Const and volatile type qualifiers
const
const means that something is not modifiable, so a data object that is declared with const as a part of its type specification must not be assigned to in any way during the run of a program.
volatile
The use of volatile ensures that the compiler always carries out the memory accesses, rather than optimizing them out (for example if the access is in a loop).
Rules of engagement
int const xxx;
| | |
| | +-----------------> xxx is a
| +----------------------> constant
+---------------------------> integer
int const *ptr;
| | | |
| | | +----------------> ptr is a
| | +------------------> pointer to a
| +---------------------> constant
+---------------------------> integer
int * const ptr;
| | | |
| | | +---------------> ptr is a
| | +---------------------> constant
| +-----------------------> pointer to an
+---------------------------> integer
int const * const ptr;
| | | | |
| | | | +---------> ptr is a
| | | +--------------> constant
| | +------------------> pointer to a
| +----------------------> constant
+---------------------------> integer
int volatile * const ptr;
| | | | |
| | | | +------> ptr is a
| | | +-----------> constant
| | +---------------> pointer to a
| +---------------------> volatile
+---------------------------> integer
int volatile const * ptr;
| | | | |
| | | | +------> ptr is a
| | | +---------> pointer to
| | +---------------> constant
| +---------------------> volatile
+---------------------------> integer
Memorymapped access to a register
Example 1
In the example below the register 32 bit register located in memory at address 0x40000 can be accessed through the uartreg pointer.
In the example below uartreg is a pointer pointing to a unsigned int 0x40000. The pointer value in memory 0x40000 is a constant not what it points to in memory. Fx. a peripheral register.
unsigned int volatile * const uartreg = (unsigned int *) 0x40000;
*uartreg = 0x80000000; // Assign 80000000 to the 32 bit register located at memeory address 0x40000
...some code...
*uartreg |= 0x1 // *uartreg's contents is OR'et to 0x1. New contents = 0x80000001
Example 2
| From the Cortex M STM32F107VCs Reference manual - See picture below - that the size of the embedded Flash can be seen in KB from the Flash size register at memory location 0x1FFFF7E0. |
 |
Method 1: Reading the Flash size register uint16_t volatile * const flashsize = (uint16_t *) 0x1ffff7e0;
printf("Flashsize: %i KB\n\r", (int) *flashsize );
On the STM32F107VC the output to stdout would be Flashsize: 256 KB |
Method 2: Reading the Flash size register#define F_SIZE ((volatile uint16_t * const) 0x1ffff7e0) // Flash Size
printf("Flashsize: %i KB\n\r", (int) *F_SIZE );
On the STM32F107VC the output to stdout would be Flashsize: 256 KB Method 3: Reading the Flash size registerPutting the pointer in the macro #define F_SIZE (*(int volatile * const) 0x1ffff7e0) // Flash Size
printf("Flashsize: %i KB\n\r", (int) F_SIZE );
On the STM32F107VC the output to stdout would be Flashsize: 256 KB |