在linux /sys/class/gpio中写入文件的错误

我现在遇到了我在linux系统中见过的最奇怪的错误，似乎只有两种可能的解释-

• 附加sudo使文件写入立即
• 或者附加sudo在执行语句时会产生短暂的延迟
• 或者我不知道我的程序发生了什么

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让我给你介绍一些背景知识。我目前正在编写一个c++程序的树莓派gpio操作。据我所知，程序中没有明显的错误&因为它可以成功地与sudo一起工作，也可以成功地与延迟一起工作。下面是rpi的gpio是如何工作的

• 首先你必须导出一个，为了保留操作，它将创建一个新目录gpio+number，其中包含几个文件。

  echo 17 > /sys/class/gpio/export

• 然后设置方向(in表示读，out表示写)

  echo "out" > /sys/class/gpio/gpio17/direction

• 然后写入值(0或1关闭和打开)

  echo 1 > /sys/class/gpio/gpio17/value

• 最后取消导出，目录将被删除。

  echo 17 > /sys/class/gpio/unexport

无论您是通过bash命令还是通过c/c++或任何其他语言的IO来执行此操作，因为在unix中这些只是文件，您只需要对它们进行读/写操作。到现在为止一切都很好。我已经手动测试了这个，它可以工作，所以我的手动测试通过了。

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现在我为我的程序编写了一个简单的测试，看起来像这样-

TEST(LEDWrites, LedDevice)
{
    Led led1(17, "MyLED");
    // auto b = sleep(1);
    EXPECT_EQ(true, led1.on());
}

Led类constructor执行导出部分- echo 17 > /sys/class/gpio/export，而.on()调用设置方向- echo "write" > /sys/class/gpio/gpio17/direction并输出值- echo 1 > /sys/class/gpio/gpio17/value。忘记这里的unexport吧，因为它是由析构函数处理的，在这里不起作用。

如果你很好奇，这些函数是这样处理I/O的-

{
    const std::string direction = _dir ? "out" : "in";
    const std::string path = GPIO_PATH + "/gpio" + std::to_string(powerPin) + "/direction";
    std::ofstream dirStream(path.c_str(), std::ofstream::trunc);
    if (dirStream) {
        dirStream << direction;
    } else {
        // LOG error here
        return false;
    }
    return true;
}

表示基本的c++文件/io。现在让我来解释一下这个bug。

---

首先，这里有3个相同的测试运行-

Normal run FAILS

[isaac@alarmpi build]$ ./test/testexe
Running main() from gtest_main.cc
[==========] Running 2 tests from 2 test cases.
[----------] Global test environment set-up.
[----------] 1 test from LEDConstruction
[ RUN      ] LEDConstruction.LedDevice
[       OK ] LEDConstruction.LedDevice (1 ms)
[----------] 1 test from LEDConstruction (1 ms total)
[----------] 1 test from LEDWrites
[ RUN      ] LEDWrites.LedDevice
../test/test.cpp:20: Failure
Value of: led1.on()
  Actual: false
Expected: true
[  FAILED  ] LEDWrites.LedDevice (2 ms)
[----------] 1 test from LEDWrites (3 ms total)
[----------] Global test environment tear-down
[==========] 2 tests from 2 test cases ran. (6 ms total)
[  PASSED  ] 1 test.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] LEDWrites.LedDevice
 1 FAILED TEST

run with sudo PASSES

[isaac@alarmpi build]$ sudo ./test/testexe
[sudo] password for isaac: 
Running main() from gtest_main.cc
[==========] Running 2 tests from 2 test cases.
[----------] Global test environment set-up.
[----------] 1 test from LEDConstruction
[ RUN      ] LEDConstruction.LedDevice
[       OK ] LEDConstruction.LedDevice (1 ms)
[----------] 1 test from LEDConstruction (2 ms total)
[----------] 1 test from LEDWrites
[ RUN      ] LEDWrites.LedDevice
[       OK ] LEDWrites.LedDevice (2 ms)
[----------] 1 test from LEDWrites (2 ms total)
[----------] Global test environment tear-down
[==========] 2 tests from 2 test cases ran. (5 ms total)
[  PASSED  ] 2 tests.

