Why do I need a Cross Compiler?: Difference between revisions

It is possible ask your compiler what target platform it is currently using by calling the command:

 

gcc -dumpmachine

 

If you are developing on 64-bit Linux, then you will get a response such as 'x86_64-unknown-linux-gnu'. This means that the compiler thinks it is creating code for Linux. If you use this GCC to build your kernel, it will use your system libraries, headers, the Linux [[libgcc]], and it will make a lot of problematic Linux assumptions. If you use a [[GCC_Cross-Compiler|cross-compiler]] such as i686-elf-gcc, then you get a response back such as 'i686-elf' that means the compiler knows it is doing something else and you can avoid a lot of problems easily and properly.

The compiler assumes it is targetting your local system, so you need a lot of options to make it behave. A trimmed down command sequence for compiling a kernel without a cross-compiler could look like this:

 

as -32 boot.s -o boot.o

gcc -m32 kernel.c -o kernel.o -ffreestanding -nostdinc

gcc -m32 my-libgcc-reimplemenation.c -o my-libgcc-reimplemenation.o -ffreestanding

gcc -m32 -T link.ld boot.o kernel.o my-libgcc-reimplemenation.o -o kernel.bin -nostdlib -ffreestanding

 

Actually, the average case is worse. People tend to add many more problematic or redundant options. With a real cross-compiler, the command sequence could look this this:

 

i686-elf-as boot.s -o boot.o

i686-elf-gcc kernel.c -o kernel.o -ffreestanding

i686-elf-gcc -T link.ld boot.o kernel.o -o kernel.bin -nostdlib -ffreestanding -lgcc

 

=== Reimplementing libgcc ===

You need to pass even more options to the command lines that build programs for your operating systems. You need a -Ipath/to/myos/include and -Lpath/to/myos/lib to use the C library, and more. If you set up an [[OS Specific Toolchain]], you just need

 

i686-myos-gcc hello.c -o hello

 

to cross-compile the hello world program to your operating system.