format COFF
use32
extrn start
call start
    

#include <stdio.h>

extern void start( void )
{
        printf ("test");
}
    

LINK : fatal error LNK1561: entry point must be defined
    

Firstly I'm not good with C, so this might be wrong.

But, when you write a boot sector you don't define an entry point because the output format should be a binary file. That is, it won't be an exe format, so there is no way to define an entry point. The BIOS will simply jump to the first byte of the sector.

But you are now trying to link with your boot sector. So the linker wants to know the entry point. But there is no way to define it.

Perhaps you can make a dummy entry point to satisfy the linker. It won't matter what you make it since it is never used.

format COFF
use32
public main as '_main'
extrn _start
main:
    call _start
    

#include <stdio.h>

extern void __cdecl start()
{
        printf ("test");
}
    

What library are you using for printf?

Also, I would recommend you make a binary file. Loading obj files is quite tricky. You could do it, but it makes the boot sector needlessly complex.

stdio.h is for the OS binding. I don't think C has a BIOS version of stdio.

I think to make your binary you would first link the obj files so that the symbol binding is done at that stage. And then you convert it to a binary as the last step.

You don't really need to link your boot sector with the second stage.

You can make your second stage binary have its entry point as the first byte. Or just put a pointer at the first dword and your boot sector can read that to jump to the entry.

I was assuming your C code was the second stage. But my comment can still apply if your C code is some other stage. If you only have one symbol to bind then it is quite easy to do. And that way you can update each stage individually without affecting the other stages.

use32
;...
jmp 0x10000 ;jump to the first byte and start executing    

use32
;...
mov eax,[0x10000] ;read the offset
add eax,0x10000 ;add the load address
jmp eax ;execute    

If we assume your boot sector is at 0:7c00 and you load the second stage C code at 0x10000 then do this:

Code:

use32
;...
jmp 0x10000 ;jump to the first byte and start executing    

Or, it you have a pointer:

Code:

use32
;...
mov eax,[0x10000] ;read the offset
add eax,0x10000 ;add the load address
jmp eax ;execute    

For the C code side of things, I don't know. But you will still need to make a binary from the obj file. But you won't need to link to the assembly.

found this. it's in nasm but if fasm produces object files that are compatible then it should work the same.

https://www.devdungeon.com/content/how-mix-c-and-assembly

extrn '__imp__HeapAlloc@12' as HeapAlloc:dword