Quality RTOS & Embedded Software

 Real time embedded FreeRTOS RSS feed 
Quick Start Supported MCUs PDF Books Trace Tools Ecosystem


taskENTER_CRITICAL()
taskEXIT_CRITICAL()
[RTOS Kernel Control]

task. h

void taskENTER_CRITICAL( void );
void taskEXIT_CRITICAL( void );

Critical sections are entered by calling taskENTER_CRITICAL(), and subsequently exited by calling taskEXIT_CRITICAL().

The taskENTER_CRITICAL() and taskEXIT_CRITICAL() macros provide a basic critical section implementation that works by simply disabling interrupts, either globally, or up to a specific interrupt priority level. See the vTaskSuspendAll() RTOS API function for information on creating a critical section without disabling interrupts.

If the FreeRTOS port being used does not make use of the configMAX_SYSCALL_INTERRUPT_PRIORITY kernel configuration constant (also called configMAX_API_CALL_INTERRUPT_PRIORITY), then calling taskENTER_CRITICAL() will leave interrupts globally disabled. If the FreeRTOS port being used does make use of the configMAX_SYSCALL_INTERRUPT_PRIORITY kernel configuration constant, then calling taskENTER_CRITICAL() will leave interrupts at and below the interrupt priority set by configMAX_SYSCALL_INTERRUPT_PRIORITY disabled, and all higher priority interrupt enabled.

Preemptive context switches only occur inside an interrupt, so will not occur when interrupts are disabled. Therefore, the task that called taskENTER_CRITICAL() is guaranteed to remain in the Running state until the critical section is exited, unless the task explicitly attempts to block or yield (which it should not do from inside a critical section).

Calls to taskENTER_CRITICAL() and taskEXIT_CRITICAL() are designed to nest. Therefore, a critical section will only be exited when one call to taskEXIT_CRITICAL() has been executed for every preceding call to taskENTER_CRITICAL().

Critical sections must be kept very short, otherwise they will adversely affect interrupt response times. Every call to taskENTER_CRITICAL() must be closely paired with a call to taskEXIT_CRITICAL().

FreeRTOS API functions must not be called from within a critical section.

taskENTER_CRITICAL() and taskEXIT_CRITICAL() must not be called from an interrupt service routine (ISR) - see taskENTER_CRITICAL_FROM_ISR() and taskEXIT_CRITICAL_FROM_ISR() for interrupt safe equivalents.

Parameters:
None
Returns:
None

Example usage:
/* A function that makes use of a critical section. */
void vDemoFunction( void )
{
    /* Enter the critical section.  In this example, this function is itself called
    from within a critical section, so entering this critical section will result
    in a nesting depth of 2. */
    taskENTER_CRITICAL();

    /* Perform the action that is being protected by the critical section here. */

    /* Exit the critical section.  In this example, this function is itself called
    from a critical section, so this call to taskEXIT_CRITICAL() will decrement the
    nesting count by one, but not result in interrupts becoming enabled. */
    taskEXIT_CRITICAL();
}

/* A task that calls vDemoFunction() from within a critical section. */
void vTask1( void * pvParameters )
{
    for( ;; )
    {
        /* Perform some functionality here. */

        /* Call taskENTER_CRITICAL() to create a critical section. */
        taskENTER_CRITICAL();


        /* Execute the code that requires the critical section here. */


        /* Calls to taskENTER_CRITICAL() can be nested so it is safe to call a
        function that includes its own calls to taskENTER_CRITICAL() and
        taskEXIT_CRITICAL(). */
        vDemoFunction();

        /* The operation that required the critical section is complete so exit the
        critical section.  After this call to taskEXIT_CRITICAL(), the nesting depth
        will be zero, so interrupts will have been re-enabled. */
        taskEXIT_CRITICAL();
    }
}






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


Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.

Latest News

NXP tweet showing LPC5500 (ARMv8-M Cortex-M33) running FreeRTOS.

Meet Richard Barry and learn about running FreeRTOS on RISC-V at FOSDEM 2019

Version 10.1.1 of the FreeRTOS kernel is available for immediate download. MIT licensed.

View a recording of the "OTA Update Security and Reliability" webinar, presented by TI and AWS.


Careers

FreeRTOS and other embedded software careers at AWS.



FreeRTOS Partners

ARM Connected RTOS partner for all ARM microcontroller cores

Espressif ESP32

IAR Partner

Microchip Premier RTOS Partner

RTOS partner of NXP for all NXP ARM microcontrollers

Renesas

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

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

OpenRTOS and SafeRTOS

Xilinx Microblaze and Zynq partner