CODEWITHSUNDEEP
c++std
jotik
jotik

Reputation: 17920

Barebones C++ without standard library?

Compilers such as GCC and Clang allow to compile C++ programs without the C++ standard library, e.g. using the -nostdlib command line flag. It seems that such often fail to link thou, for example:

void f() noexcept { throw 42; }
int main() { f(); }

Usually fails to link due to undefined symbols like __cxa_allocate_exception, typeinfo for int, __cxa_throw, __gxx_personality_v0, __clang_call_terminate, __cxa_begin_catch, std::terminate() etc.

Even a simple

int main() {}

Fails to link with

ld: warning: cannot find entry symbol _start; defaulting to 0000000000400120

and is killed by the OS upon execution. Using -c the compiler still runs the linker which blatantly fails with:

ld: error in mytest(.eh_frame); no .eh_frame_hdr table will be created.

Is it a realistic goal to program and compile C++ applications or libraries without using and linking to the standard library? How can I compile my code using GCC or Clang on Linux? What core language features would one be unable to use without the standard library?

Upvotes: 21

Views: 17039

Answers (2)

user6339982
user6339982

Reputation: 326

You will basically find all of your questions answered at osdev.org, but I'll give a brief summary anyway.

When you give GCC -nostdlib, you are saying "no startup or library files". This includes:

  • crti.o, crtbegin.o, crtend.o and crtn.o. Generally kernel developers only care about implementing crti.o and crtend.o and let GCC supply crtbegin.o and crtend.o by passing -print-file-name= to the linker. Generally these are just stubs that consist of .init and .fini respectively, leaving room for GCC to shove the contents of crtbegin.o and crtend.o respectively. These files are necessary for calling global constructors/destructors.
  • You can't avoid linking libgcc (the "low-level runtime library" (-lgcc) because even if you pass -nostdlib GCC will emit calls to its functions whenever you use it, leading to inexplicable linking errors for seemingly no reason. This is the case even when you're implementing/porting a C library.
  • You don't "need" libstdc++ no, but typically kernel developers want it. Porting a C library then implementing the C++ standard library from scratch is an extremely difficult task.

Since you only want to get rid of the "standard library", but keeping libc (on a Linux system) you're essentially programming C++ with just a C library. Of course, there's nothing wrong with this and you do you, but ultimately I don't see the point unless you plan on developing a kernel.

Required reading:

OSDev's C++ page - If you really care about RTTI/exception support, it's more annoying to implement than it sounds. Typically people just pass -fno-rtti or -fno-exceptions and then worry about it down the line or not at all.

Upvotes: 31

n. m. could be an AI
n. m. could be an AI

Reputation: 120059

"Standard" is a misnomer. In this context it doesn't mean "the library (set of functions, classes etc) as defined by the C++ standard" but "the usual set of libraries and objects (compiled files in a certain format) gcc links with by default". Some of those are necessary for most or even all programs to function.

If you use this flag, it's your responsibility to provide any missing functionality. There are several ways to do so:

  1. Cherry-pick libraries and objects that your program really needs out of the default set. (Makes little sense as the result will most probably be exactly the same as with the default link flags).
  2. Provide your own implementation of missing functionality.
  3. Explicitly disable, through compiler flags, language features your program isn't using. I know of two such features: exceptions and RTTI. This is needed because the compiler needs to generate exceptions-related code and RTTI info even if these features are not explicitly used in this module.

Upvotes: 7

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