Harnessing Volatile and Const in Embedded C 😁

This post explores the usage of the two prime keywords volatile and const in embedded applications and their significance in ensuring code correctness, reliability, and efficiency.

Let's start with the "volatile" keyword!

The volatile keyword is used to inform the compiler that a variable’s value may change unexpectedly, without the compiler’s knowledge. It tells the compiler not to consider that variable while optimizing the code. In real-time embedded systems this is crucial while dealing with hardware registers, shared memory, and interrupt service routines.

The volatile keyword also prevents variable from getting CACHED into CPU REGISTERS, but again this depends on that particular CPU’s optimization standards. Depending on it, a volatile variable can be or cannot be CACHED into CPU REGISTERS.

Consider an example of a 32-bit status register whose address is 0x40010000, let’s assume that your embedded application requires to poll this register continuously until it becomes zero. Once it becomes zero then your application is designed for doing some functionality. The code implementation for this would be something like this :

#include <stdio.h>
#include <stdint.h>

// Using “volatile” keyword with status_register.
volatile uint32_t *status_register = (uint32_t *) 0x40010000;

void main() {
    while(1)
    {
           if( *status_register == 0 )
           {
                // Application does some functionality here!
           }
    }

}

Since we are using volatile keyword for the status_register, it will make sure that the compiler fetches the value of status register from memory on each read, so it accurately fetches the hardware’s current state. The assembly (pseudocode) equivalent of the above code is given below:

Loop:
	LOAD *status_register, R0 ; Load the value of status_register from memory to register R0
	COMPARE R0, 0x0	          ; Compare R0 with 0x0
	JUMP_IF_NOT_EQUAL Loop 	  ; If not equal, jump back to Loop
	; Application does some functionality here

Now let’s write the same code without using volatile keyword with status_register. The code will look something like this:

#include <stdio.h>

// Not using “volatile” keyword with status_register.
uint32_t *status_register = (uint32_t *) 0x40010000;

int main()
{
    while(1)
    {
           if( *status_register == 0 )
           {
                // Application does some functionality here!
           }
    }

    return 0;

}

Since we are not using the volatile keyword the compiler might optimize the read operation by caching the value of the status register in a register and not fetching it from memory on every iteration. This can lead to incorrect behavior because the loop may not detect changes in the hardware status register. 

The assembly equivalent for the above code would be :

LoadStatus:
	LOAD *status_register, R0  ; Load the value of status_register from memory to register R0
Loop:
	COMPARE R0, 0x80000000 	; Compare R0 with 0x80000000
	JUMP_IF_NOT_EQUAL Loop 	; If not equal, jump back to Loop
	; Application does some functionality here!

Now, let's understand const keyword!

You can use the const keyword with a variable to inform the compiler that the value of that variable should not be changed after its first initialization. Let’s take a simple example below to understand how const works:

#include <stdio.h>
#include <stdio.h>

void main() {
  uint32_t const number = 10;
  uint32_t value;
  value = number * 2;
  printf( “ The variable value is : %d\n”, value );
}

Running the above code would give the below output, let’s try changing the variable number in the code.

❯ gcc main.c
❯ ./a.out   
The variable value is : 20

#include <stdio.h>

void main() {
  uint32_t const number = 10;
  number = number * 2;
  printf( “The variable number is : %d\n”, number );
}

Now this code would throw an error during compilation, since the variable number was qualified as const but was changed after its first initialization.

❯ gcc sample.c
❯ sample.c: In function ‘main’:
sample.c:6:10: error: assignment of read-only variable ‘number’
    6 |   number = number * 2;
      |          ^

It’s important to understand what are the different ways a const keyword could be used with a variable/pointer.

Type 1: Constant Integer

Example : uint32_t const value = 2;

The above definition of the variable tells that “value” is a constant and its variable cannot be changed in the code.

Type 2: Pointer To a Constant Integer

Example: uint32_t const *ptr = (uint8_t *)0x40000000;

The above definition of the variable tells us that ptra is a pointer pointing to a constant integer variable, the value of the ptr could be changed, but the value pointed to by ptr could not be changed.

Tip to read the above variable definition: Start reading from right to left and it goes in this way - ptr (ptr) is a pointer (*) pointing to a constant variable (const) of type integer (uint32_t). 

Type 3: Constant Pointer To an Integer

Example : uint32_t * const ptr = (uint8_t *)0x40000000;

The above definition of the variable tells us that ptr is a constant pointer pointing to an integer variable. The value of ptr is constant and the value pointed by ptr could be changed.

Type 4: Constant Pointer To a Constant integer

Example : uint32_t const * const ptr = (uint8_t *)0x40000000

In the above definition, ptr is a constant pointer to a constant integer value, in this scenario neither pointer could be modified nor the value pointed by it.

Now let’s take an example of using const in embedded systems: 

Consider there is a 32-bit register, whose address is being stored in a pointer called reg_a, so const would be used here to note that the value of the pointer cannot be changed during the course. The definition of the pointer would be as given below: