C string doesn't work when cross compile c kernel.

[yuriyzam]

Issue: strings defined as

Code: Select all

const char* greeting_msg = "Hello, world!";

are not readable (defining them as const char greeting_msg[] solve problem).
Full source code:
https://github.com/YuRaZaKa/AltOS/tree/master
C file with issue:

Code: Select all

#include <stddef.h>
#include <stdint.h>

uint8_t* video;

void cls(){
    video = 0xb8000;
    for(size_t i = 0; i < 25*80; ++i){
        *(video++) = ' ';
        *(video++) = 0x1F;
    }
    video = 0xb8000;
}

void write(const char* data){
    for(size_t i = 0; data[i] != '\0'; ++i){
        *(video++) = data[i];
        *(video++) = 0x1F;
    }
}

void kmain(void){
    cls();
    const char* greeting_msg = "Hello, world!";
    write(greeting_msg);
}

Makefile

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osname = AltOS

init:
	rm -rf $(osname).iso
	rm -rf obj
	rm -rf bin
	rm -rf iso
	mkdir obj
	mkdir bin
	mkdir iso
	mkdir iso/boot
	mkdir iso/boot/grub
compile:
	x86_64-elf-gcc src/c/kmain.c -o obj/kmain.o -std=gnu99 -ffreestanding -O2 -Wall -Wextra -nostdlib
assembly:
	nasm -f elf64 src/asm/multiboot_header.asm -o obj/multiboot_header.o
	nasm -f elf64 src/asm/boot.asm -o obj/boot.o
	nasm -f elf64 src/asm/long.asm -o obj/long.o
link:
	x86_64-elf-ld -n -o bin/kernel.bin -T src/ld/linker.ld obj/kmain.o obj/multiboot_header.o obj/boot.o obj/long.o 

grubprepare:
	cp bin/kernel.bin iso/boot
	echo 'set timeout=0' >> iso/boot/grub/grub.cfg
	echo 'set default=0' >> iso/boot/grub/grub.cfg
	echo 'menuentry "AltOS" {' >> iso/boot/grub/grub.cfg
	echo '	multiboot2 /boot/kernel.bin' >> iso/boot/grub/grub.cfg
	echo '	boot'  >> iso/boot/grub/grub.cfg
	echo '}' >> iso/boot/grub/grub.cfg

image:
	grub-mkrescue -o $(osname).iso iso

vm:
	qemu-system-x86_64 -cdrom $(osname).iso

run: init compile assembly link grubprepare image vm

I am using latest cross compiled gcc (9.1.0) and binutils (2.32)

[MollenOS]

What happens when the code runs?

[yuriyzam]

What happens when the code runs?

If const char[] used it outputs "Hello, world!" as it should. Otherwise, nothing will be displayed (only blue background will).
Before that, it checks multiboot and set long mode. Maybe the problems are in GDT?

[iansjack]

The difference between the two is where the string is stored.

const char * - the string is stored in the .rodata section
const char[] - the string is stored on the stack

This should work, except for the fact that you are giving your linker conflicting information. When you compile kmain.c you link it at the same time. So kmain.o is an executable, not a simple object file. The linker assumed that the file is loaded at 0x400000 (the default), with the .rodata section at 0x400151.

You then link the executable again, this time telling it that the program is loaded at 0x100000, with the .rodata section at 0x1001f0. When the program runs it thinks the string is at 0x400151, and prints the empty data there.

The answer is that you need to use the -c switch when compiling kmain.c to produce an object file rather than an executable.

[yuriyzam]

Thanks, it worked/
Btw, how I should solve this for multiply c files?

Code: Select all

x86_64-elf-gcc -c src/c/kmain.c src/c/vgacolor.c -o obj/kmain.o -std=gnu99 -ffreestanding -O2 -Wall -Wextra -nostdlib
x86_64-elf-gcc: fatal error: cannot specify -o with -c, -S or -E with multiple files
compilation terminated.

[iansjack]

You compile each source file to an object file separately.

[Solar]

Perhaps check out the wiki on Makefiles.