static initialization of extern pointer to function

Does the rest of your initialized file scope data work correctly? That is, you have included INIT.A51 in your program to copy the initialized data section from ROM (code space) to RAM?

(Initialized global data is optional with the Keil tools, because such take up space in both ROM and RAM, and also add the time to copy to the startup time of the program. Note that many embedded systems need some sort of relatively soft reset / reinitialization which means you have to have write code to initialize variables anyway, and the extra boot-time initialization is redundant.

Constants can often be handled by declaring them in code space. Your function pointer presumably has to be able to change, though, so it must live in RAM. So, you need to add the code to support C initializers, or initialize the pointer yourself.)

Does the rest of your initialized file scope data work correctly? That is, you
have included INIT.A51 in your program to copy the initialized data section from
ROM (code space) to RAM?

I have just discovered an interesting feature. If the modules look like this

//can.c

int (*on_CAN_msg)(void) = 0;
	...
void CAN_int_handler(void) interrupt 7 using 1 {
	...

	if (on_CAN_msg)
		on_CAN_msg();
	...
}

//main.c

int CAN_msg_handler(void){
	...
}

extern int (*on_CAN_msg)(void) = CAN_msg_handler;

//can.c

extern int (*on_CAN_msg)(void);
	...
void CAN_int_handler(void) interrupt 7 using 1 {
	...

	if (on_CAN_msg)
		on_CAN_msg();
	...
}

//main.c

int CAN_msg_handler(void){
	...
}

int (*on_CAN_msg)(void) = CAN_msg_handler;

Don't worry too much about why your essentially incorrect method miraculously works --- just use the correct one, i.e. the one in which the variable is defined and initialization in the module that has the initializer available to it, rather than the one that wants to call through hat pointer.

And while you're at it, you should consider getting rid of all 'extern' declarations in .c files. #include header files instead --- that's what they're there for. I.e. there should be a "main.h" contain that extern declaration, and both can.c and main.c should #include that.

If you're going to use function pointers, you should read the following application note:

http://www.keil.com/appnotes/docs/apnt_129.asp

Jon