jump to c++ static function from assemble error?

[jinglexy]

at the first period of my hobby os, i jump out assemble to c function,

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	lidt	idt_descr
	call	start_kernel
	cli

start_kernel define as:

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extern "C" void start_kernel(void)
{
        CKernel::start_kernel();
}

i think the c function may instead of c++ static function, in the init/main.cpp,
the start process defined as:

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class CKernel
{
public:
	static void start_kernel(void);
};

void Ckernel::start_kernel(void)
{
	......
}

after assemble, the init/main.o created, i get the Ckernel::start_kernel() function name as:

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readelf -a init/main.o > grep start_kernel

the function name mangling by g++ is "_ZN7CKernel12start_kernel",
then i jump to the kernel as:

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	lidt	idt_descr
	call	_ZN7CKernel12start_kernel
	cli

when link, an error occur, the link can not find function "_ZN7CKernel12start_kernel", why?

[Combuster]

you have been mixing CKernel with Ckernel at occasions, which are two completely different things. Try fixing that first

Also, please use

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tags. blue on blue is incredibly difficult to read. (So is white on white )

[JamesM]

You probably don't want to be calling the C++ function immediately anyway. The initial C trampoline code is an ideal place to initialise and destroy global objects as defined in the .ctor and .dtor sections of your binary.

JamesM

[jinglexy]

hi, i also puzzled what difference of c and c++ static func, how do solve it?

thank you a lot.

[os64dev]

well did you declare the kernel function extern in you assembly file?

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 loader.s
.extern __ZN7CKernel12start_kernelEv;

entry:
    call    __ZN7CKernel12start_kernelEv;
    jmp     .

compile with 'as -o loader.o loader.s'

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 kernel.cc
class CKernel
{
public:
   static void start_kernel(void);
};

void CKernel::start_kernel(void)
{
}

compile with 'g++ -o kernel.o -c kernel.cc'

link together with 'g++ -o test.exe loader.o kernel.o -nostartfiles'

no problems encountered. difference with you btw is leading underscore

[jinglexy]

in file head.S

.globl _ZN7CKernel12start_kernel

........
call call _ZN7CKernel12start_kernel

make output:
>>>>>> linking: vmjinix
ld -nostdlib -T /home/wyt/source/os/jinix-1.2.1/arch/i386/vmjinix.lds \
-o /home/wyt/source/os/jinix-1.2.1/arch/i386/vmjinix arch/i386/kernel/head.o arch/arch.t drivers/drivers.t fs/fs.t init/init.t ipc/ipc.t kernel/kernel.t lib/lib.t mm/mm.t net/net.t
arch/i386/kernel/head.o(.text+0xe6): In function `second_gdtr':
/home/wyt/source/os/jinix-1.2.1/arch/i386/kernel/head.S:121: undefined reference to `_ZN7CKernel12start_kernel'
make[1]: *** [vmjinix] error 1
make: *** [all] error 2

[os64dev]

sorry i edited the post: solution above

[jinglexy]

thank you very much, the problem resolved:

wyt@kdc-media ~/source/os/jinix-1.2.1]$ readelf -a init/main.o | grep start_kernel
22: 00000000 225 FUNC GLOBAL DEFAULT 1 _ZN7CKernel12start_kernel
[wyt@kdc-media ~/source/os/jinix-1.2.1]$ nm init/main.o | grep start_kernel
00000000 T _ZN7CKernel12start_kernelEv

the function read by readelf is truncated,
only "nm" get the correct func name, that's strange of readelf.

[JamesM]

hi, i also puzzled what difference of c and c++ static func, how do solve it?

thank you a lot.

A C++ static member function is one which is a member of a class, but is called without a "this" pointer. So it does not have access to any member variables of a class instance.

A C (or C++) static global function is one who's symbol is not exported. So, if you had 2 files:

a.c:

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extern int func();
void main() {
  func();
}

b.c:

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static int func() {
 return 1;
}

This would not link, complaining of an undefined symbol - func. If func() was declared without the static modifier, it would link.

JamesM