在边沿触发的GPIO输入上使用boost :: asio :: posix :: stream_descriptor可以做到这一点吗？
我配置了GPIO输入,如下所示：
echo XX >/sys/class/gpio/export echo in >/sys/class/gpio/gpioXX/direction echo both >/sys/class/gpio/gpioXX/edge
我设法写了一个基于epoll的测试应用程序,它阻止GPIO文件描述符,直到GPIO信号改变但boost :: asio似乎无法正常阻塞.对boost :: asio :: async_read的调用总是立即调用处理程序(当然只在io_service.run()中)和EOF或 – 如果文件指针被设置回 – 2字节数据.
我不是boost :: asio内部的专家,但原因可能是在:: posix :: stream_descriptor的情况下,boost :: asio epoll reactor是水平触发而不是边缘触发？
这是我的代码：
#include <fcntl.h>
#include <algorithm>
#include <iterator>
#include <stdexcept>
#include <boost/asio.hpp>
boost::asio::io_service io_service;
boost::asio::posix::stream_descriptor sd(io_service);
boost::asio::streambuf streambuf;
void read_handler(const boost::system::error_code& error,std::size_t bytes_transferred)
{
if (error.value() == boost::asio::error::eof) {
// If we don't reset the file pointer we only get EOFs
lseek(sd.native_handle(),SEEK_SET);
} else if (error)
throw std::runtime_error(std::string("Error ") + std::to_string(error.value()) + " occurred (" + error.message() + ")");
std::copy_n(std::istreambuf_iterator<char>(&streambuf),bytes_transferred,std::ostreambuf_iterator<char>(std::cout));
streambuf.consume(bytes_transferred);
boost::asio::async_read(sd,streambuf,&read_handler);
}
int main(int argc,char *argv[])
{
if (argc != 2)
return 1;
int fd = open(argv[1],O_RDONLY);
if (fd < 1)
return 1;
try {
sd.assign(fd);
boost::asio::async_read(sd,&read_handler);
io_service.run();
} catch (...) {
close(fd);
return 1;
}
close(fd);
return 0;
}
解决方法
boost::asio::posix::stream_descriptor及其操作所期望的类似流的行为.
Boost.Asio的epoll反应器是边缘触发的(参见Boost.Asio 1.43 revision history notes).在某些条件下1,Boost.Asio将在启动功能的上下文中尝试I / O操作(例如async_read()).如果I / O操作完成(成功或失败),则完成处理程序将按照io_service.post()发布到io_service中.否则,文件描述符将被添加到事件多路分解器以进行监视.该文档暗示了这种行为：
Regardless of whether the asynchronous operation completes immediately or not,the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using
boost::asio::io_service::post().
对于组合操作,例如async_read(),EOF is treated as an error,因为它表示操作合同中存在违规(即永远不会满足完成条件,因为没有更多数据可用).在这种特殊情况下,I / O系统调用将在async_read()启动函数中发生,从文件的开头(偏移0)读取到文件的末尾,导致操作失败,使用boost :: asio ::错误:: EOF.操作完成后,它永远不会添加到事件多路分解器中以进行边缘触发监视：
boost::asio::io_service io_service;
boost::asio::posix::stream_descriptor stream_descriptor(io_service);
void read_handler(const boost::system::error_code& error,...)
{
if (error.value() == boost::asio::error::eof)
{
// Reset to start of file.
lseek(sd.native_handle(),SEEK_SET);
}
// Same as below. ::readv() will occur within this context,reading
// from the start of file to end-of-file,causing the operation to
// complete with failure.
boost::asio::async_read(stream_descriptor,...,&read_handler);
}
int main()
{
int fd = open( /* sysfs file */,O_RDONLY);
// This would throw an exception for normal files,as they are not
// poll-able. However,the kernel flags some files on procfs and
// sysfs as pollable.
stream_descriptor.assign(fd);
// The underlying ::readv() system call will occur within the
// following function (not deferred until edge-triggered notification
// by the reactor). The operation will read from start of file to
// end-of-file,causing the operation to complete with failure.
boost::asio::async_read(stream_descriptor,&read_handler);
// Run will invoke the ready-to-run completion handler from the above
// operation.
io_service.run();
}
在内部,Boost.Asio将此行为称为推测性操作.这是一个实现细节,但如果操作可能不需要事件通知(例如,它可以立即尝试非阻塞I / O调用),则将在启动函数内尝试I / O操作,并且既没有挂起I / O对象上的相同类型的操作或未决的带外操作.没有自定义挂钩来防止此行为.
