From OSDev Wiki
Jump to: navigation, search

This page is under construction! This page is a work in progress and may thus be incomplete. Its content may be changed in the near future.

The factual accuracy of this article or section is disputed.
Please see the relevant discussion on the talk page.

Difficulty level
Difficulty 0.png
Not rated

Libsupc++ is a support library for g++ that contains functions dealing with run-time type information (RTTI) and exception handling. If you attempt to use either exceptions or RTTI in a C++ kernel you have compiled with a GCC Cross-Compiler you will also need the libsupc++ library. In general, you should be able to use the one provided as part of a Linux distribution. If, however, you run into problems and need to compile your own, you can follow these steps.


Compiling libsupc++

Create a working GCC Cross-Compiler.

This tutorial assumes it is entitled 'i686-elf-gcc'

Configure gcc

Enter the gcc source directory, run

   ./configure --target=i686-elf --prefix=/usr/local/cross --enable-languages=c,c++ \
       --without-headers --disable-nls
   cd libstdc++-v3

Edit the libstdc++ configure script

Now you need to edit the configure file in the libstdc++-v3 directory. Open it up in the editor of your choice (which preserves unix style line endings) and find a section similar to (it is around line 108,000 in gcc 4.2.1, searching for 'combination' is probably the easiest way to find it):

   { { echo "$as_me:$LINENO: error: No support for this host/target combination." >&5
   echo "$as_me: error: No support for this host/target combination." >&2;}
   { (exit 1); exit 1; }; }

and alter the third line so that it reads:

   { { echo "$as_me:$LINENO: error: No support for this host/target combination." >&5
   echo "$as_me: error: No support for this host/target combination." >&2;}

Configure and make libsupc++

   CPP=i686-elf-cpp ./configure --host=i686-elf --prefix=/usr/local/cross --disable-hosted-libstdcxx \
   cd include
   make install
   cd ../libsupc++
   make install


Libsupc++ should now be installed into /usr/local/cross/lib. To use it, you will need to add

   -L/usr/local/cross/lib -lsupc++

to your linker command line.

Additional requirements

Libsupc++ also requires that libgcc.a be included in your link as well. This is usually found (if you followed the cross compiler directions) in /usr/local/cross/lib/gcc/i686-elf/<gcc version>. Finally, it has a number of dependencies which your kernel must provide, including (but not limited to) malloc, free, abort and strlen.

Tested on

These steps were tested on g++ 4.2.1 under Cygwin with a cross compiler targeting i686-elf

Full C++ Runtime Support Using libgcc And libsupc++

The following description is valid for i386, GCC 3.2 and libgcc/libsupc++ compiled for Linux/glibc (you can use the static libgcc/libsupc++ libraries compiled for your Linux for your kernel).

If you want Exceptions, RTTI, new and delete altogether, you also could use libgcc and libsupc++. libgcc contains the unwinder (for exceptions), while libsupc++ contains the C++ support. These functions look very complex (gcc_sources/gcc/unwind*, gcc_sources/libstdc++-v3/libsupc++/*), so it might be better to port them instead of trying to write them yourself.

To get full C++ support, you only have to do the following:

  • Provide some libc functions (e.g. abort, malloc, free, ...) because libsupc++ needs them. There are even more functions you could support, like pthread_*, but since these are weak symbols, you don't need to define them.
  • There's also a strange function dl_iterate_phdrs. You don't need this so let it simply return -1. It's usually used to find exception frames for dynamically linked objects. You could also remove calls to this function from the library.
  • To make use of exception handling, you also have to tell libsupc++ where the .eh_frame section begins. Before you throw any exception: <verbatim>__register_frame(address_of_eh_frames); </verbatim>.
  • Terminate the .eh_frame section with 4 bytes of zeros (somehow). If you forget this, libsupc++ will never find the end of .eh_frame and generate stupid page faults.

Please note that you still have to call the constructors/destructors by yourself as documented in Calling Global Constructors.

Linking a kernel with libsupc++

You can use your libsupc++ to get exception handling and RTTI in a C++ kernel (no more passing -fno-exceptions -fno-rtti to g++!) so you can use things like throw and dynamic_cast<>. Libsupc++ depends upon libgcc for stack unwinding support. Passing the -nostdlib option to gcc when linking caused libgcc.a and libsupc++.a to not be included, so you need to specify -lgcc -lsupc++ on the command line (no need to specify the directories; gcc knows where it installed them to). In addition, you need to include a .eh_frame section in your linker script and terminate it with 32 bits of zeros (QUAD(0) is a useful linker script command). The symbol start_eh_frame should point to the start of the eh_frame section, and it should be aligned by 4. In addition you need to include your constructors and destructors in the link (see C++ for details). You also need to provide __register_frame() (or call the function provided by libgcc with the start of your .eh_frame section), void *__dso_handle;, __cxa_atexit() and __cxa_finalize (again see C++). Something along the lines of

 #include <reent.h>
 static struct _reent global_reent;
 struct _reent *_impure_ptr = &global_reent;

somewhere in your kernel will keep libgcc happy, because it expects these bits to be provided by the standard library (which you aren't linking into your kernel - but you can provide them in your libk). Libgcc expects a number of (simple) C library functions to be provided by your kernel, including abort, malloc, free, memcpy, memset and strlen. Libsupc++ also requires write, fputs, fputc, fwrite, strcpy and strcat for debugging output.

Personal tools