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catomicvolatile

Pass non-volatile variable's address to a volatile object pointer

Suppose I have the following function:

void atomic_add(volatile unsigned * loc, unsigned incr );

And I have want to pass a variable to the first argument of this function.

unsigned a;
atomic_add(&a, 1);

volatile unsigned b;
atomic_add(&b, 1);

From volatile type qualifier, it should can be compiled.
Q1: Do both of them get the same results?
Q2: Does the pointer returned from mmap() can also be used as the first argument of this function?
Thanks!

Solution

  • Different between volatile pointer pointing to volatile variable and non-volatile variable

    1. First of all loc is not a volatile pointer. It is a pointer referencing a volatile object

    2. You can pass volatile and not volatile to this function, bout localy in the function your ocject will be treated as volatile. It means that it will be read before every use, and saved to its storage after every change. It will not be "buffered" in he registers. Even if you *loc + *loc the referenced object will be read twice.

    To have a volatile pointer you need to declare it different way:

    unsigned * volatile loc; // volatile pointer to not volatile unsigned int object
volatile unsigned * volatile loc; // volatile pointer to volatile unsigned int object
volatile unsigned * loc; // non volatile pointer to volatile unsigned int object

    Q1 - yes

    Q2 - yes