wtf delay run PASSES has uncomment // auto b = sleep(1);

[isaac@alarmpi build]$ ./test/testexe
Running main() from gtest_main.cc
[==========] Running 2 tests from 2 test cases.
[----------] Global test environment set-up.
[----------] 1 test from LEDConstruction
[ RUN      ] LEDConstruction.LedDevice
[       OK ] LEDConstruction.LedDevice (1 ms)
[----------] 1 test from LEDConstruction (2 ms total)
[----------] 1 test from LEDWrites
[ RUN      ] LEDWrites.LedDevice
[       OK ] LEDWrites.LedDevice (1001 ms)
[----------] 1 test from LEDWrites (1003 ms total)
[----------] Global test environment tear-down
[==========] 2 tests from 2 test cases ran. (1005 ms total)
[  PASSED  ] 2 tests.

唯一的区别b/w延迟和正常运行是一个没有注释的行- // auto b = sleep(1);一切都是一样的，包括设备，目录结构，构建配置和一切。唯一能解释这一点的是linux可能会稍后创建该文件和它的朋友，或者需要一些时间?在此之前我调用.on()。这就可以解释了…

但是为什么sudo调用没有延迟通过?它是使那些写更快/即时还是单独放置延迟语句?这是某种缓冲的原因吗?请说不:/

如果有关系，我使用以下dev规则来获得非sudo访问gpio目录-

SUBSYSTEM=="bcm2835-gpiomem", KERNEL=="gpiomem", GROUP="gpio", MODE="0660"
SUBSYSTEM=="gpio", KERNEL=="gpiochip*", ACTION=="add", PROGRAM="/bin/sh -c 'chown root:gpio /sys/class/gpio/export /sys/class/gpio/unexport ; chmod 220 /sys/class/gpio/export /sys/class/gpio/unexport'"
SUBSYSTEM=="gpio", KERNEL=="gpio*", ACTION=="add", PROGRAM="/bin/sh -c 'chown root:gpio /sys%p/active_low /sys%p/direction /sys%p/edge /sys%p/value ; chmod 660 /sys%p/active_low /sys%p/direction /sys%p/edge /sys%p/value'"

EDIT -正如@charles所提到的，我在每次写I/O操作后都使用std::flush。仍然失败。

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紧急救援

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让我们看看失败的构建命令的执行-

open("/sys/class/gpio/export", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/unexport", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/export", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/gpio17/value", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = -1 EACCES (Permission denied)
open("/sys/class/gpio/gpio17/direction", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = -1 EACCES (Permission denied)
open("/sys/class/gpio/unexport", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3

..., 0666) = -1 EACCES (Permission denied)

好的，这里有一些东西，解释了为什么它是用sudo传递的。但为什么它迟迟没有通过呢?我们再检查一下

open("/sys/class/gpio/export", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/unexport", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/export", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/gpio17/value", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/gpio17/direction", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 4
open("/sys/class/gpio/unexport", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3

不，等等，怎么了?这意味着，如果当时没有创建文件，则必须拒绝该权限。但如何使用sudo解决这个问题?

以下是sudo - 的相关输出

open("/sys/class/gpio/export", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/unexport", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/export", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/gpio17/value", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
open("/sys/class/gpio/gpio17/direction", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 4
open("/sys/class/gpio/unexport", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3

open("/sys/class/gpio/gpio17/value", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = -1 EACCES (Permission denied)
open("/sys/class/gpio/gpio17/direction", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = -1 EACCES (Permission denied)

if (dirStream) {
    dirStream << direction;
} else {
    // LOG error here
    return false;
}

if (dirStream) {
    dirStream << direction << std::endl;
} else {
    // LOG error here
    return false;
}