Rename symbol at link time

Rename symbol at link time

[DamienDaG]

Hi

I'm trying to build a very simple project composed of one source file and a
linker script : 

file f1.c :
#include <stdio.h>

extern void fct1_wrongname();

void fct9()
{
	printf("I'm fct9\n");
}

int main( int argc, char** argv)
{
	fct1_wrongname();
	return 0;
}
end of file f1.c

This file is compiled with command :
gcc -O0 -xc -g -Wall -c f1.c -of1.o
then build with :
ld -T'Link.cmd' -O0 -Map out.map -o rename.exe f1.o

file Link.cmd contains :

fct1_wrongname = fct9 ;

end of file Link.cmd


Function fct1_wrongname doesn't exist, and I want to replace the call of
this function by a call of fct9.

I always get this error message :
Link.cmd:4: undefined symbol `fct9' referenced in expression

I need to use a linker script (here Link.cmd) to apply this to a larger
project, and I can't use symbol definition (i.e. -D"fct1_wrongname=fct9")
because there are to many symbols.
I've been looking for a solution for a long time, but I couldn't find
anything.

Can anybody help me ?

Thanks in advance

Damien
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Re: Rename symbol at link time

[Václav Zeman]

On 6 June 2012 09:27, DamienDaG wrote:
>
> Hi
>
> I'm trying to build a very simple project composed of one source file and a
> linker script :
>
> file f1.c :
> #include <stdio.h>
>
> extern void fct1_wrongname();
>
> void fct9()
> {
>        printf("I'm fct9\n");
> }
>
> int main( int argc, char** argv)
> {
>        fct1_wrongname();
>        return 0;
> }
> end of file f1.c
>
> This file is compiled with command :
> gcc -O0 -xc -g -Wall -c f1.c -of1.o
> then build with :
> ld -T'Link.cmd' -O0 -Map out.map -o rename.exe f1.o
>
> file Link.cmd contains :
>
> fct1_wrongname = fct9 ;
>
> end of file Link.cmd
>
>
> Function fct1_wrongname doesn't exist, and I want to replace the call of
> this function by a call of fct9.
>
> I always get this error message :
> Link.cmd:4: undefined symbol `fct9' referenced in expression
>
> I need to use a linker script (here Link.cmd) to apply this to a larger
> project, and I can't use symbol definition (i.e. -D"fct1_wrongname=fct9")
> because there are to many symbols.
> I've been looking for a solution for a long time, but I couldn't find
> anything.
Is it possible that the linker script should contain leading
underscores for the symbols? E.g., _fct1_wrongname=_fct9? Take a look
at nm -AB output for the f1.o file.

-- 
VZ

Re: Rename symbol at link time

[DamienDaG]

Václav Zeman wrote:
> 
> On 6 June 2012 09:27, DamienDaG wrote:
>>
>> Hi
>>
>> I'm trying to build a very simple project composed of one source file and
>> a
>> linker script :
>>
>> file f1.c :
>> #include <stdio.h>
>>
>> extern void fct1_wrongname();
>>
>> void fct9()
>> {
>>        printf("I'm fct9\n");
>> }
>>
>> int main( int argc, char** argv)
>> {
>>        fct1_wrongname();
>>        return 0;
>> }
>> end of file f1.c
>>
>> This file is compiled with command :
>> gcc -O0 -xc -g -Wall -c f1.c -of1.o
>> then build with :
>> ld -T'Link.cmd' -O0 -Map out.map -o rename.exe f1.o
>>
>> file Link.cmd contains :
>>
>> fct1_wrongname = fct9 ;
>>
>> end of file Link.cmd
>>
>>
>> Function fct1_wrongname doesn't exist, and I want to replace the call of
>> this function by a call of fct9.
>>
>> I always get this error message :
>> Link.cmd:4: undefined symbol `fct9' referenced in expression
>>
>> I need to use a linker script (here Link.cmd) to apply this to a larger
>> project, and I can't use symbol definition (i.e. -D"fct1_wrongname=fct9")
>> because there are to many symbols.
>> I've been looking for a solution for a long time, but I couldn't find
>> anything.
> Is it possible that the linker script should contain leading
> underscores for the symbols? E.g., _fct1_wrongname=_fct9? Take a look
> at nm -AB output for the f1.o file.
> 
> -- 
> VZ
> 
> 

