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.. highlight:: cpp
.. _lang-volatile:
``volatile``
============
The ``volatile`` keyword known is a variable *qualifier*. It is
usually used before the datatype of a variable, to modify the way in
which the compiler treats the variable.
Declaring a variable ``volatile`` is a directive to the compiler. The
compiler is software which translates your C++ code into the machine
code, which are the real instructions for the STM32 chip in the
Maple. (The particular compiler we provide for use with the Maple is a
version of :ref:`GCC <arm-gcc>`).
Specifically, it directs the compiler to read the variable's value
fresh every time it is used, rather than "backing up" the value and
reading from its backup copy. (Compilers often "back up" a variable's
value in RAM into a storage location called a *register*; this is done
for efficiency).
A variable should be declared ``volatile`` whenever its value can be
changed by something beyond the control of the code section in which
it appears, such as an :ref:`external interrupt
<external-interrupts>`. On the Maple, the only place that this is
likely to occur is in sections of code associated with interrupts.
Example
-------
::
// toggles LED when interrupt pin changes state
int pin = 13;
volatile int state = LOW;
void setup() {
pinMode(pin, OUTPUT);
attachInterrupt(0, blink, CHANGE);
}
void loop() {
digitalWrite(pin, state);
}
void blink() {
if (state == HIGH) {
state = LOW;
} else {
// state must be HIGH
state = HIGH;
}
}
See also
--------
- :ref:`External Interrupts <external-interrupts>`
- :ref:`lang-attachinterrupt`
- :ref:`lang-detachinterrupt`
.. include:: cc-attribution.txt
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