Now, a is local to the main function. You are trying to access it as a global variable in inc(). Try this instead:
Code:
#include <iostream>

void inc(int* b)
{
    *b += 50;
}

int main()
{
    int a = 100;
    std::cout << "a = " << a << std::endl;
    inc(&a);
    std::cout << "a = " << a << std::endl;
}