I tried many combinations with/without leanding underscores :
with
_fct1_wrongname = fct9 ;
I get 
Link.cmd:4: undefined symbol `fct9' referenced in expression

with
_fct1_wrongname = _fct9 ;
and
fct1_wrongname = _fct9 ;
I get 
ld: BFD 2.13 assertion fail ../../src/bfd/stabs.c:783

nm -AB f1.o outputs :
$ nm -AB f1.o
f1.o:00000000 b .bss
f1.o:00000000 d .data
f1.o:00000000 N .stab
f1.o:00000000 N .stabstr
f1.o:00000000 t .text
f1.o:         U ___main
f1.o:         U __alloca
f1.o:         U _fct1_wrongname
f1.o:         U _fct2
f1.o:00000012 T _fct9
f1.o:0000002a T _main
f1.o:         U _printf

Thanks

Damien
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Re: Rename symbol at link time

[Václav Zeman]

On 6 June 2012 10:12, DamienDaG wrote:
>
>
>
> Václav Zeman wrote:
>>
>> On 6 June 2012 09:27, DamienDaG wrote:
>>>
>>> Hi
>>>
>>> I'm trying to build a very simple project composed of one source file and
>>> a
>>> linker script :
>>>
>>> file f1.c :
>>> #include <stdio.h>
>>>
>>> extern void fct1_wrongname();
>>>
>>> void fct9()
>>> {
>>>        printf("I'm fct9\n");
>>> }
>>>
>>> int main( int argc, char** argv)
>>> {
>>>        fct1_wrongname();
>>>        return 0;
>>> }
>>> end of file f1.c
>>>
>>> This file is compiled with command :
>>> gcc -O0 -xc -g -Wall -c f1.c -of1.o
>>> then build with :
>>> ld -T'Link.cmd' -O0 -Map out.map -o rename.exe f1.o
>>>
>>> file Link.cmd contains :
>>>
>>> fct1_wrongname = fct9 ;
>>>
>>> end of file Link.cmd
>>>
>>>
>>> Function fct1_wrongname doesn't exist, and I want to replace the call of
>>> this function by a call of fct9.
>>>
>>> I always get this error message :
>>> Link.cmd:4: undefined symbol `fct9' referenced in expression
>>>
>>> I need to use a linker script (here Link.cmd) to apply this to a larger
>>> project, and I can't use symbol definition (i.e. -D"fct1_wrongname=fct9")
>>> because there are to many symbols.
>>> I've been looking for a solution for a long time, but I couldn't find
>>> anything.
>> Is it possible that the linker script should contain leading
>> underscores for the symbols? E.g., _fct1_wrongname=_fct9? Take a look
>> at nm -AB output for the f1.o file.
>>
>> --
>> VZ
>>
>>
>
> I tried many combinations with/without leanding underscores :
> with
> _fct1_wrongname = fct9 ;
> I get
> Link.cmd:4: undefined symbol `fct9' referenced in expression
>
> with
> _fct1_wrongname = _fct9 ;
> and
> fct1_wrongname = _fct9 ;
> I get
> ld: BFD 2.13 assertion fail ../../src/bfd/stabs.c:783
>
> nm -AB f1.o outputs :
> $ nm -AB f1.o
> f1.o:00000000 b .bss
> f1.o:00000000 d .data
> f1.o:00000000 N .stab
> f1.o:00000000 N .stabstr
> f1.o:00000000 t .text
> f1.o:         U ___main
> f1.o:         U __alloca
> f1.o:         U _fct1_wrongname
> f1.o:         U _fct2
> f1.o:00000012 T _fct9
> f1.o:0000002a T _main
> f1.o:         U _printf
I think I do not entirely understand your use case but wouldn't using
the alias attribute (see
http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Function-Attributes.html#Function-Attributes)
do what you want?

