Quality RTOS & Embedded Software

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




xSemaphoreTake
[Semaphores]

semphr. h
xSemaphoreTake( SemaphoreHandle_t xSemaphore, TickType_t xTicksToWait );

Macro to obtain a semaphore. The semaphore must have previously been created with a call to xSemaphoreCreateBinary(), xSemaphoreCreateMutex() or xSemaphoreCreateCounting().

This macro must not be called from an ISR. xQueueReceiveFromISR() can be used to take a semaphore from within an interrupt if required, although this would not be a normal operation. Semaphores use queues as their underlying mechanism, so functions are to some extent interoperable.

Parameters:
xSemaphore A handle to the semaphore being taken - obtained when the semaphore was created.
xTicksToWait The time in ticks to wait for the semaphore to become available. The macro portTICK_PERIOD_MS can be used to convert this to a real time. A block time of zero can be used to poll the semaphore.

If INCLUDE_vTaskSuspend is set to '1' then specifying the block time as portMAX_DELAY will cause the task to block indefinitely (without a timeout).

Returns:
pdTRUE if the semaphore was obtained. pdFALSE if xTicksToWait expired without the semaphore becoming available.
Example usage:
SemaphoreHandle_t xSemaphore = NULL;

/* A task that creates a semaphore. */
void vATask( void * pvParameters )
{
    /* Create the semaphore to guard a shared resource.  As we are using
    the semaphore for mutual exclusion we create a mutex semaphore
    rather than a binary semaphore. */
    xSemaphore = xSemaphoreCreateMutex();
}

/* A task that uses the semaphore. */
void vAnotherTask( void * pvParameters )
{
    /* ... Do other things. */

    if( xSemaphore != NULL )
    {
        /* See if we can obtain the semaphore.  If the semaphore is not
        available wait 10 ticks to see if it becomes free. */
        if( xSemaphoreTake( xSemaphore, ( TickType_t ) 10 ) == pdTRUE )
        {
            /* We were able to obtain the semaphore and can now access the
            shared resource. */

            /* ... */

            /* We have finished accessing the shared resource.  Release the
            semaphore. */
            xSemaphoreGive( xSemaphore );
        }
        else
        {
            /* We could not obtain the semaphore and can therefore not access
            the shared resource safely. */
        }
    }
}






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




Copyright (C) 2004-2010 Richard Barry. Copyright (C) 2010-2016 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:

FreeRTOS V9.0.0 is now available for download.


Free TCP/IP and file system demos for the RTOS


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

Silicon Labs low power RTOS 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