Thread: Trying to modify array in a shared library function

Really straight forward, but I'm trying to build a shared object and I'm testing it as I'm writing it (with a test exectuable I built using dlopen()). I've gotten to where I can print something from the shared object but when I pass an array I want modified it misbehaves.

Here is the shared object's source code:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <libconfig.h>
#include <sys/stat.h>
#include <sys/acl.h>
#include <dlfcn.h>
#include <string.h>
#include <libgen.h>
#include <grp.h>
#include <unistd.h>


int** scratch_modArr ( int ** givenList ){


        int numgroups=3;


        *givenList=(int*)malloc(  numgroups * sizeof(int) * 6 );
        **givenList=500;


        printf("modArr Ping: %d\n", (*givenList)[0]);


        return givenList;


}


void scratch_ping(){
        printf("main ping\n");
}

And the test executable's code:

Code:

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


int main (){


        void *(*scratch_ping)();
        int **(*scratch_modArr)();
        void *libHandle;
        int newArr[1]= {1};


        libHandle=dlopen("./libscratch.so", RTLD_LAZY);
        scratch_ping=dlsym(libHandle, "scratch_ping");
        scratch_modArr=dlsym(libHandle, "scratch_modArr");


        printf("Before Call: %d\n", newArr[0]);


        (*scratch_ping)();
        (*scratch_modArr)(newArr);


        printf("After Call: %d\n", newArr[0]);


        return 0;
}

This is the executable's command line output:

Code:

[root@thatch scratch]# ./scratch-test
Before Call: 1
main ping
modArr Ping: 500
After Call: 7986464
[root@thatch scratch]#

Obviously, something's gone awry. My assumption was that malloc gave me a new segment of memory which killed off my old pointer. But I'm not sure how to modify the return type on the functions so that it can do that.

Originally Posted by AndiPersti

Code:

int** scratch_modArr ( int ** givenList ){   // in shared library
...
int **(*scratch_modArr)();   // in executable

Shouldn't you also declare the correct input parameter in your executable?

Bye, Andreas

Thanks for the quick response!

I made the updates but now I'm getting the following error when I try to compile:

Code:

[root@thatch scratch]# make
cc scratch-test.c -o scratch-test -ldl
scratch-test.c: In function âmainâ:
scratch-test.c:18: error: incompatible types when assigning to type âint[1]â from type âint **â
make: *** [scratch-test] Error 1
[root@thatch scratch]#

This is line 18 from the executable:

Code:

 newArr=(*scratch_modArr)(newArr);

My understanding is that with "int **" I should be returning the array itself, so since I don't ever re-define newArr in the executable, it should be alright to just pass it back to newArr. Is that not correct?

Originally Posted by JulietBoy

Another question related to this, I was under the impression that in C arrays are pointers. When I typecast to either "int*" or "int**" it tells me that same thing about newArr being int[1]. Are they somehow different data types?

In C, arrays are not pointers, though in most contexts, an array will be converted to a pointer to its first element.