如何解决如何使用poll2或select2服务调用来查看伪文件与Kotlin的更改
我正在使用使用Android 5.1和Kotlin的DragonBoard 410C来测试40针低功耗连接器上的GPIO针。我正在使用的库正在使用sysfs接口与GPIO引脚进行交互,这需要在/sys/class/gpio/
目录树中打开各种伪文件并对这些文件读写值,请参见accessing GPIO low power connector on DragonBoard 410C running Android
我的理解是,我可以提供一个GPIO引脚作为输入和边沿触发,这将允许我用瞬时接触开关连接一个简单的电路,并能够检测何时按下该开关。
但是,我发现的文档表明我需要在GPIO引脚的/value
伪文件的文件描述符上使用poll(2)系统服务或select(2)系统服务我正在使用以检测何时检测到边缘,例如/sys/class/gpio/gpio910/value
。
如何在Kotlin中将poll(2)
或select(2)
系统服务与文件描述符一起使用? poll(2)
与ready()
的{{1}}方法相同吗?
也许需要类似于Java FileReader
功能的东西? http://www.java2s.com/Tutorials/Java/java.nio.file/WatchService/0060__WatchService.poll_.htm
除非是错误的方法,否则我正在计划要具有实用程序功能,例如:
WatchService
https://www.kernel.org/doc/Documentation/gpio/sysfs.txt
“值” ...读为0(低)或1(高)。如果GPIO是 配置为输出,可以写入该值;任何非零 值被视为高。
如果该引脚可以配置为产生中断的中断,并且 如果已将其配置为生成中断(请参见 描述“ edge”),则可以对该文件执行poll(2)和poll(2) 每当中断被触发时将返回。如果您使用 poll(2),设置事件POLLPRI和POLLERR。如果您使用select(2), 在exceptfds中设置文件描述符。在poll(2)返回之后, lseek(2)到sysfs文件的开头并读取新值 或关闭文件,然后重新打开以读取值。
“边缘” ...读作“无”,“上升”,“下降”或“两者”。 编写这些字符串以选择将使 返回“值”文件中的poll(2)。
仅当该引脚可以配置为中断时,此文件才存在 生成输入引脚。
附加说明
注意1:使用// pollPseudoFile() polls the specified GPIO pin using the sysfs interface in order
// to wait for a pin value due to an external event such as a momentary switch press
//
// Returns:
// - 0 -> an error of some kind
// - 1 -> time out expired with no value
// - 2 -> value change detected
public fun pollPseudoFile (pinPathFull : String,timeOut : Int) : Int {
println(" pollPseudoFile - String")
var iStatus : Int = 0 // status return indicating if poll() completed or not.
try {
val br = FileReader(pinPathFull)
br.poll(timeOut) // wait for data to be available or time out. how to do poll?
iStatus = 2 // indicate value change unless the poll() timed out
if (br.pollExpired) iStatus = 1 // poll timed out. how to check for poll status?
