C ++递减单字节(易失性)数组的元素不是原子的！为什么? (另外:我如何在Atmel AVR mcus/Arduino中强制原子化) [英] C++ decrementing an element of a single-byte (volatile) array is not atomic! WHY? (Also: how do I force atomicity in Atmel AVR mcus/Arduino)

问题描述

我只是失去了天，从字面上看，约25个小时的工作，因为要调试我的代码在简单的东西，我不知道.

I just lost days, literally, ~25 hrs of work, due to trying to debug my code over something simple that I didn't know.

事实证明，在AVR ATmega328 8位微控制器(Arduino)上，递减C ++中单字节数组的元素不是原子操作，并且需要原子访问保护(即关闭中断).为什么是这样???另外，什么是C语言技术来确保原子自动访问Atmel AVR微控制器上的变量?

It turns out decrementing an element of a single-byte array in C++, on an AVR ATmega328 8-bit microcontroller (Arduino) is not an atomic operation, and requires atomic access guards (namely, turning off interrupts). Why is this??? Also, what are all of the C techniques to ensure atomic access to variables on an Atmel AVR microcontroller?

这是我所做工作的简明版本:

Here's a dumbed down version of what I did:

//global vars:
const uint8_t NUM_INPUT_PORTS = 3;
volatile uint8_t numElementsInBuf[NUM_INPUT_PORTS];

ISR(PCINT0_vect) //external pin change interrupt service routine on input port 0
{
  //do stuff here
  for (uint8_t i=0; i<NUM_INPUT_PORTS; i++)
    numElementsInBuf[i]++;
}

loop()
{
  for (uint8_t i=0; i<NUM_INPUT_PORTS; i++)
  {
    //do stuff here
    numElementsInBuf[i]--; //<--THIS CAUSES ERRORS!!!!! THE COUNTER GETS CORRUPTED.
  }
}

这是很好的循环版本:

loop()
{
  for (uint8_t i=0; i<NUM_INPUT_PORTS; i++)
  {
    //do stuff here
    noInterrupts(); //globally disable interrupts 
    numElementsInBuf[i]--; //now it's ok...30 hrs of debugging....
    interrupts(); //globally re-enable interrupts 
  }
}

请注意原子访问保护"，即:在递减之前禁用中断，然后在中断之后重新启用它们.

Notice the "atomic access guards", ie: disabling interrupts before decrementing, then re-enabling them after.

由于我在这里处理一个字节，所以我不知道我需要原子访问保护.为什么在这种情况下需要它们?这是典型的行为吗?我知道如果这是2个字节的值的数组，我会需要它们，但是为什么要使用1个字节的值呢?通常对于1字节的值，这里不需要原子访问保护...

因此，我们知道在AVR 8位元文件上读取或写入任何单字节变量是原子操作，但是STM32 32位元文件又如何呢? 哪些变量在STM32上具有自动的原子读写功能?答案在这里:哪些变量类型/大小是原子的在STM32微控制器上?.

So we know that reading from or writing to any single-byte variable on AVR 8-bit mcus is an atomic operation, but what about STM32 32-bit mcus? Which variables have automatic atomic reads and writes on STM32? The answer is here: Which variable types/sizes are atomic on STM32 microcontrollers?.

推荐答案

好，答案是为什么要增加/减少单字节变量不是原子的?" 此处的伊沙梅尔和

Ok, the answer to "Why is incrementing/decrementing a single byte variable NOT atomic?" is answered very well here by Ishamael here, and Michael Burr here.

现在我得到了答案，即使对字节值进行了操作，--减量和++增量操作也永远不会是原子的(请参见上面的答案和

Now that I got my answer that -- decrement and ++ increment operations are never atomic, even when done on byte values (see answers above and Nick Gammon's link here), I'd like to ensure the follow-up question of how do I force atomicity on Atmel AVR microcontrollers is also answered so this question becomes somewhat of a resource:

1)选项1(首选方法):

uint8_t SREG_bak = SREG; //save global interrupt state
noInterrupts(); //disable interrupts (for Arduino only; this is an alias of AVR's "cli()")
//atomic variable-access code here
SREG = SREG_bak; //restore interrupt state

2)选项2(不太安全，不建议使用的方法，因为如果您不小心在ISR内部调用的代码块或库中使用此方法，可能会导致您无意中启用嵌套中断):

noInterrupts(); //disable interrupts (Arduino only; is an alias to AVR's "cli()" call)
//atomic variable-access code here
interrupts(); //enable interrupts (Arduino only; is an alias to AVR's "sei()" call)

替代方法2:

cli(); //clear (disable) interrupts flag; noInterrupts() is simply a macro for this
//atomic variable-access code here
sei(); //set (enable) interrupts flag; interrupts() is simply a macro for this

3)选项3(与选项1基本相同；只是使用avr-libc库中保留的宏，并且在括号内应用了可变范围)
来源: http://www.nongnu.org/avr-libc/user-manual/group__util__atomic.html

3) Option 3 (essentially the same as option 1; just using a macro held in an avr-libc library instead, and with variable scope applied within the braces of course)
source: http://www.nongnu.org/avr-libc/user-manual/group__util__atomic.html

#include <util/atomic.h> //(place at top of code)
ATOMIC_BLOCK(ATOMIC_RESTORESTATE)
{
  //atomic access code here
}

相关:

1. 哪些变量类型/大小在STM32上是原子的微控制器?
2. https://stm32f4-discovery.net/2015/06/如何正确启用isable-interrupts-in-arm-cortex-m/

1. Which variable types/sizes are atomic on STM32 microcontrollers?
2. https://stm32f4-discovery.net/2015/06/how-to-properly-enabledisable-interrupts-in-arm-cortex-m/

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