Chapter 4: Backbone of the OS and C++ runtime
C++ kernel run-time
A kernel can be written in C++ just as it can be in C, with the exception of a few pitfalls that come with using C++ (runtime support, constructors, etc).
The compiler will assume that all the necessary C++ runtime support is available by default, but as we are not linking libsupc++ into your C++ kernel, we need to add some basic functions that can be found in the cxx.cc file.
Caution: The operators
delete cannot be used before virtual memory and pagination have been initialized.
Basic C/C++ functions
The kernel code can't use functions from the standard libraries so we need to add some basic functions for managing memory and strings:
void itoa(char *buf, unsigned long int n, int base); void * memset(char *dst,char src, int n); void * memcpy(char *dst, char *src, int n); int strlen(char *s); int strcmp(const char *dst, char *src); int strcpy(char *dst,const char *src); void strcat(void *dest,const void *src); char * strncpy(char *destString, const char *sourceString,int maxLength); int strncmp( const char* s1, const char* s2, int c );
In the next step, we're going to define different types we're going to use in our code. Most of our variable types are going to be unsigned. This means that all the bits are used to store the integer. Signed variables use their first bit to indicate their sign.
typedef unsigned char u8; typedef unsigned short u16; typedef unsigned int u32; typedef unsigned long long u64; typedef signed char s8; typedef signed short s16; typedef signed int s32; typedef signed long long s64;
Compile our kernel
Compiling a kernel is not the same thing as compiling a linux executable, we can't use a standard library and should have no dependencies to the system.
Our Makefile will define the process to compile and link our kernel.
For x86 architecture, the followings arguments will be used for gcc/g++/ld:
# Linker LD=ld LDFLAG= -melf_i386 -static -L ./ -T ./arch/$(ARCH)/linker.ld # C++ compiler SC=g++ FLAG= $(INCDIR) -g -O2 -w -trigraphs -fno-builtin -fno-exceptions -fno-stack-protector -O0 -m32 -fno-rtti -nostdlib -nodefaultlibs # Assembly compiler ASM=nasm ASMFLAG=-f elf -o