How to use boost::asio with Linux GPIOs
I have a single-threaded Linux application using boost::asio for asynchronous input/output. Now I need to extend this application to read in GPIO inputs on /sys/class/gpio/gpioXX/value .
It is possible to do that with boost::asio::posix::stream_descriptor on edge-triggered GPIO inputs?
I configured the GPIO input like follows:
echo XX >/sys/class/gpio/export
echo in >/sys/class/gpio/gpioXX/direction
echo both >/sys/class/gpio/gpioXX/edge
I managed to write a epoll based test application that blocks on the GPIO file descriptor until the GPIO signal changes but boost::asio does not seem to be able to block properly. A call to boost::asio::async_read always immediately invokes the handler (of course only within io_service.run() ) with either EOF or - in case the file pointer was set back - 2 bytes data.
I'm not an expert in boost::asio internals but could the reason be that the boost::asio epoll reactor is level triggered instead of edge triggered in case of posix::stream_descriptor ?
Here is my code:
#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(), 0, 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, streambuf, &read_handler);
io_service.run();
} catch (...) {
close(fd);
return 1;
}
close(fd);
return 0;
}
As far as I know, it is not possible to get this particular behavior with Boost.Asio. While the kernel flags some files on the procfs and sysfs as pollable, they do not provide the stream-like behavior that is expected from boost::asio::posix::stream_descriptor and its operations.
Boost.Asio's epoll reactor is edge-triggered (see Boost.Asio 1.43 revision history notes). Under certain conditions1, Boost.Asio will attempt the I/O operation within the context of the initiating function (eg async_read() ). If the I/O operation completes (success or failure), then the completion handler is posted into the io_service as-if by io_service.post() . Otherwise, the file descriptor will be added to the event demultiplexer for monitoring. The documentation alludes to this behavior:
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() .
For composed operations, such as async_read() , EOF is treated as an error, as it indicates a violation in the operation's contract (ie completion condition will never be satisfied because no more data will be available). In this particular case, the I/O system call will occur within the async_read() initiating function, reading from the start of the file (offset 0) to the end of file, causing the operation to fail with boost::asio::error::eof . As the operation has completed, it is never added to the event demultiplexer for edge-triggered monitoring:
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(), 0, 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();
}
1. Internally, Boost.Asio refers to this behavior as speculative operations. It is an implementation detail, but the I/O operation will be attempted within the initiating function if the operation may not need event notification (eg it can immediately attempt to a non-blocking I/O call), and and there are neither pending operations of the same type nor pending out-of-band operations on the I/O object. There are no customization hooks to prevent this behavior.
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