Search code examples
cpointerspointer-to-pointer

What is the difference between a , &a and *a?

I am trying to understand the difference between a and &a when a is a pointer.

in the following example code :

int main()
{
    int b = 100;
    int *a;
    a = &b;
    printf("%d %d %d", a , &a , *a);
    return 0;
}

According to my understanding, a is a name given to the address of a. That is :

enter image description here

Therefore I am expecting a and &a to be same when a is a pointer. But in the output, I am getting the first two values ( a and &a ) as different.

Where am I going wrong ?

Solution

  • First of all, use %p and cast the argument to void * for printing a pointer. Passing an incompatible (mismatched) type of argument for any conversion specification is undefined behavior.

    That said, even a pointer variable, is a variable, and has to be "stored" in an address. So, it's the address of a pointer type variable.

    In other words,

    • b is a variable (of type int) and it has an address.
    • a is a variable (of type int *) and it also has an address.

    To add some reference, quoting C11, chapter §6.5.3.2,

    The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier.

    and, from §6.3.2.1,

    An lvalue is an expression (with an object type other than void) that potentially designates an object. [...]