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cscopeglobal-variables
vitaly.v.ch
vitaly.v.ch

Reputation: 2532

How can I access a shadowed global variable in C?

How can I access a shadowed global variable in C? In C++ I can use :: for the global namespace.

Upvotes: 39

Views: 14463

Answers (6)

David Neiss
David Neiss

Reputation: 8237

Yet another option is to reference the global before defining your local, or at least get a pointer to it first so you can access it after defining your local.

#include <stdio.h>

int x = 1234;
int main()
{
   printf("%d\n",x); // prints global
   int x = 456;
   printf("%d\n",x); // prints local
}

Upvotes: 1

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 507105

If your file-scope variable is not static, then you can use a declaration that uses extern in a nested scope:

int c;

int main() {
    {
        int c = 0;
        // now, c shadows ::c. just re-declare ::c in a 
        // nested scope:
        {
            extern int c;
            c = 1;
        }
        // outputs 0
        printf("%d\n", c);
    }
    // outputs 1
    printf("%d\n", c);
    return 0;
}

If the variable is declared with static, i don't see a way to refer to it.

Upvotes: 57

qrdl
qrdl

Reputation: 34998

If you are talking about shadowed global var, then (on Linux) you can use dlsym() to find an address of the global variable, like this:

int myvar = 5;    // global

{
    int myvar = 6;    // local var shadows global
    int *pglob_myvar = (int *)dlsym(RTLD_NEXT, "myvar");
    printf("Local: %d, global: %d\n", myvar, *pglob_myvar);
}

If you want your code to look sexy, use macro:

#define GLOBAL_ADDR(a,b)  b =(typeof(b))dlsym(RTLD_NEXT, #a)
...
int *pglob_myvar;
GLOBAL_ADDR(myvar, pglob_myvar);
...

Upvotes: 6

mouviciel
mouviciel

Reputation: 67859

Depending on what you call shielded global variable in C, different answers are possible.

If you mean a global variable defined in another source file or a linked library, you only have to declare it again with the extern prefix:

extern int aGlobalDefinedElsewhere;

If you mean a global variable shadowed (or eclipsed, choose the terminology you prefer) by a local variable of the same name), there is no builtin way to do this in C. So you have either not to do it or to work around it. Possible solutions are:

  • getter/setter functions for accessing global variable (which is a good practice, in particular in multithreaded situations)

  • aliases to globals by way of a pointer defined before the local variable:

    int noName;
{
    int * aliasToNoName = &noName; /* reference to global */
    int noName;                    /* declaration of local */
    *aliasToNoName = noName;       /* assign local to global */
}

Upvotes: 2

Dutow
Dutow

Reputation: 5668

There is no :: in c but you can use a getter function

#include <stdio.h>

int L=3;

inline int getL()
{
   return L;
}

int main();
{
   int L = 5;

   printf("%d, %d", L, getL());
}

Upvotes: 27

Peter Miehle
Peter Miehle

Reputation: 6070

what is a "shielded global variable" in pure C?

in C you have local variables, file local/global variables (static) and global variables (extern)

so file1.c:
int bla;

file2.c
extern int bla;

Upvotes: 1

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