Posts Volatile keyword used in C programming

Volatile keyword used in C programming

Embedded Programmers are often asked about “volatile” keyword. I have tried explaining it in this article.So let’s start off with the definition.What does volatile keyword mean to C compiler? It meas “dont perform any sort of optimisation please. Specifically, reload the variable every time I access it”. Eg:

volatile int IamVolatile;
int IamNotVolatile;
int temp;
temp = IamVolatile; /* Fetches from the actual memory address.*/
temp = IamNotVolatile; /* Fetches from the actual memory address. */
temp = IamNotVolatile; /* Compiler MIGHT optimise this line. It MIGHT not go and fetch 
from memory address of ‘IamNotVolatile’…Runtime Performance optimistaion!.*/

temp = IamVolatile; /* Fetches from the actual memory address */


From the Programmer perspective ”volatile” variable is the one that can change unexpectedly. But why do you need this at all?If “IamVolatile” were to be a hardware register whose value is dependent on external h/w,there is no way we can do something in C without “volatile”.Here is a list of potential candidates for “volatile”:

1>Hardware registers. 2>Variables referenced in an ISR(interrupt service routine) and main code. 3>Variables shared across tasks in OS based Code. 4>Variables used across SetJump/Longjumps.

The point is where ever compiler tends to make optimisation due to of lack of static analysis shortcoming, it is potential candidate if need be.

“volatile” used along with other C keywords:

1> extern volatile unsigned int externalvar; /* volatile variable outside a module.*/

2> volatile const unsigned int PSW; /* An example is a read-only status register. It is volatile because it can change unexpectedly. It is const because the program should not attempt to modify it. / 3> volatile int ptr; /”ptr” points to VOLATILE memory./ Following is the scenario of excessive usage of volatile which introduces bugs in software:


volatile unsigned char *sptr = (unsigned char *) (0×1000);

unsigned int res;

res = acessserialportandMulBy3(sptr);


/* read serial port and return cube of it */
unsigned int acessserialportandMulBy3(volatile unsigned char *p)

unsigned int res;

res = ((*sptr) * (*sptr) * (*sptr) );

return res;


I leave it to you to find the bug in the above code.

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