[cfe-dev] C++ language linkage problem

[Seth Cantrell]

The issue is that  language linkage isn't part of a function type, and so the two functions appear to be declarations of the same function rather than overloads where one takes a pointer to a "C" linkage function and the other takes a pointer to a "C++" linkage function.

The reason this error is occurring now is that language linkage support is improving; clang can diagnose an error where a function is declared different times with different linkages, but support is not yet good enough to figure out that these two declarations are of different functions.

There's already a bug filed for this issue: http://llvm.org/bugs/show_bug.cgi?id=15563


On Jul 14, 2013, at 9:43 AM, Dmitri Shubin <sbn at tbricks.com> wrote:

> Hi list!
> 
> In C++ standard (C++11) I can read the following about bsearch() library function (25.5.3 [alg.c.library]):
> 
> The function signature:
> bsearch(const void *, const void *, size_t, size_t,
> int (*)(const void *, const void *));
> 
> is replaced by the two declarations:
> 
> extern "C" void *bsearch(const void *key, const void *base,
> size_t nmemb, size_t size,
> int (*compar)(const void *, const void *));
> extern "C++" void *bsearch(const void *key, const void *base,
> size_t nmemb, size_t size,
> int (*compar)(const void *, const void *));
> 
> But clang (3.4 from trunk) rejects to compile these 2 declarations:
> 
> $ cat a.cpp
> typedef unsigned long size_t;
> 
> extern "C" void *bsearch(const void *key, const void *base,
>        size_t nmemb, size_t size,
>        int (*compar)(const void *, const void *));
> 
> extern "C++" void *bsearch(const void *key, const void *base,
>        size_t nmemb, size_t size,
>        int (*compar)(const void *, const void *));
> 
> $ ../build-clang-cmake/bin/clang++ -v -std=c++11 -c a.cpp
> clang version 3.4
> Target: i386-pc-solaris2.10
> Thread model: posix
> "/export/home/sbn/src/build-clang-cmake/bin/clang-3.4" -cc1 -triple i386-pc-solaris2.10 -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -main-file-name a.cpp -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -target-cpu pentium4 -v -coverage-file /export/home/sbn/src/extern-cpp/a.o -resource-dir /export/home/sbn/src/build-clang-cmake/bin/../lib/clang/3.4 -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /export/home/sbn/src/extern-cpp -ferror-limit 19 -fmessage-length 237 -mstackrealign -fobjc-runtime=gcc -fobjc-default-synthesize-properties -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -vectorize-loops -o a.o -x c++ a.cpp
> clang -cc1 version 3.4 based upon LLVM 3.4svn default target i386-pc-solaris2.10
> #include "..." search starts here:
> #include <...> search starts here:
> /opt/csw/include/c++/4.8.0
> /opt/csw/include/c++/4.8.0/i386-pc-solaris2.10
> /opt/csw/include/c++/4.8.0/backward
> /export/home/sbn/src/build-clang-cmake/bin/../lib/clang/3.4/include
> /usr/include
> End of search list.
> a.cpp:7:20: error: declaration of 'bsearch' has a different language linkage
> extern "C++" void *bsearch(const void *key, const void *base,
>                   ^
> a.cpp:3:18: note: previous declaration is here
> extern "C" void *bsearch(const void *key, const void *base,
>                 ^
> 1 error generated.
> 
> BTW this file is perfectly compile-able using clang 3.2:
> 
> $ /tb/soft/sun/x86_64/clang/bin/clang++ -v -std=c++11 -c a.cpp
> clang version 3.2 (branches/release_32 174327)
> Target: i386-pc-solaris2.10
> Thread model: posix
> "/tb/soft/sun/i386/clang/bin/clang-3.2" -cc1 -triple i386-pc-solaris2.10 -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -main-file-name a.cpp -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -target-cpu pentium4 -momit-leaf-frame-pointer -v -coverage-file /export/home/sbn/src/extern-cpp/a.o -resource-dir /tb/soft/sun/i386/clang/bin/../lib/clang/3.2 -fmodule-cache-path /var/tmp/clang-module-cache -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /export/home/sbn/src/extern-cpp -ferror-limit 19 -fmessage-length 237 -mstackrealign -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -o a.o -x c++ a.cpp
> clang -cc1 version 3.2 based upon LLVM 3.2svn default target i386-pc-solaris2.10
> ignoring nonexistent directory "/opt/csw/include/c++/4.7.2"
> ignoring nonexistent directory "/opt/csw/include/c++/4.7.2/i386-pc-solaris2.10/"
> ignoring nonexistent directory "/opt/csw/include/c++/4.7.2/backward"
> ignoring nonexistent directory "/tb/soft/sun/i386/clang/bin/../lib/clang/3.2/include"
> #include "..." search starts here:
> #include <...> search starts here:
> /usr/include
> End of search list.
> 
> Is it some known bug/regression/fix or should I file a case?
> Thanks!
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