C

Last updated Nov 8, 2022 Edit Source

1

int volatile foo;

Volatile is a qualifier that is applied to a variable when it is declared. It tells the compiler that the value of the variable may change at any time-without any action being taken by the code the compiler finds nearby.

These registers may have their values changed asynchronously during program flow. Code without this keyword can be optimised by the compiler into an infinite loop.

1
2
3
4
5

UINT1 * ptr = (UINT1 *) 0x1234;

// Wait for register to become non-zero.  
while (*ptr == 0);  
// Do something else.

Compiler interprets the ptr value is being always 0, as it has already loaded the value in the second line, resulting in an infinite loop:

1
2
3

mov ptr, #0x1234
mov a, @ptr
loop bz loop

Same situations can occur for variables that may be modified in ISRs or by multi-threaded applications.