Re: Figuring out start and end of sections

Re: Figuring out start and end of sections

[Felipe Balbi]

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Added back the list. Sorry for that, I had forgotten.

-- 
balbi

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From: Felipe Balbi <me@felipebalbi.com>
To: Ian Lance Taylor <iant@google.com>
Subject: Re: Figuring out start and end of sections
Date: Wed, 14 Apr 2010 09:17:33 +0300
Message-ID: <20100414061733.GA4285@gandalf>

Hi Ian,

I guess my mailer is fishy, didn't get your mail either

Ian Taylor <iant@google.com> writes:
>> 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.
>>
>> Do I need a specific linker script to achieve that or does the default
>> scripts give me possibility to find that out ?
> 
> If you are using the GNU linker, or gold, and you make the section
> name a valid C identifier, then the linker will automatically define
> symbols __start_SECNAME and __stop_SECNAME which you can use.

so you're saying that something if I name my section _init then I should
have __start__init and __stop__init identifiers ??

The following isn't compiling:

#include <stdio.h>

typedef int (*initcall_t)(void);

static int my_init(void)
{
	printf("hello world from _init\n");
}
static initcall_t __my_initcall_my_init
	__attribute__((__used__))
	__attribute__((section("_initcall"))) = my_init;

int main(int argc, char *argv[])
{
	initcall_t	*fn;

	for (fn = __start__initcall; fn <
			__stop__initcall; fn++)
		(void) fn();

	return 0;
}

do I need to pass any particular option to gcc/ld to get that working ??

-- 
balbi

Re: Figuring out start and end of sections

[Felipe Balbi]

Hi,

On Wed, Apr 14, 2010 at 09:22:35AM +0300, Felipe Balbi wrote:
> #include <stdio.h>
> 
> typedef int (*initcall_t)(void);
> 
> static int my_init(void)
> {
> 	printf("hello world from _init\n");
> }
> static initcall_t __my_initcall_my_init
> 	__attribute__((__used__))
> 	__attribute__((section("_initcall"))) = my_init;
> 
> int main(int argc, char *argv[])
> {
> 	initcall_t	*fn;
> 
> 	for (fn = __start__initcall; fn <
> 			__stop__initcall; fn++)
> 		(void) fn();
> 
> 	return 0;
> }

I commented the for loop just to get the thing compiling and nm doesn't
show any __start_ symbols:

$ nm tst | grep init
080494cc d __init_array_end
080494cc d __init_array_start
08048400 T __libc_csu_init
080495d4 d __my_initcall_my_init
08048298 T _init
080483c4 t my_init

-- 
balbi

Re: Figuring out start and end of sections

[Felipe Balbi]

Hi,

On Wed, Apr 14, 2010 at 09:30:43AM +0300, Felipe Balbi wrote:
> I commented the for loop just to get the thing compiling and nm doesn't
> show any __start_ symbols:

now I got it working:

#include <stdio.h>

typedef int (*initcall_t)(void);

extern initcall_t __start__initcall[];
extern initcall_t __stop__initcall[];

static int my_init(void)
{
	printf("hello world from _init\n");
}
static initcall_t __my_initcall_my_init
	__attribute__((__used__))
	__attribute__((section("_initcall"))) = my_init;

static void call_initcall(initcall_t fn)
{
	fn();
}

int main(int argc, char *argv[])
{
	initcall_t	*fn;

	for (fn = __start__initcall; fn
			< __stop__initcall; fn++)
		call_initcall(*fn);

	return
		0;
}

thanks a lot Ian and Andrew.

-- 
balbi

Re: Figuring out start and end of sections

[Fabian Cenedese]

>I commented the for loop just to get the thing compiling and nm doesn't
>show any __start_ symbols:
>
>$ nm tst | grep init
>080494cc d __init_array_end
>080494cc d __init_array_start
>08048400 T __libc_csu_init
>080495d4 d __my_initcall_my_init
>08048298 T _init
>080483c4 t my_init

Here's what we do for structures:

In the linker script:
        PROVIDE (__HWDEVICE_DESC_BEGIN = .);
        .struct_hw_dev_desc : {*(.struct_hw_dev_desc)} > ram
        PROVIDE (__HWDEVICE_DESC_END = .);

In the code:
#define STRUCT_HW_DEV_DESC __attribute__ ((section (".struct_hw_dev_desc")))

STRUCT_HW_DEV_DESC SINOSHwDeviceDesc { definition }

extern char* __HWDEVICE_DESC_BEGIN[];
extern char* __HWDEVICE_DESC_END[];

        SINOSHwDeviceDesc* desc = (SINOSHwDeviceDesc*) __HWDEVICE_DESC_BEGIN;
        desc++;

And then you can use these values for whatever you want
until you reach __HWDEVICE_DESC_END.

bye  Fabi

Re: Figuring out start and end of sections

[Andrew Haley]

On 04/14/2010 01:24 AM, Ian Lance Taylor wrote:
> Felipe Balbi <me@felipebalbi.com> writes:
> 
>> 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.
>>
>> Do I need a specific linker script to achieve that or does the default
>> scripts give me possibility to find that out ?
> 
> If you are using the GNU linker, or gold, and you make the section
> name a valid C identifier, then the linker will automatically define
> symbols __start_SECNAME and __stop_SECNAME which you can use.

Ah, that's a much better idea.  Forget my suggestion.

Andrew.

Re: Figuring out start and end of sections

[Ian Lance Taylor]

Felipe Balbi <me@felipebalbi.com> writes:

> 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.
>
> Do I need a specific linker script to achieve that or does the default
> scripts give me possibility to find that out ?

If you are using the GNU linker, or gold, and you make the section
name a valid C identifier, then the linker will automatically define
symbols __start_SECNAME and __stop_SECNAME which you can use.

Ian

Re: Figuring out start and end of sections

[Andrew Haley]

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.

Andrew.

Figuring out start and end of sections

[Felipe Balbi]

Hi all,

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.

Do I need a specific linker script to achieve that or does the default
scripts give me possibility to find that out ?

-- 
balbi