Cannot view std::string when compiled with clang

Question

g++ (GCC) 5.2.0

clang version 3.7.1 (tags/RELEASE_371/final)

GNU gdb (GDB) 7.12

Gdb is unable to locate the definition of std::string when compiled with clang for some reason. I have custom compiled and build gcc and clang as Centos 6.5 comes with older version of gcc.

Example code

#include <string>

int main()
{
    std::string s("This is a string");

    return 0;
}

Compile with g++ and debug - works just fine

[~]$ g++ -ggdb3 -std=c++14 stl.cpp 
[~]$ gdb a.out
GNU gdb (GDB) 7.12
Reading symbols from a.out...done.
(gdb) break main
Breakpoint 1 at 0x400841: file stl.cpp, line 5.
(gdb) r
Starting program: /home/vagrant/a.out 

Breakpoint 1, main () at stl.cpp:5
5       std::string s("This is a string");
(gdb) n
7       return 0;
(gdb) p s
$1 = {static npos = <optimized out>, 
  _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x612c20 "This is a string"}, _M_string_length = 16, {
    _M_local_buf = "\020\000\000\000\000\000\000\000\300\b@\000\000\000\000", _M_allocated_capacity = 16}}
(gdb) 

Check that it is linking with my rpm build version of libstdc++ and not system

[~]$ ldd a.out
    linux-vdso.so.1 =>  (0x00007ffd709e0000)
    libstdc++.so.6 => /opt/spotx-gcc/lib64/libstdc++.so.6 (0x00007f29318fa000)
    libm.so.6 => /lib64/libm.so.6 (0x00007f2931676000)
    libgcc_s.so.1 => /opt/spotx-gcc/lib64/libgcc_s.so.1 (0x00007f293145f000)
    libc.so.6 => /lib64/libc.so.6 (0x00007f29310cb000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f2931c93000)

[~]$ objdump -T -C a.out
a.out:     file format elf64-x86-64
DYNAMIC SYMBOL TABLE:
0000000000000000  w   D  *UND*  0000000000000000              __gmon_start__
0000000000000000  w   D  *UND*  0000000000000000              _Jv_RegisterClasses
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 __libc_start_main
0000000000000000  w   D  *UND*  0000000000000000              _ITM_deregisterTMCloneTable
0000000000000000  w   D  *UND*  0000000000000000              _ITM_registerTMCloneTable
0000000000000000      DF *UND*  0000000000000000  GLIBCXX_3.4.21 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()
0000000000000000      DF *UND*  0000000000000000  GLIBCXX_3.4 std::allocator<char>::~allocator()
0000000000000000      DF *UND*  0000000000000000  GLIBCXX_3.4.21 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)
0000000000000000      DF *UND*  0000000000000000  GLIBCXX_3.4 std::allocator<char>::allocator()
0000000000000000      DF *UND*  0000000000000000  GCC_3.0     _Unwind_Resume
0000000000400700      DF *UND*  0000000000000000  CXXABI_1.3  __gxx_personality_v0

All looks good now if I try the same with clang

[~]$ clang++ -std=c++14 -g stl.cpp
[~]$ gdb a.out
GNU gdb (GDB) 7.12
Reading symbols from a.out...done.
(gdb) break  main
Breakpoint 1 at 0x400853: file stl.cpp, line 5.
(gdb) r
Starting program: /home/vagrant/a.out 

Breakpoint 1, main () at stl.cpp:5
5       std::string s("This is a string");
(gdb) n
7       return 0;
(gdb) p s
$1 = <incomplete type>
(gdb) 

Now I get an incomplete type - but the same libraries are being used

[~]$ ldd a.out
    linux-vdso.so.1 =>  (0x00007fff5352d000)
    libstdc++.so.6 => /opt/spotx-gcc/lib64/libstdc++.so.6 (0x00007f76b4023000)
    libm.so.6 => /lib64/libm.so.6 (0x00007f76b3d9f000)
    libgcc_s.so.1 => /opt/spotx-gcc/lib64/libgcc_s.so.1 (0x00007f76b3b88000)
    libc.so.6 => /lib64/libc.so.6 (0x00007f76b37f4000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f76b43bc000)


[~]$ objdump -T -C a.out
a.out:     file format elf64-x86-64
DYNAMIC SYMBOL TABLE:
0000000000000000  w   D  *UND*  0000000000000000              __gmon_start__
0000000000000000  w   D  *UND*  0000000000000000              _Jv_RegisterClasses
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 __libc_start_main
0000000000000000  w   D  *UND*  0000000000000000              _ITM_deregisterTMCloneTable
0000000000000000  w   D  *UND*  0000000000000000              _ITM_registerTMCloneTable
0000000000000000      DF *UND*  0000000000000000  GLIBCXX_3.4.21 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()
0000000000000000      DF *UND*  0000000000000000  GLIBCXX_3.4 std::allocator<char>::~allocator()
0000000000000000      DF *UND*  0000000000000000  GLIBCXX_3.4.21 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)
0000000000000000      DF *UND*  0000000000000000  GLIBCXX_3.4 std::allocator<char>::allocator()
0000000000000000      DF *UND*  0000000000000000  GCC_3.0     _Unwind_Resume
0000000000400700      DF *UND*  0000000000000000  CXXABI_1.3  __gxx_personality_v0

Does anyone have any advice on where to look or something that I've missed. Both compilers are bootstrapped when building them - everything seems fine - it just appears to be std::string is not defined when using clang.

Answer 1

As ks1322 mentioned, this is because clang has decided not to emit debug information for libstc++.

You can force clang to do so by providing the following flag: -D_GLIBCXX_DEBUG

I would only provide the flag for debug builds, but if debug is the default and release builds are a special target you should remove it:

release: CXXFLAGS := $(filter-out -D_GLIBCXX_DEBUG,$(CXXFLAGS)) -O2

This has fixed the same problem for me.

Answer 2

The last workaround mentioned in bug 24202 as linked by ks1322 is worth having a look at:

-fno-limit-debug-info will make your debug info larger, slow link (if you're not using -gsplit-dwarf) and debugger performance. But, yes, will address this.

Using -fno-limit-debug-info forces Clang to emit debug information for e.g. std::string at the cost of a larger binary while preserving compatibility with other libraries and the rest of the system/SDK.

As ks1322 and Kevin mentioned, one can instead use -D_GLIBCXX_DEBUG to switch libstdc++ into debug mode but this comes at a heavy price: any library you link against and with which you exchange STL containers (string, vector, etc.) must also be built with -D_GLIBCXX_DEBUG. Meaning: your system/SDK must either support this with a separate set of libraries or you will have to rebuild them yourself.

Answer 3

I've reproduced this issue on Fedora with system clang.

It appears that clang is not emitting debug information for std::string because it was told that libstdc++ provides it. See this comment from bug 24202:

Looks like you don't have debug information for libstdc++ installed:

Missing separate debuginfos, use: dnf debuginfo-install libgcc-5.1.1-4.fc22.x86_64 libstdc++-5.1.1-4.fc22.x86_64

Clang is not emitting debug information for std::string because it was told that libstdc++ provides it (but in your case, it's not installed); this is a debug size optimization that GCC apparently doesn't perform.

Does this work if you install the debug information for libstdc++?

I've installed debug info for libstdc++ with command dnf debuginfo-install libstdc++-6.2.1-2.fc25.x86_64 and that resolved the issue.