Quality RTOS & Embedded Software
About   Contact   Support   FAQ

 Real time embedded FreeRTOS RSS feed 
Real time embedded FreeRTOS mailing list 
Quick Start Supported MCUs Books & Kits Trace Tools Ecosystem TCP & FAT Training

Last site update June 02 2015

Queues, Mutexes, Semaphores...
[Inter-task communication and synchronisation]

[See also Blocking on Multiple RTOS Objects]

The FreeRTOS tutorial book provides additional information on queues, binary semaphores, mutexes, counting semaphores and recursive semaphores, along with simple worked examples in a set of accompanying example projects.

FreeRTOS Counting Semaphores

Just as binary semaphores can be thought of as queues of length one, counting semaphores can be thought of as queues of length greater than one. Again, users of the semaphore are not interested in the data that is stored in the queue - just whether or not the queue is empty or not.

Counting semaphores are typically used for two things:

  1. Counting events.

    In this usage scenario an event handler will 'give' a semaphore each time an event occurs (incrementing the semaphore count value), and a handler task will 'take' a semaphore each time it processes an event (decrementing the semaphore count value). The count value is therefore the difference between the number of events that have occurred and the number that have been processed. In this case it is desirable for the count value to be zero when the semaphore is created.

  2. Resource management.

    In this usage scenario the count value indicates the number of resources available. To obtain control of a resource a task must first obtain a semaphore - decrementing the semaphore count value. When the count value reaches zero there are no free resources. When a task finishes with the resource it 'gives' the semaphore back - incrementing the semaphore count value. In this case it is desirable for the count value to be equal the maximum count value when the semaphore is created.

See the Semaphores/Mutexes section of the user documentation for a list of semaphore related API functions. Searching the files in the FreeRTOS/Demo/Common/Minimal directory will reveal multiple examples of their usage. Note that interrupts must NOT use API functions that do not end in "FromISR".

[ Back to the top ]    [ About FreeRTOS ]    [ Sitemap ]    [ ]

Copyright (C) 2004-2010 Richard Barry. Copyright (C) 2010-2015 Real Time Engineers Ltd.
Any and all data, files, source code, html content and documentation included in the FreeRTOSTM distribution or available on this site are the exclusive property of Real Time Engineers Ltd.. See the files license.txt (included in the distribution) and this copyright notice for more information. FreeRTOSTM and FreeRTOS.orgTM are trade marks of Real Time Engineers Ltd.

Latest News - New Book Edition!

Receive a complimentary pre-release pdf of our up-coming book "Mastering the FreeRTOS Real Time Kernel, a Hands-On Tutorial Guide" with every purchased copy of the existing "Using the FreeRTOS Real Time Kernel" book.

Sponsored Links

⇓ Now With No Code Size Limit! ⇓
⇑ Free Download Without Registering ⇑

FreeRTOS Partners

ARM Connected RTOS partner for all ARM microcontroller cores

Renesas Electronics Gold Alliance RTOS Partner.jpg

Microchip Premier RTOS Partner

RTOS partner of NXP for all NXP ARM microcontrollers

Atmel RTOS partner supporting ARM Cortex-M3 and AVR32 microcontrollers

STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0

Xilinx Microblaze and Zynq partner

Altera RTOS partner for Nios II and Cortex-A9 SoC

Freescale Alliance RTOS Member supporting ARM and ColdFire microcontrollers

Infineon ARM Cortex-M microcontrollers

Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers

Cypress RTOS partner supporting ARM Cortex-M3

Fujitsu RTOS partner supporting ARM Cortex-M3 and FM3

Microsemi (previously Actel) RTOS partner supporting ARM Cortex-M3

Atollic Partner

IAR Partner

Keil ARM Partner

Embedded Artists