Volatile (keyword): Difference between revisions

The <code>volatile</code> keyword can be used in type definitions as well as function arguments, variable definitions and typecasts. Similar to the <code>const</code> keyword, it either makes the variable itself or the data it points to <code>volatile</code>, or both.

 

/* Ensures the changes to/reads from x are always performed */

volatile int x;

volatile int * volatile ptr;

int volatile * volatile ptr;

 

== Examples ==

This example is supposed to poll a byte until it is <code>0</code> (e.g. waiting for a [[Spinlock]] to be released). Obviously another thread is supposed to change that memory, but since the compiler has no clue about this, we need to ensure that the code isn't optimized away in any case:

 

typedef unsigned char * spin_lock;

/* Should be defined as '''typedef volatile unsigned char * spin_lock;'''

}

}

 

=== Dereferencing memory ===

This is an example of code that is supposed to touch a piece of memory (e.g. to make it become resident or to check if it's valid and doesn't raise a page fault):

 

static void touch_mem(void *ptr)

{

*data = *data;

}

 

Also see the [[APIC#IO_APIC_Configuration|APIC example code]] where the <code>volatile</code> keyword is crucial (reading/writing to hardware registers).