dlopen - calling functions other way round

[Matt Churchyard]

I am using the dlopen,dlsym functions to load in an external module in my
program which works fine. I can call functions inside the loaded module and
all is well.
The problem is, when i try and call a function inside the main program from
the module,
it won't load giving 'Undefined symbol "getnumber"' errors. I can think of
no way round this.

Below is the test program I am using.
Passing a pointer for the getnumber() function is out of the question as the
real application
contains 30+ api functions which may all need to be accessible.

load.c
----------------------------------------------------------------------------
#include <stdio.h>
#include <dlfcn.h>
#include "load.h"

typedef int (*pointer)();

int main(void)
{
void *modhandle;
pointer symhandle;

modhandle = dlopen("./testmod.so", RTLD_LAZY);

if( modhandle == NULL )
{
printf("Load failed \"%s\"\n", dlerror());
exit(1);
}

symhandle = (pointer)dlsym(modhandle, "testfunc");

if( symhandle == NULL )
{
printf("Sym failed \"%s\"\n", dlerror());
exit(1);
}

(*symhandle)();

dlclose(modhandle);
}

int getnumber()
{
return 10;
}
----------------------------------------------------------------------------

load.h
----------------------------------------------------------------------------
int getnumber();
----------------------------------------------------------------------------

testmod.c
----------------------------------------------------------------------------
#include <stdio.h>
#include "load.h"

int testfunc()
{
int i;

i = getnumber();

printf("Hello %d\n", i);
}
----------------------------------------------------------------------------

--

Regards,
Matt Churchyard
_______________________________
Project Development Manager
Userve Internet
[email]matt@userve.net[/email]
[url]http://www.userve.net/[/url]

[Jens.Toerring@physik.fu-berlin.de]

Matt Churchyard <matt@userve.net> wrote:

> I am using the dlopen,dlsym functions to load in an external module in my
> program which works fine. I can call functions inside the loaded module and
> all is well.
> The problem is, when i try and call a function inside the main program from
> the module,
> it won't load giving 'Undefined symbol "getnumber"' errors. I can think of
> no way round this.

You must tell the linker that your main program exports symbols. If you're
e.g. the gcc and ld combination you need to do

gcc -Wl,-E -o load load.c -ldl

where the -Wl,-E tells gcc to invoke the linker with the -E option,
which in turn will make ld add all symbols to the dynamic symbol
table, so that they can be found by the dlopen'ed library. Of
course, you may need some other flags when you're using a different
compiler and linker.
Regards, Jens
--
_ _____ _____
| ||_ _||_ _| [email]Jens.Toerring@physik.fu-berlin.de[/email]
_ | | | | | |
| |_| | | | | | [url]http://www.physik.fu-berlin.de/~toerring[/url]
\___/ens|_|homs|_|oerring

[Måns Rullgård]

[email]Jens.Toerring@physik.fu-berlin.de[/email] writes:

>> I am using the dlopen,dlsym functions to load in an external module in my
>> program which works fine. I can call functions inside the loaded module and
>> all is well.
>> The problem is, when i try and call a function inside the main program from
>> the module,
>> it won't load giving 'Undefined symbol "getnumber"' errors. I can think of
>> no way round this.

>
> You must tell the linker that your main program exports symbols. If you're
> e.g. the gcc and ld combination you need to do
>
> gcc -Wl,-E -o load load.c -ldl
>
> where the -Wl,-E tells gcc to invoke the linker with the -E option,
> which in turn will make ld add all symbols to the dynamic symbol
> table, so that they can be found by the dlopen'ed library. Of
> course, you may need some other flags when you're using a different
> compiler and linker.

gcc -rdynamic does the same thing.

--
Måns Rullgård
[email]mru@users.sf.net[/email]

[Matt Churchyard]

thanks,
its all works perfectly with the -rdynamic switch

"Måns Rullgård" <mru@users.sourceforge.net> wrote in message
news:yw1xhe5kl1it.fsf@users.sourceforge.net...

> [email]Jens.Toerring@physik.fu-berlin.de[/email] writes:
>

> >> I am using the dlopen,dlsym functions to load in an external module in

my

> >> program which works fine. I can call functions inside the loaded module

and

> >> all is well.
> >> The problem is, when i try and call a function inside the main program

from

> >> the module,
> >> it won't load giving 'Undefined symbol "getnumber"' errors. I can think

of

> >> no way round this.

> >
> > You must tell the linker that your main program exports symbols. If

you're

> > e.g. the gcc and ld combination you need to do
> >
> > gcc -Wl,-E -o load load.c -ldl
> >
> > where the -Wl,-E tells gcc to invoke the linker with the -E option,
> > which in turn will make ld add all symbols to the dynamic symbol
> > table, so that they can be found by the dlopen'ed library. Of
> > course, you may need some other flags when you're using a different
> > compiler and linker.

>
> gcc -rdynamic does the same thing.
>
> --
> Måns Rullgård
> [email]mru@users.sf.net[/email]