How is the value of global variable changing in my code?

Question

I have declared global variable again after the main function but It still affects main function. I know that C allows a global variable to be declared again when first declaration doesn’t initialize the variable(It will not work in c++). If I assign the value after the main function it still works with two warning in c but gives error in c++.

I have debugged the code but it never reaches the line int a=10;.

#include <stdio.h>
#include <string.h>

int a;

int main()
{
    printf("%d",a);
    return 0;
}
/*a=10  works fine with following warnings in c.
        warning: data definition has no type or storage class
        warning: type defaults to 'int' in declaration of 'a' [-Wimplicit-int]|

        but c++ gives the following error
        error: 'a' does not name a type|
*/
int a=10;

The output is 10.

Answer 1

Several things:

• The first int a; is a tentative declaration; the second int a = 10; is a defining declaration.

• a is declared at file scope, so it will have static storage duration - this means that storage for it will be set aside and initialized at program startup (before main executes), even though the defining declaration occurs later in the source code.

• Older versions of C allow for implicit int declaration - if a variable or function call appears without a declaration, it is assumed to have type int. C++ does not support implicit declarations, so you will get an error.