-- 
VZ

Re: Rename symbol at link time

[DamienDaG]

Václav Zeman wrote:
> 
> On 6 June 2012 10:12, DamienDaG wrote:
>>
>>
>>
>> Václav Zeman wrote:
>>>
>>> On 6 June 2012 09:27, DamienDaG wrote:
>>>>
>>>> Hi
>>>>
>>>> I'm trying to build a very simple project composed of one source file
>>>> and
>>>> a
>>>> linker script :
>>>>
>>>> file f1.c :
>>>> #include <stdio.h>
>>>>
>>>> extern void fct1_wrongname();
>>>>
>>>> void fct9()
>>>> {
>>>>        printf("I'm fct9\n");
>>>> }
>>>>
>>>> int main( int argc, char** argv)
>>>> {
>>>>        fct1_wrongname();
>>>>        return 0;
>>>> }
>>>> end of file f1.c
>>>>
>>>> This file is compiled with command :
>>>> gcc -O0 -xc -g -Wall -c f1.c -of1.o
>>>> then build with :
>>>> ld -T'Link.cmd' -O0 -Map out.map -o rename.exe f1.o
>>>>
>>>> file Link.cmd contains :
>>>>
>>>> fct1_wrongname = fct9 ;
>>>>
>>>> end of file Link.cmd
>>>>
>>>>
>>>> Function fct1_wrongname doesn't exist, and I want to replace the call
>>>> of
>>>> this function by a call of fct9.
>>>>
>>>> I always get this error message :
>>>> Link.cmd:4: undefined symbol `fct9' referenced in expression
>>>>
>>>> I need to use a linker script (here Link.cmd) to apply this to a larger
>>>> project, and I can't use symbol definition (i.e.
>>>> -D"fct1_wrongname=fct9")
>>>> because there are to many symbols.
>>>> I've been looking for a solution for a long time, but I couldn't find
>>>> anything.
>>> Is it possible that the linker script should contain leading
>>> underscores for the symbols? E.g., _fct1_wrongname=_fct9? Take a look
>>> at nm -AB output for the f1.o file.
>>>
>>> --
>>> VZ
>>>
>>>
>>
>> I tried many combinations with/without leanding underscores :
>> with
>> _fct1_wrongname = fct9 ;
>> I get
>> Link.cmd:4: undefined symbol `fct9' referenced in expression
>>
>> with
>> _fct1_wrongname = _fct9 ;
>> and
>> fct1_wrongname = _fct9 ;
>> I get
>> ld: BFD 2.13 assertion fail ../../src/bfd/stabs.c:783
>>
>> nm -AB f1.o outputs :
>> $ nm -AB f1.o
>> f1.o:00000000 b .bss
>> f1.o:00000000 d .data
>> f1.o:00000000 N .stab
>> f1.o:00000000 N .stabstr
>> f1.o:00000000 t .text
>> f1.o:         U ___main
>> f1.o:         U __alloca
>> f1.o:         U _fct1_wrongname
>> f1.o:         U _fct2
>> f1.o:00000012 T _fct9
>> f1.o:0000002a T _main
>> f1.o:         U _printf
> I think I do not entirely understand your use case but wouldn't using
> the alias attribute (see
> http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Function-Attributes.html#Function-Attributes)
> do what you want?
> 
> -- 
> VZ
> 
> 

I my case I'd like to replace a call to a function that doesn't exist by a
call to an existing function. This is to be applied to a project where a
function is called with a name (e.g. fct1_wrongname in my example), but is
implemented with another name (e.g. fct9). Since I cannot modify source
code, I'd like to make this with the help of a linker script.
It looks strange but in the final project, a part of the code calling
fct1_wrongname is generated automatically, so I cannot modify it.

I'm afraid I cannot use alias as you mentioned because it makes me modify
source code. 
In my example, I need to "rename" function. Eventually I'll need to "rename"
variables as well.

