Semaphore Library

When multiple tasks running simultaneously, how to avoid two tasks from using the same single resource at the same time? 

Let's take an example with the Serial port and examine the results.

• Without semaphore
  

3641 : mySemaphore3      3 (-)

3660 : mySemaphore1    1   (-)

3699 : mySemaphore2     2  (-)

3781 : mySemap32  hore1    1   (-)

3813 : mySemaphore3      3 (-)

3862 : mySemaphore2     2  (-)

3904 : mySemaphore1    1   (-)

3944 : mySemaphore2     2  (-)

3985 : mySemaphore3      3 (-)

4022Syrr

)

4107 : mySemaphore2     2  (-)

4147 : mySemaphore1    1   (-)

4157 : mySemaphore3      3 (-)

4189 : mySemaphore2     2  (-)

4269 : mySemaphore1    1 4-: mySemaphore2     2  (-)

4329 : mySemaphore3      3 (-)

4353 : mySemaphore2     2  (-)

• With semaphore
  

0 : mySemaphore1    1   (0)

52 : mySemaphore2     2  (0)

104 : mySemaphore3      3 (0)

156 : mySemaphore1    1   (0)

208 : mySemaphore2     2  (0)

276 : mySemaphore3      3 (0)

328 : mySemaphore1    1   (0)

380 : mySemaphore2     2  (0)

448 : mySemaphore3      3 (0)

500 : mySemaphore1    1   (0)

552 : mySemaphore2     2  (0)

620 : mySemaphore3      3 (0)

672 : mySemaphore1    1   (0)

724 : mySemaphore2     2  (0)

792 : mySemaphore3      3 (0)

844 : mySemaphore1    1   (0)

896 : mySemaphore2     2  (0)

964 : mySemaphore3      3 (0)

What's happening? 

• Without semaphore, the three tasks send text to Serial.print(), and the result is scrambled.
  
• With semaphore, only one tasks can send text to Serial.print(), and the result is clean.
  

Using a Semaphore

The project includes four files: a header file .h and three sketches .ino.

The header file rtosGlobals.h contains the global variables, here the semaphore mySemaphore, shared across the three tasks, 

// *** Header rtosGlobals.h

// Core library

#include "Energia.h"

#ifndef rtosGlobals_h

#define rtosGlobals_h

// Include application, user and local libraries

#include "Semaphore.h"

// Semaphore for Serial

Semaphore mySemaphore;

#endif

The three sketches are very similar.

The setup() function on the main sketch initialises the Serial port and the semaphore mySemaphore.  

The parameter 1 means there's only one resource to share, here the Serial port.

The loop() function is explained below.

// *** Sketch SemaphoreLibrary.ino

// Core library 

#include "Energia.h"

// Include application, user and local libraries

#include "rtosGlobals.h"

// Setup

void Semaphore_setup()

{

    // 1 is the recommended value as there is one single Serial port.

    // Try with 3 to show the remaining count

    mySemaphore.begin(1);

    Serial.begin(115200);

}

// Loop

void Semaphore_loop()

{

    mySemaphore.waitFor();

    Serial.print(millis(), DEC);

    Serial.print("\t: mySemaphore2     2  (");

    Serial.print(mySemaphore.available(), DEC);

    Serial.print("-");

    Serial.println(")");

    mySemaphore.post();

    delay(30);

}

The second and third sketches are identical to the first one, except 

• on the setup() function, the semaphore isn't initialised again —once suffices— and a delay() has been added, and  
  
• on the loop() function, the message and the delay are adapted.
  

On the loop() function, there are two key elements.

• mySemaphore.waitFor() waits for the semaphore to be available and then turns it as busy,
  
• mySemaphore.posts() frees the semaphore and makes it available to the other tasks.
  

The result is, each task has an exclusive access to the Serial port and prints the message nicely.

// *** Sketch Semaphore2.ino

// *** Same for Semaphore3.ino with 3 instead of 2

// Core library 

#include "Energia.h"

// Include application, user and local libraries

#include "rtosGlobals.h"

// Setup

void Semaphore2_setup()

{

    Serial.begin(115200);

}

// Loop

void Semaphore2_loop()

{

    mySemaphore.waitFor();

    Serial.print(millis(), DEC);

    Serial.print("\t: mySemaphore2     2  (");

    Serial.print(mySemaphore.available(), DEC);

    Serial.print("-");

    Serial.println(")");

    mySemaphore.post();

    delay(30);

}

Try another value, for example 3, on mySemaphore.begin(3) and see the results! 

The value is the number of resources available for and to be shared by the tasks.