Figurig out start and end of sections

Andrew Haley aph@redhat.com
Wed Apr 14 08:49:00 GMT 2010


On 04/14/2010 06:08 AM, Felipe Balbi wrote:
> Hi Andrew,
> 
> for some reason I didn't receive your reply.
> 
>> On 04/13/2010 06:40 PM, Felipe Balbi wrote:
>>> is there any way to figure out where a section starts and ends ?
>>>
>>> I added a specific section to my program using
>>> __attribute__((section "<section name>")) and now I want to figure out
>>> where that section starts so I can iterate over it and call the
>>> function
>>> pointers I'm adding to it.
>>
>> Terminate the list with a null pointer; AFAIK that's what everyone else
>> does.  To get the start address, you just need to define a global
>> variable
>> in that section.  Of course this means you have to link everything in
>> the
>> correct order.
> 
> I think you're talking about something like:
> 
> void (*func_ptr)(void)[] __attribute__((section ".my_section")) = {
> 	func1,
> 	func2,
> 	func3,
> 	NULL,
> };

No.

void (*func_ptr)(void) __attribute__((section ".my_section") = foo;

void (*func_ptr)(void) __attribute__((section ".my_section") = bar;

etc, etc.  I'm assuming the entries are in different files.
And in the last file,

void *poo __attribute__((section ".my_section") = NULL;

> is that right ? (the code above isn't really correct).
> 
> I don't really have an array. I'm trying to mimic what the kernel does
> with the modules, but then again, the kernel declares the constants for
> the start and end of initcalls in constants within the ld script.
> 
> So what I have right now is:
> 
> main.c:
> 
> [..]
> 
> int __attribute__((section(".init"))) register_struct(struct my_struct *ptr)
> {
> 	[..]
> }
> 
> [..]
> 
> static int call_initcalls(void)
> {
> 	initcall_t	*fn;
> 
> 	for (fn = __start_initcall; fn; fn++)
> 		fn();

I don't understand this.  How can you increment a function pointer?


> 
> 	return 0;
> }
> 
> then on another source code, I define an initcall_t.
> 
> initcall.c:
> 
> [..]
> 
> static int __attribute__((section(".init"))) init(void)
> {
> 	return register_struct(ptr);
> }
> 
> static initcall_t initcall_t_init __attribute__((section(".my.init")))
> 	__attribute__((__used__)) = init;
> 
> then several little "plugins" would define that and all those pointers
> are going to the right section. Now how to fetch them and call them ??
> currently I'm getting segfault. :-p
> 



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