Libsupcxx

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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.

Contents

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 \
       --disable-nls
   cd include
   make
   make install
   cd ../libsupc++
   make
   make install

Usage

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.

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