Shared libraries: How to share global variaables?

Shared libraries: How to share global variaables?

[Naga Raju]

Is it possible to share global variables such that all applications
which use shared libraries can see the changes made to the global
variables by the other applications.

I use gcc and compiled
      gcc -shared -Wl,-soname,xyz.so.1 -o libxyz.so.1.0 -lxyz2  abc.o

Regards,
Nagaraju.

Re: Shared libraries: How to share global variaables?

[Robin Doer]

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Am Samstag, 30. Oktober 2004 07:21 schrieb Naga Raju:
> Is it possible to share global variables such that all applications
> which use shared libraries can see the changes made to the global
> variables by the other applications.
>
> I use gcc and compiled
>       gcc -shared -Wl,-soname,xyz.so.1 -o libxyz.so.1.0 -lxyz2  abc.o
>

Well, imho you can use the "extern" keyword.

See the following example:

bash-2.05b$ cat foo.c
const char* foo = "Hello World"; /* Global variable foo */

bash-2.05b$ cat bar.c
#include <stdio.h>

extern const char* foo;

int main(int argc, char* argv[]) {
  printf("%s\n", foo);

  return 0;
}

bash-2.05b$ gcc -shared -Wl,-soname,libfoo.so -o libfoo.so foo.c
bash-2.05b$ gcc -Wall bar.c -L. -lfoo -o bar
bash-2.05b$ ./bar
Hello World

> Regards,
> Nagaraju.

Bye,
Robin

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Re: Shared libraries: How to share global variaables?

[Naga Raju]

Hi Robin,
Thanks for the help.
But my problem is little different.
In the example you have given, I initialized

char foo[100]="";

and in one application I set

application 1:

extern char foo[100];
strcpy(foo,"Test message");
printf("%s",foo);

and in applications 2:
extern char foo[100];
printf("%s",foo);

But application1 disaplays

Test message.

application2:

(No output ..it prints '\0' )

I want application2 to display "Test message"

Why the variables in two applications are different?
Why are they not using the same global variable?

Regards,
Nagaraju



On Sat, 30 Oct 2004 14:50:11 +0200, Robin Doer <robin@robind.de> wrote:
> Am Samstag, 30. Oktober 2004 07:21 schrieb Naga Raju:
> 
> 
> > Is it possible to share global variables such that all applications
> > which use shared libraries can see the changes made to the global
> > variables by the other applications.
> >
> > I use gcc and compiled
> >       gcc -shared -Wl,-soname,xyz.so.1 -o libxyz.so.1.0 -lxyz2  abc.o
> >
> 
> Well, imho you can use the "extern" keyword.
> 
> See the following example:
> 
> bash-2.05b$ cat foo.c
> const char* foo = "Hello World"; /* Global variable foo */
> 
> bash-2.05b$ cat bar.c
> #include <stdio.h>
> 
> extern const char* foo;
> 
> int main(int argc, char* argv[]) {
>  printf("%s\n", foo);
> 
>  return 0;
> }
> 
> bash-2.05b$ gcc -shared -Wl,-soname,libfoo.so -o libfoo.so foo.c
> bash-2.05b$ gcc -Wall bar.c -L. -lfoo -o bar
> bash-2.05b$ ./bar
> Hello World
> 
> > Regards,
> > Nagaraju.
> 
> Bye,
> Robin
> 
> 
>

Re: Shared libraries: How to share global variaables?

[Jan-Benedict Glaw]

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On Sat, 2004-10-30 14:50:11 +0200, Robin Doer <robin@robind.de>
wrote in message <200410301450.15154.robin@robind.de>:
> Am Samstag, 30. Oktober 2004 07:21 schrieb Naga Raju:
> > Is it possible to share global variables such that all applications
> > which use shared libraries can see the changes made to the global
> > variables by the other applications.
> 
> Well, imho you can use the "extern" keyword.
> See the following example:
> bash-2.05b$ cat foo.c
> const char* foo = "Hello World"; /* Global variable foo */
> 
> bash-2.05b$ cat bar.c
> #include <stdio.h>
> 
> extern const char* foo;
> 
> int main(int argc, char* argv[]) {
>   printf("%s\n", foo);
> 
>   return 0;
> }
> 
> bash-2.05b$ gcc -shared -Wl,-soname,libfoo.so -o libfoo.so foo.c
> bash-2.05b$ gcc -Wall bar.c -L. -lfoo -o bar
> bash-2.05b$ ./bar
> Hello World

As stated, the initial author had something different in mind: to have
two programs linking one common shared library in a way that one program
(in it's VM space) changes a (common to both programs) variable and
automagically, this variable also changes in the 2nd program.

Actually, early Windows versions had something like that.

It's possibly to achieve this by some hacks, but I won't tell you how to
do that--it's evil.

Just use shared memory; asking Google for "linux programming shared
memory" will give you some examples and book tips.

MfG, JBG

-- 
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