libbmcxx - bare metal C++ lib

Work-in-progress.

You may use and redistribute this code, in any form, without restriction. There is no warranty expressed or implied.

Testing has been minimal. Use at your own risk.

Provides various C++ (and C) standard library components that are suitable for bare metal applications. Requires G++/Clang++ or something very compatible, and uses the C headers provided by the compiler (in -ffreestanding mode). Aims roughly for C++14 (but with a bit of later standards thrown in for good measure too).

Note: no floating-point support or thread support (yet; both may be added as options later).

The intention is to provide a useful subset of the C++ standard library, with various options useful for bare metal including exception-less mode and float-less mode.

Currently provides (at least partial implementations of):

• C "string" functions: memcpy, memset, memmove, memcmp, strlen, strcpy, strcmp, etc
• various C++ wrapper headers (cstddef, cstdint, etc)
• string_view and variants, string and variants (rudimentary)
• vector (rudimentary)
• unique_ptr and shared_ptr
• pair (but not yet tuple)
• to_string and from_chars for (some) integer types
• terminate() (calls abort)
• a little bit of atomic
• various standard new/delete overloads (which ultimately call malloc/free)
• some of type_traits, limits, utility
• some standard exception types via exception, stdexcept
• very little (but some) of algorithm

Client application must provide:

• abort()
• malloc()/free()
• C++ ABI runtime support (see eg bmcxxabi, bmunwind)

Configuration

The "bmcxx_config.h" file contains configuration options for the library and can be edited or supplanted by an alternative version earlier on the include path. Currently the supported options are:

• BMCXX_DISABLE_EXCEPTIONS - if defined true, will prevent exceptions from being thrown by the library.

  This is not implemented in all headers/functions. However with this set it should be possible to compile with -fno-exceptions, and in that case use of headers which throw exceptions should generate a compilation error.

  In general, where an exception would have been thrown, std::terminate() will be called instead. One exception is that calls to new() will return nullptr on failure (as if new(nothrow) had been used) rather than terminating. To enable checking its return value the -fcheck-new option should be used when compiling code that does so.

• BMCXX_DISABLE_FLOAT - if defined true, floating-point functions (returning floating-point values or taking them as arguments) will be unavailable.

  This is currently a no-op since at present there are no such functions implemented anyway...

For build-time configuration, see comments in the top-level Makefile.

Incorporation

To use, build via "make" and specify any desired parameters on the command line. Specify OUTDIR=... to indicate where the final library (libbmcxx.a) should be placed:

make CC=gcc CXX=g++ OUTDIR=/some/project/directory

If not specified, defaults to using gcc/g++ and dropping the library in the "src" subdirectory.

In general building for a bare-metal target may require compilation flags to be specified (via CFLAGS/CXXFLAGS).

To change library configuration without modifying the included bmcxx_config.h file, copy the file to another directory and add that to the compiler's #include path (via CPPFLAGS/CXXPPFLAGS).

Limitations / known issues

• No floating point support
• No thread support
• Library headers are not "namespace pollution safe". For example, variable names (for local variables in functions defined in a header) do not begin with leading '__' as is done in mainstream implementations. The issue is that a user may define a macro with the same name as the variable before including the header and cause chaos. Probably won't fix; code is easier to read this way, and users just "shouldn't do that".

TODO

• optional thread support
• optional floating-point support
• optional heap allocator implementation (i.e. implementation of malloc/free etc)
• custom allocator support for collections/strings
• make "assert" useful
• option for non-replaceable versions of new/delete variants which act sensibly. (see for example nothrow new, which to be standard-conformant has to be implemented by calling the throwing new, and catching the exception, ugh).
• audit use of new(), make sure to check result for nullptr if exceptions are disabled