reassign 1000974 xfslibs-dev
severity 1000974 important
retitle 1000974 xfs/linux.h defines common word "fallthrough" breaking
unrelated headers
thanks
Hi,
On 04/12/21 23:36, Thomas Goirand wrote:
On 12/4/21 5:11 PM, Giovanni Mascellani wrote:
Could you try running that compilation command with g++ -E, so you can
see what BOOST_FALLTHROUGH is actually begin replaced with?
Oh, sorry, now I get what you meant (did a c++ --help to understand what
-E was doing). Here's the result in
CMakeFiles/seastar.dir/src/core/file.cc.o:
__attribute__((__attribute__((__fallthrough__))));
Probably, there's a mistake, and it should really be:
__attribute__((__fallthrough__));
instead, which is the source of the trouble?
It seems that the problem goes this way:
* Boost defines in /usr/include/boost/config/compiler/gcc.hpp:
#define BOOST_FALLTHROUGH __attribute__((fallthrough))
* But /usr/include/xfs/linux.h defines:
#define fallthrough __attribute__((__fallthrough__))
So the "fallthrough" in Boost's definition is expanded again and causes
the problem.
I think xfs/linux.h is at fault here, because it aggressively defines a
common word (while Boost namespaces all its definitions with a BOOST_
prefix). Unfortunately the C/C++ preprocessor makes it easy to break
other headers if you define stuff too liberally. This wold also, for
example, break any program that use the name "fallthrough" for a
variable, which is a completely reasonable name to use. A simple example
could be:
---
#include <xfs/linux.h>
int main() {
int fallthrough = 0;
return fallthrough;
}
---
which fails compilation with:
---
$ LANG=C gcc test.c
test.c: In function 'main':
test.c:4:5: warning: 'fallthrough' attribute not followed by ';'
[-Wattributes]
4 | int fallthrough = 0;
| ^~~
test.c:4:21: error: expected identifier or '(' before '=' token
4 | int fallthrough = 0;
| ^
In file included from test.c:1:
test.c:5:12: error: expected expression before '__attribute__'
5 | return fallthrough;
| ^~~~~~~~~~~
---
You can probably work around this problem by undefining "fallthrough"
just after the xfs/linux.h header. In the meantime I am reassigning this
bug to xfslibs-dev.
Giovanni.
--
Giovanni Mascellani <g.mascell...@gmail.com>
