January 31, 2016

nullprogram.com/blog/2016/01/31/

Recently I’ve been experimenting with freestanding C programs on Windows. Freestanding refers to programs that don’t link, either statically or dynamically, against a standard library (i.e. libc). This is typical for operating systems and similar, bare metal situations. Normally a C compiler can make assumptions about the semantics of functions provided by the C standard library. For example, the compiler will likely replace a call to a small, fixed-size memmove() with move instructions. Since a freestanding program would supply its own, it may have different semantics.

My usual go to for C/C++ on Windows is Mingw-w64, which has greatly suited my needs the past couple of years. It’s packaged on Debian, and, when combined with Wine, allows me to fully develop Windows applications on Linux. Being GCC, it’s also great for cross-platform development since it’s essentially the same compiler as the other platforms. The primary difference is the interface to the operating system (POSIX vs. Win32).

However, it has one glaring flaw inherited from MinGW: it links against msvcrt.dll, an ancient version of the Microsoft C runtime library that currently ships with Windows. Besides being dated and quirky, it’s not an official part of Windows and never has been, despite its inclusion with every release since Windows 95. Mingw-w64 doesn’t have a C library of its own, instead patching over some of the flaws of msvcrt.dll and linking against it.

Since so much depends on msvcrt.dll despite its unofficial nature, it’s unlikely Microsoft will ever drop it from future releases of Windows. However, if strict correctness is a concern, we must ask Mingw-w64 not to link against it. An alternative would be PlibC, though the LGPL licensing is unfortunate. Another is Cygwin, which is a very complete POSIX environment, but is heavy and GPL-encumbered.

Sometimes I’d prefer to be more direct: skip the C library altogether and talk directly to the operating system. On Windows that’s the Win32 API. Ultimately I want a tiny, standalone .exe that only links against system DLLs.

Linux vs. Windows

The most important benefit of a standard library like libc is a portable, uniform interface to the host system. So long as the standard library suits its needs, the same program can run anywhere. Without it, the programs needs an implementation of each host-specific interface.

On Linux, operating system requests at the lowest level are made directly via system calls. This requires a bit of assembly language for each supported architecture ( int 0x80 on x86, syscall on x86-64, swi on ARM, etc.). The POSIX functions of the various Linux libc implementations are built on top of this mechanism.

For example, here’s a function for a 1-argument system call on x86-64.

long syscall1 ( long n , long arg ) { long result ; __asm__ volatile ( "syscall" : "=a" ( result ) : "a" ( n ), "D" ( arg ) ); return result ; }

Then exit() is implemented on top. Note: A real libc would do cleanup before exiting, like calling registered atexit() functions.

#include <syscall.h> // defines SYS_exit void exit ( int code ) { syscall1 ( SYS_exit , code ); }

The situation is simpler on Windows. Its low level system calls are undocumented and unstable, changing across even minor updates. The formal, stable interface is through the exported functions in kernel32.dll. In fact, kernel32.dll is essentially a standard library on its own (making the term “freestanding” in this case dubious). It includes functions usually found only in user-space, like string manipulation, formatted output, font handling, and heap management (similar to malloc() ). It’s not POSIX, but it has analogs to much of the same functionality.

Program Entry

The standard entry for a C program is main() . However, this is not the application’s true entry. The entry is in the C library, which does some initialization before calling your main() . When main() returns, it performs cleanup and exits. Without a C library, programs don’t start at main() .

On Linux the default entry is the symbol _start . It’s prototype would look like so:

void _start ( void );

Returning from this function leads to a segmentation fault, so it’s up to your application to perform the exit system call rather than return.

On Windows, the entry depends on the type of application. The two relevant subsystems today are the console and windows subsystems. The former is for console applications (duh). These programs may still create windows and such, but must always have a controlling console. The latter is primarily for programs that don’t run in a console, though they can still create an associated console if they like. In Mingw-w64, give -mconsole (default) or -mwindows to the linker to choose the subsystem.

The default entry for each is slightly different.

int WINAPI mainCRTStartup ( void ); int WINAPI WinMainCRTStartup ( void );

Unlike Linux’s _start , Windows programs can safely return from these functions, similar to main() , hence the int return. The WINAPI macro means the function may have a special calling convention, depending on the platform.

On any system, you can choose a different entry symbol or address using the --entry option to the GNU linker.

Disabling libgcc

One problem I’ve run into is Mingw-w64 generating code that calls __chkstk_ms() from libgcc. I believe this is a long-standing bug, since -ffreestanding should prevent these sorts of helper functions from being used. The workaround I’ve found is to disable stack protection.

-fno-stack-check -fno-stack-protector -mno-stack-arg-probe

(If you always write perfect code you don’t need this, amiright?)

Alternatively you could link against libgcc (statically) with -lgcc , but, again, I’m going for a tiny executable.

A freestanding example

Here’s an example of a Windows “Hello, World” that doesn’t use a C library.

#include <windows.h> int WINAPI mainCRTStartup ( void ) { char msg [] = "Hello, world!

" ; HANDLE stdout = GetStdHandle ( STD_OUTPUT_HANDLE ); WriteFile ( stdout , msg , sizeof ( msg ), ( DWORD []){ 0 }, NULL ); return 0 ; }

To build it:

x86_64-w64-mingw32-gcc -std=c99 -Wall -Wextra \ -nostdlib -ffreestanding -mconsole -Os \ -fno-stack-check -fno-stack-protector -mno-stack-arg-probe \ -o example.exe example.c \ -lkernel32

Notice I manually linked against kernel32.dll. The stripped final result is only 4kB, mostly PE padding. There are techniques to trim this down even further, but for a substantial program it wouldn’t make a significant difference.

From here you could create a GUI by linking against user32.dll and gdi32.dll (both also part of Win32) and calling the appropriate functions. I already ported my OpenGL demo to a freestanding .exe, dropping GLFW and directly using Win32 and WGL. It’s much less portable, but the final .exe is only 4kB, down from the original 104kB (static linking against GLFW).

I may go this route for the upcoming 7DRL 2016 in March.