In one C file you can declare a global:

    int x;

and in another:

    int* x;

and you'll be mixing pointers and ints and it won't be detected.

I was not talking about this, but about aliasing a variable on the same translation unit.

    int x = 7;
    void f() { /* do things using x */ }
    void insidious_function(int *p) { *p = 3; }

now, inside f you cannot be sure that x equals 7, even if you never write into it. You may call some functions, that in turn call the insidious function that receives the address of x as a parameter. There's no way to be sure that the value of x is not changed, just by looking at your code.