Damien
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Re: Rename symbol at link time

[Ángel González]

On 06/06/12 09:27, DamienDaG wrote:
> Hi
>
> I'm trying to build a very simple project composed of one source file and a
> linker script : 
> (...)
> This file is compiled with command :
> gcc -O0 -xc -g -Wall -c f1.c -of1.o
> then build with :
> ld -T'Link.cmd' -O0 -Map out.map -o rename.exe f1.o
>
> file Link.cmd contains :
>
> fct1_wrongname = fct9 ;
>
> end of file Link.cmd
>
>
> Function fct1_wrongname doesn't exist, and I want to replace the call of
> this function by a call of fct9.
>
> I always get this error message :
> Link.cmd:4: undefined symbol `fct9' referenced in expression
>
> I need to use a linker script (here Link.cmd) to apply this to a larger
> project, and I can't use symbol definition (i.e. -D"fct1_wrongname=fct9")
> because there are to many symbols.
> I've been looking for a solution for a long time, but I couldn't find
> anything.
>
> Can anybody help me ?
>
> Thanks in advance
>
> Damien
Works for me on Linux.
Just with that code I get undefined reference to `puts' (puts instead of
printf due to a gcc optimization),
which was solved just by adding -L/usr/lib -lc, and then I needed to add
-dynamic-linker because the default interpreter was the missing
/lib/ld64.so.1 (that's ELF specific, as you're building a PE you don't
need to worry).

It dies by sigkill as soon as i try to run it, though :S
Adding crti.o crtbegin.o crtend.o crtn.o doesn't solve the crashing.

I tried adding a dummy to make an alias, and it worked:
Just create a file link.s with content:
-- begin file --
.global fct1_wrongname
.set fct1_wrongname, fct9
fct1_wrongname:
-- end of file--

And add it to the compilation *before* f1.c:
gcc link.s f1.c -o rename.exe


(there are probably many things wrong with this approach, but it
apparently works)

Re: Rename symbol at link time

[Thomas Martitz]

Am 06.06.2012 12:58, schrieb DamienDaG:
> I my case I'd like to replace a call to a function that doesn't exist by a
> call to an existing function. This is to be applied to a project where a
> function is called with a name (e.g. fct1_wrongname in my example), but is
> implemented with another name (e.g. fct9). Since I cannot modify source
> code, I'd like to make this with the help of a linker script.
> It looks strange but in the final project, a part of the code calling
> fct1_wrongname is generated automatically, so I cannot modify it.
>
> I'm afraid I cannot use alias as you mentioned because it makes me modify
> source code.
> In my example, I need to "rename" function. Eventually I'll need to "rename"
> variables as well.
>

Can you not simply implement fctl_wrongname() and have it call fct9()? 
Perhaps by the means of linking an additional .o file that includes the 
wrapper? Since you can modify the linker script I assume you can change 
the linker command line too.

i.e. "void fctl_wrongname(void) { fct9(); }"

Best regards

Re: Rename symbol at link time

[Ian Lance Taylor]

DamienDaG <deusnei-div@yahoo.fr> writes:

> with
> _fct1_wrongname = _fct9 ;

This is the version you want, since this:

> nm -AB f1.o outputs :
> $ nm -AB f1.o
> f1.o:00000000 b .bss
> f1.o:00000000 d .data
> f1.o:00000000 N .stab
> f1.o:00000000 N .stabstr
> f1.o:00000000 t .text
> f1.o:         U ___main
> f1.o:         U __alloca
> f1.o:         U _fct1_wrongname
> f1.o:         U _fct2
> f1.o:00000012 T _fct9
> f1.o:0000002a T _main
> f1.o:         U _printf

prints leading underscores.

You should tell us that version of the GNU binutils you are using.  You
should tell us what type of system you are running on.

> I get 
> ld: BFD 2.13 assertion fail ../../src/bfd/stabs.c:783

The error message suggests that you are using GNU binutils 2.13, which
is very very old.  So the first thing I would suggest is using a current
version.

