` will be needed when building
with MSVC at least. Seems gcc, clang and msvc each wants a different
trick :/
Thanks,
Xavier Claessens.
Le mercredi 02 novembre 2022 à 00:06 +0800, LIU Hao a écrit :
> 在 2022-11-01 23:38, xclae...@gmail.com 写道:
> > Thanks a lot for your help.
> >
> > Sorry for late reply, but I gave your trick a try and it did not
> > work:
> > ```
> > /home/xclaesse/programmation/inline-example/builddir/../foo.h:7:
> > multiple definition of `g_strcmp0';
> > libfoo.dll.p/foo.c.obj:/home/xclaesse/programmation/inline-
> > example/builddir/../foo.h:7: first defined here
> > ```
> >
>
> Apologies for the typo. The `__dllexport__` in 'foo.h' should have
> been `__gnu_inline__`. If, for
> some reason, it needs to be specified explicitly, you may do that in
> 'foo.c' instead.
>
> 'foo.h':
> ```
> #ifndef INSIDE_FOO_C
> __attribute__((__gnu_inline__)) __inline__
> #endif
> extern
> int g_strcmp0(const char*str1, const char*str2) {
> ...
> ```
>
> 'foo.c':
> ```
> __attribute__((__dllexport__)) /* This is optional. */
> extern int g_strcmp0 (const char *str1, const char *str2);
> ```
>
>
>
>
> --
> Best regards,
> LIU Hao
>
