GCC ignores function attributes

[Nikos Chantziaras]

According to the documentation:

https://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Function-Attributes.html#Function-Attributes

and more specific, the example about the visibility attribute:

void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }

doesn't work for me. I have this C++ function prototype:

const SDL_AudioSpec& __attribute__((visibility("default")))
spec();

Compiling this with g++ 4.8.2 gives me:

warning: 'visibility' attribute ignored on non-class types

I do not understand :-/ I found this bug on GCC's bugzilla:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30361

but it is very old and unconfirmed.

In any event, I have no idea what I am doing wrong here.

[Jonathan Wakely]

Post by Nikos Chantziaras
https://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Function-Attributes.html#Function-Attributes
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
const SDL_AudioSpec& __attribute__((visibility("default")))
spec();
warning: 'visibility' attribute ignored on non-class types

I think the attribute binds to the return type here, which is a
reference (not a class type) so can't be given visibility.

This applies the attribute to the function, not the return type:

const SDL_AudioSpec& spec() __attribute__((visibility("hidden")));

[Nikos Chantziaras]

Post by Jonathan Wakely

Post by Nikos Chantziaras
https://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Function-Attributes.html#Function-Attributes
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
const SDL_AudioSpec& __attribute__((visibility("default")))
spec();
warning: 'visibility' attribute ignored on non-class types

I think the attribute binds to the return type here, which is a
reference (not a class type) so can't be given visibility.
const SDL_AudioSpec& spec() __attribute__((visibility("hidden")));

I've found that this also works:

__attribute__((visibility("hidden"))) const SDL_AudioSpec& spec();

The only one that doesn't, is the documented one :-/ Documentation bug?

[Jonathan Wakely]

Post by Nikos Chantziaras

Post by Jonathan Wakely

Post by Nikos Chantziaras
https://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Function-Attributes.html#Function-Attributes
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
const SDL_AudioSpec& __attribute__((visibility("default")))
spec();
warning: 'visibility' attribute ignored on non-class types

I think the attribute binds to the return type here, which is a
reference (not a class type) so can't be given visibility.
const SDL_AudioSpec& spec() __attribute__((visibility("hidden")));

__attribute__((visibility("hidden"))) const SDL_AudioSpec& spec();
The only one that doesn't, is the documented one :-/ Documentation bug?

No, the example in the documentation is a function with a void return,
and in that case the attribute binds to the function not the return
type. Your function does not have a void return type. I don't know
why that's different, but both the C and C++ compilers treat the
example the same.

[Nikos Chantziaras]

Post by Jonathan Wakely

Post by Nikos Chantziaras

Post by Jonathan Wakely

Post by Nikos Chantziaras
https://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Function-Attributes.html#Function-Attributes
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
const SDL_AudioSpec& __attribute__((visibility("default")))
spec();
warning: 'visibility' attribute ignored on non-class types

I think the attribute binds to the return type here, which is a
reference (not a class type) so can't be given visibility.
const SDL_AudioSpec& spec() __attribute__((visibility("hidden")));

__attribute__((visibility("hidden"))) const SDL_AudioSpec& spec();
The only one that doesn't, is the documented one :-/ Documentation bug?

No, the example in the documentation is a function with a void return,
and in that case the attribute binds to the function not the return
type.

That can't be. This works:

int __attribute__((visibility("hidden"))) spec();

spec() is indeed hidden, so the attribute is not applied to the return
type at all (if that was even possible; how can you hide a return type.)

[Jonathan Wakely]

Post by Nikos Chantziaras
int __attribute__((visibility("hidden"))) spec();

But this isn't hidden:

int* __attribute__((visibility("hidden"))) spec();

because the attribute binds to the return type, which is what the
warning tells you.

[Nikos Chantziaras]

Post by Jonathan Wakely

Post by Nikos Chantziaras
int __attribute__((visibility("hidden"))) spec();

int* __attribute__((visibility("hidden"))) spec();
because the attribute binds to the return type, which is what the
warning tells you.

So what's the difference between 'int' and 'int*' here? Why does:

int HIDE spec();

bind to 'spec' but

int* HIDE spec();

doesn't?