Ian

Re: Rename symbol at link time

[DamienDaG]

Ian Lance Taylor-3 wrote:
> 
> DamienDaG <deusnei-div@yahoo.fr> writes:
> 
>> with
>> _fct1_wrongname = _fct9 ;
> 
> This is the version you want, since this:
> 
>> nm -AB f1.o outputs :
>> $ nm -AB f1.o
>> f1.o:00000000 b .bss
>> f1.o:00000000 d .data
>> f1.o:00000000 N .stab
>> f1.o:00000000 N .stabstr
>> f1.o:00000000 t .text
>> f1.o:         U ___main
>> f1.o:         U __alloca
>> f1.o:         U _fct1_wrongname
>> f1.o:         U _fct2
>> f1.o:00000012 T _fct9
>> f1.o:0000002a T _main
>> f1.o:         U _printf
> 
> prints leading underscores.
> 
> You should tell us that version of the GNU binutils you are using.  You
> should tell us what type of system you are running on.
> 
>> I get 
>> ld: BFD 2.13 assertion fail ../../src/bfd/stabs.c:783
> 
> The error message suggests that you are using GNU binutils 2.13, which
> is very very old.  So the first thing I would suggest is using a current
> version.
> 
> Ian
> 
> 


Hi

Thank you for your answers.

I'm trying to build on WinXP 32 Pro, with MSys (1.0.10) / MinGW 2.0.
I'm currently trying to update MSys and MinGW.

@Thomas 
I could write a wrapper to solve my problem (i.e. void fctl_wrongname(void)
{ fct9(); } ), but I also need to rename global variables. I'm afraid a
wrapper wouldn't help.

@Ian
I'm actually using a quite old version of GNU binutils... I'm trying to
update.

Thanks again

Damien
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Re: Rename symbol at link time

[Václav Zeman]

On 7 June 2012 10:59, DamienDaG wrote:
>
>
>
> Ian Lance Taylor-3 wrote:
>>
>> DamienDaG <deusnei-div@yahoo.fr> writes:
>>
>>> with
>>> _fct1_wrongname = _fct9 ;
>>
>> This is the version you want, since this:
>>
>>> nm -AB f1.o outputs :
>>> $ nm -AB f1.o
>>> f1.o:00000000 b .bss
>>> f1.o:00000000 d .data
>>> f1.o:00000000 N .stab
>>> f1.o:00000000 N .stabstr
>>> f1.o:00000000 t .text
>>> f1.o:         U ___main
>>> f1.o:         U __alloca
>>> f1.o:         U _fct1_wrongname
>>> f1.o:         U _fct2
>>> f1.o:00000012 T _fct9
>>> f1.o:0000002a T _main
>>> f1.o:         U _printf
>>
>> prints leading underscores.
>>
>> You should tell us that version of the GNU binutils you are using.  You
>> should tell us what type of system you are running on.
>>
>>> I get
>>> ld: BFD 2.13 assertion fail ../../src/bfd/stabs.c:783
>>
>> The error message suggests that you are using GNU binutils 2.13, which
>> is very very old.  So the first thing I would suggest is using a current
>> version.
>>
>> Ian
>>
>>
>
>
> Hi
>
> Thank you for your answers.
>
> I'm trying to build on WinXP 32 Pro, with MSys (1.0.10) / MinGW 2.0.
> I'm currently trying to update MSys and MinGW.
>
> @Thomas
> I could write a wrapper to solve my problem (i.e. void fctl_wrongname(void)
> { fct9(); } ), but I also need to rename global variables. I'm afraid a
> wrapper wouldn't help.
I could agree with renaming handful of symbols to patch things up but
if you need to rename a lot more then you should probably focus on
fixing the source of the wrong symbols instead.

>
> @Ian
> I'm actually using a quite old version of GNU binutils... I'm trying to
> update.

-- 
VZ

Re: Rename symbol at link time

[DamienDaG]

Hi

As suggested by Ian, y tried this with another version of MinGW, and it
worked with 

_fct1_wrongname = _fct9

It also works when a variable is renamed.

Václav : of course I could rename symbols in source files, but I need to
compile code for an embedded target. This code is written either by
following strict coding rules, or by automatic generation. The target linker
script renames symbols (no problem) and I wanted to do the same to build an
executable on PC. Since the target compiler is based on gcc, I supposed it
was possible to build on PC.

Thanks again for your help.

Damien


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