Is clang++ ignoring extern "C" for some deprecation warnings?

Question

If I use clang 3.8.1 to compile:

extern "C" {
int foo(int x) { register int y = x; return y; }
}

int main() { return foo(123); }

I get the warning:

a.cpp:3:18: warning: 'register' storage class specifier is deprecated and incompatible with C++1z [-Wdeprecated-register]
int foo(int x) { register int y = x; return y; }
                 ^~~~~~~~~

... which I really shouldn't be getting this, since the inner function is C code. If I use GCC 6.3.1, even with -Wall, I don't get this warning.

Is this a clang bug or am I doing something wrong?

The code ist still C++ code! extern "C" just specifies the ABI and nameing conventions. (did you even read the warning?) — too honest for this site
– too honest for this site, Commented Apr 18, 2017 at 14:16
extern "C" only has effect on linkage of external names - it doesn't' change the compiler to 'C-mode'. — Michael Burr
– Michael Burr, Commented Apr 18, 2017 at 14:16
either fix it, or compile this part in C - whichever makes sense. — Karoly Horvath
– Karoly Horvath, Commented Apr 18, 2017 at 14:26

Pete Becker · Accepted Answer · 2017-04-18 14:18:28Z

8

extern "C" does not mean "compile this code as C". It means "make this function (or functions) callable from C code", which typically means changing name mangling and, sometimes, calling convention.

edited Apr 18, 2017 at 14:18

answered Apr 18, 2017 at 14:16

Pete Becker

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3 Comments

John Zwinck Over a year ago

Another way to look at it: it means "the name of this function is in C," rather than the body.

Pete Becker Over a year ago

@JohnZwinck - it can involve more than the name.

too honest for this site Over a year ago

@JohnZwinck: Not exactly. It also implys C calling conventions.

Swiss Frank · Accepted Answer · 2018-01-03 03:34:53Z

0

Perhaps the error has nothing to do with the extern "C"? It looks like it says, not, "register is incompatible with C" but rather "register is incompatible with C++1z". (I assume C++1x means C++11/14/17.)

answered Jan 3, 2018 at 3:34

Swiss Frank

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