br.close()
} catch (e: Exception) {
println("Error: " + e.message)
}
return iStatus;
}
public fun pollGetValue (pinPathFull : String) : Int {
println(" pollGetValue - String")
var line = ""
try {
val br = BufferedReader(FileReader(pinPathFull))
line = br.readLine()
br.close()
} catch (e: Exception) {
println("Error: " + e.message)
}
return line.toInt()
}
实用程序,我可以adb
进入我的DragonBoard 410C,并测试了配置shell
的物理引脚26 GPIO971的配置direction
和in
设置为edge
。在连接到物理引脚23 GPIO938的面包板上使用简单的LED电路,并添加一条从物理引脚26到物理引脚23管理的LED的电线,我能够使用rising
和然后echo 1 > gpio938/value
看到物理引脚26的值变高并且读为cat gpio971/value
。然后,我用1
断开了连接到物理引脚23的LED,然后echo 0 > gpio938/value
如期返回了cat gpio971/value
的值。
但是,这个实验并没有告诉我0
是否会指示打开和关闭LED时poll(2)
上的更改。
注释1a::我有一个本机C ++ JNI函数的第一版来实现gpio971/value
服务调用,并且已经在我的DragonBoard 410C上进行了测试。我看到的是poll(2)
函数将立即返回,并在poll(2)
数组的POLLIN
成员中同时设置POLLERR
和revents
。
测试使用的是连接到面包板行的物理引脚26,而LED的一条腿连接到物理引脚23,我可以打开和关闭它。当我尝试以10000毫秒的超时时间打开轮询时,无论两个指示灯均已设置,无论LED点亮(引脚26的值为1)还是不点亮(引脚26的值为0),呼叫都会立即返回。
我的期望是,因为我将struct pollfd
设置为edge
,所以只有在LED熄灭然后打开电源或经过10秒钟后,我才应该看到rising
返回
在继续调查的过程中,我可能会发现我如何使用在应用程序的Kotlin端编写的Native C ++函数。
注释2:我试图在我的Kotlin应用程序中使用poll(2)
,但遇到一个错误,提示WatchService
需要API级别26,而我在Android Studio中的最低目标是API级别22。似乎WatchService
需要Android 8.0(Oreo),而DragonBoard是Android 5.1(Lollipop),因此无法使用WatchService
来监视文件状态。
解决方法
我采用的方法是创建本机C ++ JNI函数,以提供一种实现poll(2) Linux service call的方法。
我在开发和测试期间遇到的一个有趣问题是poll()
立即返回,而不是等待超时或GPIO输入引脚的电压。在DragonBoard 410C的96Boards.org论坛上发布How to use poll() with sysfs interface to input GPIO pin to handle a switch press event之后,有人提出了可行的解决方案,可以在启动poll(2)之前读取伪文件。
要使用此功能,我需要具有某种Kotlin协程或侧线程,以便在主UI处理按钮单击时开始对GPIO输入引脚的轮询,而主UI线程不是直到函数返回GPIO事件或超时返回为止。
我还无法辨别如何制作这样的协程,因此这仍在进行中。经过一番思考,看来某种事件侦听器体系结构将是最合适的方法。
但是测试表明,当使用1.8v电源的电线手动施加电压时,通过超时或返回pollPseudoFile()
中的值,函数/value
可以正常工作(引脚38)连接到GPIO输入引脚,该引脚通过rising
伪文件中的falling
或/edge
设置。
以下是Native C ++ JNI函数的源代码。我将其与以下Kotlin源代码一起使用。
首先,在我的MainActivity.kt
源文件中,使Native C ++库可用于以下源程序:
// See the StackOverFlow question with answer at URL:
// https://stackoverflow.com/questions/36932662/android-how-to-call-ndk-function-from-kotlin
init {
System.loadLibrary("pollfileservice")
}
external fun pollFileWithTimeOut(pathPseudo : String,timeOutMs : Int): Int
external fun pollGetLastRevents() : Int
接下来,我将在Kotlin源文件Gpio.kt
中使用此功能来实际对伪文件执行poll()
服务调用。
class Gpio(pin: Int) {
private val pin : Int
private val pinGpio : GpioFile = GpioFile()
/*
* The GPIO pins are represented by folders in the Linux file system
* within the folder /sys/class/gpio. Each pin is represented by a folder
* whose name is the prefix "gpio" followed by the pin number.
* Within the folder representing the pin are two files,"value" used to
* set or get the value of the pin and "direction" used to set or get
* the direction of the pin.
*
* This function creates the path to the Linux file which represents a particular
* GPIO pin function,"value" or "direction".
