setenv and dlopen

setenv and dlopen

[Henry Margies]

Hi all,

I have a problem using dlopen and setting the LD_LIBRARY_PATH with
putenv or setenv. 


Here is my test code:

#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
#include <unistd.h>

int main(int argn, char** argv)
{
        void* handle;
        int   ret;

        putenv("LD_LIBRARY_PATH=.");
        putenv("PATH=.");

        handle = dlopen("libmylib.so", RTLD_LAZY);

        if (NULL == handle) {
                printf("error: %s\n", dlerror());
        }

        if (2 == argn)
                return 0;

        ret = execlp("dltest", "dltest", "break", NULL);

        if (ret)
                perror("Error in starting dltest: ");
	return ret;
}


I also tried setenv and absolute path-names for LD_LIBRARY_PATH. Setting
LD_LIBRARY_PATH on the console works perfectly. And as you can see,
setting the PATH variable using putenv is also working for execlp.


Any suggestions?


Henry


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Re: setenv and dlopen

[Christoph Bussenius]

On Tue, Oct 25, 2005 at 07:52:04PM +0200, Henry Margies wrote:
> Hi all,

Hi Henry,

> I have a problem using dlopen and setting the LD_LIBRARY_PATH with
> putenv or setenv. 

I think this is because putenv does not really modify your process'es
environment (which is not possible afaik), but rather remembers the
given variable until you call some exec* function.  libc will then pass
the new environment to execve so that the new program will have the
modified environment.

Regards,
Christoph

-- 
``There's no dark side of the moon, really
Matter of fact, it's all dark''

--Pink Floyd

Re: setenv and dlopen

[Henry Margies]

Hi Christoph,

thank you for you answer.

> I think this is because putenv does not really modify your process'es
> environment (which is not possible afaik), but rather remembers the
> given variable until you call some exec* function.  libc will then pass
> the new environment to execve so that the new program will have the
> modified environment.

Well, depends on the point of view. For the user-space processes side
the environment values are all stored in char **environ and I can
remove, add and change values. But you are right, the kernel also holds
the original values I cannot change (which are shown
in /proc/PID/environ).

So it seems like execlp uses the local values from char **environ
(because it reads my PATH value) as dlopen uses the kernel version of
the environment variables (and do not know about my changes to
LD_LIBARAY_PATH).

Maybe dlopen runs far beyond my local processes scope and has no access
to my local environment or it just don't care about :-)

Any solutions to tell dlopen within my c program to search also in my
paths? 


Greetings,

Henry

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Re: setenv and dlopen

[Glynn Clements]

Henry Margies wrote:

> I have a problem using dlopen and setting the LD_LIBRARY_PATH with
> putenv or setenv. 

ld-linux.so.2 reads the value into a variable at startup, and uses the
stored value thereafter.

-- 
Glynn Clements <glynn@gclements.plus.com>