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what is static/volatile function mean

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ep.hobbyiest

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hi
i am bit confuse in static and volatile fnction. means where it is use and what is use of this. Most of the time i used variable's only. But i function i am watching first time.
 

Vbase

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Static

The static keyword is used to create variables that are visible to only one function. However unlike local variables that get created and destroyed every time a function is called, static variables persist beyond the function call, preserving their data between function calls.
Variables declared as static will only be created and initialized the first time a function is called.
Example


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/* RandomWalk
* Paul Badger 2007
* RandomWalk wanders up and down randomly between two
* endpoints. The maximum move in one loop is governed by
* the parameter "stepsize".
* A static variable is moved up and down a random amount.
* This technique is also known as "pink noise" and "drunken walk".
*/
 
#define randomWalkLowRange -20
#define randomWalkHighRange 20
int stepsize;
 
int thisTime;
int total;
 
void setup()
{
  Serial.begin(9600);
}
 
void loop()
{        //  tetst randomWalk function
  stepsize = 5;
  thisTime = randomWalk(stepsize);
  Serial.println(thisTime);
   delay(10);
}
 
int randomWalk(int moveSize){
  static int  place;     // variable to store value in random walk - declared static so that it stores
                         // values in between function calls, but no other functions can change its value
 
  place = place + (random(-moveSize, moveSize + 1));
 
  if (place < randomWalkLowRange){                    // check lower and upper limits
    place = place + (randomWalkLowRange - place);     // reflect number back in positive direction
  }
  else if(place > randomWalkHighRange){
    place = place - (place - randomWalkHighRange);     // reflect number back in negative direction
  }
 
  return place;
}



volatile keyword

volatile is a keyword known as a variable qualifier, it is usually used before the datatype of a variable, to modify the way in which the compiler and subsequent program treats the variable.
Declaring a variable volatile is a directive to the compiler. The compiler is software which translates your C/C++ code into the machine code, which are the real instructions for the Atmega chip in the Arduino.
Specifically, it directs the compiler to load the variable from RAM and not from a storage register, which is a temporary memory location where program variables are stored and manipulated. Under certain conditions, the value for a variable stored in registers can be inaccurate.
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 a concurrently executing thread. In the Arduino, the only place that this is likely to occur is in sections of code associated with interrupts, called an interrupt service routine.
Example

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// 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()
{
  state = !state;
}

 
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