*/
private fun MakeFileName(pin: Int,op: String): String {
return "/sys/class/gpio/gpio$pin$op"
}
// ....... other source code in the Kotlin class Gpio
fun pinPoll (timeMs: Int) : Int {
val iStatus : Int = pinGpio.pollPseudoFile (MakeFileName(pin,"/value"),timeMs)
return iStatus
}
上面的Gpio类用于实际的UI按钮单击侦听器,如下所示:
val gpioProcessor = GpioProcessor()
// Get reference of GPIO23.
val gpioPin26 = gpioProcessor.pin26
// Set GPIO26 as input.
gpioPin26.pinIn()
gpioPin26.pinEdgeRising()
var xStatus: Int = gpioPin26.pinPoll(10000)
val xvalue = gpioPin26.value
PollFileService.h
//
// Created by rchamber on 9/24/2020.
//
#ifndef MY_APPLICATION_POLLFILESERVICE_H
#define MY_APPLICATION_POLLFILESERVICE_H
class PollFileService {
private:
int iValue;
int fd; /* file descriptor */
public:
// See poll(2) man page at https://linux.die.net/man/2/poll
static const int PollSuccess = 0;
static const int PollTimeOut = 1;
static const int PollErrorEFAULT = -1;
static const int PollErrorEINTR = -2;
static const int PollErrorEINVAL = -3;
static const int PollErrorENOMEM = -4;
static const int PollErrorPOLLERR = -5;
static const int PollErrorPOLLNVAL = -6;
static const int PollErrorPOLLERRNVAL = -7;
static const int PollErrorPOLLHUP = -8;
static const int PollErrorPOLLERRDEFLT = -9;
static const int PollErrorUNKNOWN = -100;
static int iPollStatus;
static int iPollRet;
static int iPollRevents;
PollFileService(const char *pathName = nullptr,int timeMilliSec = -1);
~PollFileService();
int PollFileCheck (const char *pathName,int timeMilliSec = -1);
int PollFileRead (const char *pathName = nullptr);
};
extern "C"
JNIEXPORT jint JNICALL
Java_com_example_myapplication_MainActivity_pollFileWithTimeOut (JNIEnv* pEnv,jobject pThis,jstring pKey,jint timeMS);
#endif //MY_APPLICATION_POLLFILESERVICE_H
PollFileService.cpp
//
// Created by rchamber on 9/24/2020.
//
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <math.h>
#include <errno.h>
#include <poll.h>
#include <jni.h>
#include "PollFileService.h"
int PollFileService::iPollStatus = 0;
int PollFileService::iPollRet = 0;
int PollFileService::iPollRevents = 0;
PollFileService::PollFileService(const char *pathName /* = nullptr */,int timeMilliSec /* = -1 */) : iValue(23),fd(-1)
{
iPollStatus = 0;
if (pathName) {
fd = open (pathName,O_RDONLY);
}
}
PollFileService::~PollFileService()
{
if (fd >= 0) {
close (fd);
fd = -1;
}
}
int PollFileService::PollFileCheck(const char *pathName,int timeMilliSec /* = -1 */)
{
struct pollfd fdList[] = {
{fd,POLLPRI | POLLERR,0},{0}
};
nfds_t nfds = 1;
unsigned char tempbuff[256] = {0};
if (fd < 0 && pathName) {
fd = open (pathName,O_RDONLY);
fdList[0].fd = fd;
}
// with a edge triggered GPIO that we are going to use the poll(2)
// function to wait on an event,we need to read from the
// pin before we do the poll(2). If the read is not done then
// the poll(2) returns with both POLLPRI and POLLERR set in the
// revents member. however if we read first then do the poll2()
// the poll(2) will wait for the event,input voltage change with
// either a rising edge or a falling edge,depending on the setting
// in the /edge pseudo file.
ssize_t iCount = read (fdList[0].fd,tempbuff,255);
iPollStatus = PollErrorUNKNOWN;
int iRet = poll(fdList,nfds,timeMilliSec);
if (iRet == 0) {
iPollStatus = PollTimeOut;
} else if (iRet < 0) {
switch (errno) {
case EFAULT:
iPollStatus = PollErrorEFAULT;
break;
case EINTR:
iPollStatus = PollErrorEINTR;
break;
case EINVAL:
iPollStatus = PollErrorEINVAL;
break;
case ENOMEM:
iPollStatus = PollErrorENOMEM;
break;
default:
iPollStatus = PollErrorUNKNOWN;
break;
}
} else if (iRet > 0) {
// successful call now determine what we should return.
