GCC question

[Sascha Hauer]

On Wed, Jan 09, 2013 at 10:02:32AM +0100, Franck Jullien wrote:
> Hi,
> 
> I have a question not directly related to Barebox but I think I can
> find some answer here: )
> 
> I would like to use initcalls in a Linux user's land program on a x86 target.
> 
> I'm doing something like this:
> 
> #ifndef _INIT_H
> #define _INIT_H
> 
> typedef int (*initcall_t)(void);
> 
> extern initcall_t __start_target, __stop_target;
> 
> #define target_initcall(fn)	static initcall_t _##fn \
> 				__attribute__((used)) \
> 				__attribute__ ((section("target"))) = fn
> 
> #endif
> 
> then:
> 
> 	initcall_t *initcall;
> 
> 	for (initcall = &__start_target;
> 			initcall < &__stop_target; initcall++) {
> 		printf("initcall-> %p\n", *initcall);
> 		ret = (*initcall)();
> 		if (ret)
> 			printf("initcall %p failed: %d\n", *initcall, ret);
> 	}
> 
> Everything looks fine except the linker removes the function
> "initcalled" because it is not
> referenced anywhere and this is normal.
> 
> I have not modified the linker script (I'm using the default one). I'm
> using auto generated
> __start_target and __stop_target symbols generated by the linker.
> 
> My question is: why does it work in barebox ? For example, in
> nios2/generic.c we have only
> static function and initcalls. So why the linker does optimize out
> those functions ? Is it
> because we have initcall corresponding sections in the linker script ?

Yes. In nios2 this is:

	__barebox_initcalls_start = .;
	.barebox_initcalls : { INITCALLS }
	__barebox_initcalls_end = .;

With INITCALLS being defined as:

#define INITCALLS			\
	KEEP(*(.initcall.0))			\
	KEEP(*(.initcall.1))			\
	KEEP(*(.initcall.2))			\
	KEEP(*(.initcall.3))			\
	KEEP(*(.initcall.4))			\
	KEEP(*(.initcall.5))			\
	KEEP(*(.initcall.6))			\
	KEEP(*(.initcall.7))			\
	KEEP(*(.initcall.8))			\
	KEEP(*(.initcall.9))			\
	KEEP(*(.initcall.10))			\
	KEEP(*(.initcall.11))

The 'KEEP' keyword keeps the linker from throwing away these.

Sascha

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