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.