iPollRevents = fdList[0].revents; /* & (POLLIN | POLLPRI | POLLERR); */
switch (fdList[0].revents & (POLLIN | POLLPRI | POLLERR /* | POLLNVAL | POLLHUP*/)) {
case (POLLIN): // value of 1,There is data to read.
case (POLLPRI): // value of 2,There is urgent data to read
case (POLLOUT): //,Writing now will not block.
case (POLLIN | POLLPRI): // value of 3
iPollStatus = PollSuccess;
break;
// testing with a DragonBoard 410C indicates that we may
// see the POLLERR indicator set in revents along with
// the POLLIN and/or POLLPRI indicator set indicating there
// is data to be read.
// see as well poll(2) man page which states:
// POLLERR Error condition (output only).
case (POLLIN | POLLERR): // value of 9
case (POLLPRI | POLLERR): // value of 10
case (POLLIN | POLLPRI | POLLERR): // value of 11
iPollStatus = PollSuccess;
break;
case (POLLHUP): //,Hang up (output only).
iPollStatus = PollErrorPOLLHUP;
break;
case (POLLERR): // value of 8,Error condition (output only).
iPollStatus = PollErrorPOLLERR;
break;
case (POLLNVAL): //,Invalid request: fd not open (output only).
iPollStatus = PollErrorPOLLNVAL;
break;
case (POLLERR | POLLNVAL):
iPollStatus = PollErrorPOLLERRNVAL;
break;
default:
iPollStatus = PollErrorPOLLERRDEFLT;
break;
}
}
return iPollStatus;
}
int PollFileService::PollFileRead (const char *pathName /* = nullptr */)
{
char buffer[12] = {0};
int iRet = -1;
if (fd < 0 && pathName) {
fd = open (pathName,O_RDONLY);
}
int nCount = read (fd,buffer,10);
if (nCount > 0) {
iRet = atoi (buffer);
}
return iRet;
}
// Check the specified file using the poll(2) service and
// return a status as follows:
// - 0 -> poll(2) success indicating something is available
// - 1 -> poll(2) failed with time out before anything available
// - -1 -> poll(2) error - EFAULT
// - -2 -> poll(2) error - EINTR
// - -3 -> poll(2) error - EINVAL
// - -4 -> poll(2) error - ENOMEM
// - -5 -> poll(2) error - POLLERR
// - -6 -> poll(2) error - POLLNVAL
// - -7 -> poll(2) error - POLLERR | POLLNVAL
// - -8 -> poll(2) error - POLLHUP
// - -9 -> poll(2) error - poll(2) revent indicator Unknown
// - -100 -> poll(2) error - Unknown error
//
static int lastRevents = 0;
extern "C"
JNIEXPORT jint JNICALL
Java_com_example_myapplication_MainActivity_pollFileWithTimeOut (JNIEnv* pEnv,jint timeMS)
{
char *pathName;
int timeMilliSec;
PollFileService myPoll;
const char *str = pEnv->GetStringUTFChars(pKey,0);
int timeMSint = 10000; // timeMS;
#if 1
int iStatus = myPoll.PollFileCheck(str,timeMSint);
#else
int iStatus = myPoll.PollFileRead(str);
#endif
pEnv->ReleaseStringUTFChars(pKey,str);
lastRevents = myPoll.iPollRevents;
return iStatus;
}
#if 0
extern "C"
JNIEXPORT jint JNICALL
Java_com_example_myapplication_MainActivity_pollGetLastStatus (JNIEnv* pEnv,jobject pThis) {
return PollFileService::iPollStatus;
}
#endif
extern "C"
JNIEXPORT jint JNICALL
Java_com_example_myapplication_MainActivity_pollGetLastRevents (JNIEnv* pEnv,jobject pThis)
{
return lastRevents;
}
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