volatile variable

[sekhar_ps]

can any one tell me whats the use of volatile variable,i know by declaring variable as volatile makes compiler not to optimize while compilation,but whats the use of not optimizing.can any one tell me with giving example.

[Banfa]

Easy if you work in embedded software (as I do) when you are accessing hardware external to the processor then you may wish to read or write to the same address more than once like so

#define CONFIG_REGISTER    ((ULONG *)0xC4000000)
#define INPUT_REGISTER    ((ULONG *)0xC4000004)
 
*CONFIG_REGISTER = 0x80003000;
*CONFIG_REGISTER = 0x40050000;
 
char data[10];
int x;
 
for( x=0; x<10; x++ )
{
    data[x] = *INPUT_REGISTER;
}
 

As declared this wont work because the optomiser will optomise out the multiple consecutive reads or writes to the same location, the volitile specifier stops this by informaing the compiler that it can assume nothing about how that variable works so correctly written this is

#define CONFIG_REGISTER    ((volatile ULONG *)0xC4000000)
#define INPUT_REGISTER    ((volatile ULONG *)0xC4000004)
 
*CONFIG_REGISTER = 0x80003000;
*CONFIG_REGISTER = 0x40050000;
 
char data[10];
int x;
 
for( x=0; x<10; x++ )
{
    data[x] = *INPUT_REGISTER;
}