Accessing from outside the global variables declared inside a shared library.

I have a simple c file (mylib.so)compiled as a .so on Mac. This file has two global variables, one is an integer and the other is a pointer pointing to heap. From outside the .so, i.e..from another file (myapp.c) I see that I can access the global integer variable defined inside the .so but when I try to access the pointer, its value is NULL. I have declared both as externs in myapp.c. Was wondering why the global pointer is NULL ?

Comments

[Loki Le DEV]

Who is responsible to initializing your pointer?
You should have something like an init method in mylib.c to initialize it.

Could you share some code?
It's a bad design anyway, why do you need a global variable in your library?
A better design would be to have a "context structure" containing the data and make the methods of your library work on this structure.

// myLib.h

struct my_lib_context
{
   int* p; // some pointer
};

void my_lib_init(my_lib_context* ctx);
void my_lib_deinit(my_lib_context* ctx);

void my_lib_do_something(my_lib_context* ctx);
// etc

// myLib.c

#include <stdlib.h> 

#include "myLib.h"

void my_lib_init(my_lib_context* ctx)
{
   ctx->p = malloc(sizeof(int)); // init your data here
}

void my_lib_do_something(my_lib_context* ctx)
{
  // Do something on ctx here
}

void my_lib_init(my_lib_context* ctx)
{
   free(ctx->p);
}

Now you can use this context in your app.

// myApp.c

#include "myLib.h"

int main()
{

struct my_lib_context ctx;

my_lib_init(ctx)

// work here.

return 0;
}

Note: it lacks error handling in my example.

I'm not sure it answers your question here but in a general sense you shouldn't rely on global variables. You could make one static context struct for your library in myLib.cc also.

[vg283]

Thanks a lot for your reply. This is definitely much better than using global variables directly. Since it didnt work, I too had something similar to what you have mentioned. I had getter functions to access the global pointer. I still want to know why exactly the global pointer was NULL. Since the lib and the app both are part of same memory space. Here is a snippet which had the problem.

mylib.c

struct node *pHead;

node *add_new_head(int val)
{
pHead = malloc();
...
...
return pHead;
}

myapp.c

extern node *pHead;

int main()
{
//add, move , delete nodes
node *itr = pHead
while(itr) {
//do something
}
}

[Loki Le DEV]

global variables are initialized to 0 by default because they are stored in a different section of memory than automatic variables.

stackoverflow.com/a/14052894/5547634