Hi there!
The recent commit "posix: allow headers compilation with GCC pedantic
flag" added the "__extension__" attribute in front of #include_next
directives, likely to flag them as GNU extensions. This, however,
introduced compiler errors for us when we include those headers from C++
code.
This can be easily reproduced with the following code snippet using the
POSIX skin:
--------
extern "C" {
# include <signal.h>
}
int main() {}
--------
At least with gcc 4.6 (which is the most current version available in
Ubuntu 12.04 LTS), this gives:
# g++ `xeno-config --skin posix --cflags --ldflags` test.cpp
test.cpp:3:1: error: expected unqualified-id before '}' token
Having a closer look (e.g. by looking at the preprocessor output with
-E), we found that the __extension__ attribute is not understood by the
preprocessor and thus not removed when resolving #include_next. So after
the preprocessor run, #include_next is replaced by the actual content of
the signal.h header, but __extension__ stays there.
Usually, this doesn't harm too much as some code will likely follow from
the included header which is accepted after __extension__.
In the special case of signal.h, however, as it has two subsequent
#include_next<signal.h> statements, the latter is replaced by an empty
string, so that __extension__ ends up right in front of the } token of
the extern "C" block. Thus, we see the compiler error cited above.
Even if it doesn't harm in most cases, I think that __extension__ is
not the correct way to tag GNU preprocessor extensions (you don't see
this in libstdc++ neither wget or other popular code snippets using
#include_next).
The "official" way I found is to tell the compiler that your header is a
system header - in that case, #include_next is also accepted in pedantic
mode. This is either reached by using -isystem for the search path of
such headers or by adding this line to the header:
#pragma GCC system_header
Both solutions worked for me and prevented any warning in gcc pedantic
mode. However, I'm not really sure about other possible side effects
they could have, so any further insight is greatly welcome...
--
Regards,
Gernot
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux
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