Different platforms choose whether or not to automatically prefix
underscores and whether start or _start is the default symbol. On a
linux system, I was able to compile and link the following program:
void _start() { }
Using just:
gcc -nostdlib nm.c
The resulting program crashes, however, since it does not have what is
typically contained within crt0. The important thing to keep in mind,
is that start() should never return. Adding a trivial loop proves
this, as with the following variant:
void start() { while (1) { } }
Since you are running it within M5, I would suggest putting a trailing
m5_exit to keep it from running of into no-man's land.
- Clint
On Nov 12, 2008, at 7:18 PM, nathan binkert wrote:
> You should be able to just create a function called _start. Does that
> not work? What does nm say when you do that?
>
> I think the thing you're missing here is that you need to tell the
> linker that you're using _start as the entry point for your program.
> The kernel doesn't actually look for _start (you should be able to
> call it whatever you want). It looks in the elf header for the
> address of an entry point. Because you've told it not to use the
> stdlib, I'm guessing that the linker might not add the entry point if
> you don't tell it which symbol it is.
>
> Good luck.
>
> Nate
>
> On Wed, Nov 12, 2008 at 4:12 PM, Shoaib Akram <[EMAIL PROTECTED]>
> wrote:
>> Kind of not related to m5, but do you know how do I define my own
>> _start symbol in a c program?
>>
>> ---- Original message ----
>>> Date: Tue, 11 Nov 2008 01:05:35 -0800
>>> From: Gabe Black <[EMAIL PROTECTED]>
>>> Subject: Re: [m5-users] statictics
>>> To: M5 users mailing list <[email protected]>
>>>
>>> I believe you use the -nostdlib option to leave out all of the
>>> standard
>>> library stuff, including _start which kicks off all the things Ali
>>> mentioned and more and then finally calls main(). In your program
>>> you'll
>>> then need to define _start yourself. If you're writing your
>>> program in
>>> assembly, then all you need to do is include a label called _start
>>> and
>>> put everything you want to run after it. If your program uses any
>>> system
>>> calls you'll have to implement stubs for those since those are
>>> part of
>>> the standard library too.
>>>
>>> Gabe
>>>
>>> Ali Saidi wrote:
>>>> No it's probably from all the libc _start code that is executed.
>>>> Even
>>>> if main() is a single line there is much more code that is
>>>> included in
>>>> the binary to setup the environment properly, get the arguments in
>>>> order, see if the terminal is character or line buffered, etc. You
>>>> should be able to convince the linker to not include all of this -
>>>> fno-
>>>> builtin, but you'll need to define a _start symbol.
>>>>
>>>> Ali
>>>>
>>>> On Nov 10, 2008, at 11:21 PM, Shoaib Akram wrote:
>>>>
>>>>
>>>>> I wrote a simple program with 3 branches and ran it on m5. The
>>>>> statstics show much more branches and the number of instructions
>>>>> executed are also more than the assembly generated. Is it
>>>>> because of
>>>>> noise from emulated system calls?
>>>>> _______________________________________________
>>>>> m5-users mailing list
>>>>> [email protected]
>>>>> http://m5sim.org/cgi-bin/mailman/listinfo/m5-users
>>>>>
>>>>>
>>>>
>>>> _______________________________________________
>>>> m5-users mailing list
>>>> [email protected]
>>>> http://m5sim.org/cgi-bin/mailman/listinfo/m5-users
>>>>
>>>
>>> _______________________________________________
>>> m5-users mailing list
>>> [email protected]
>>> http://m5sim.org/cgi-bin/mailman/listinfo/m5-users
>> _______________________________________________
>> m5-users mailing list
>> [email protected]
>> http://m5sim.org/cgi-bin/mailman/listinfo/m5-users
>>
>>
> _______________________________________________
> m5-users mailing list
> [email protected]
> http://m5sim.org/cgi-bin/mailman/listinfo/m5-users
_______________________________________________
m5-users mailing list
[email protected]
http://m5sim.org/cgi-bin/mailman/listinfo/m5-users
