pipe-example.c

#include <stdio.h>
#include <string.h>
#include <windows.h>

static CALLBACK DWORD discard_thread(void *arg)
{
	HANDLE pipe = arg;
	OVERLAPPED ol = { 0 };
	ol.hEvent = CreateEventW(NULL, TRUE, TRUE, NULL);

	// Incoming messages are read into this buffer and discarded
	char outbuf[128];

	for (;;) {
		DWORD r;
		DWORD err = ReadFile(pipe, outbuf, 128, NULL, &ol) ? 0 : GetLastError();

		// ERROR_IO_PENDING means GetOverlappedResult will block and wait for a
		// response. If err is 0, GetOverlappedResult will return immediately.
		if (err && err != ERROR_IO_PENDING)
			break;
		if (!GetOverlappedResult(pipe, &ol, &r, TRUE))
			break;
	}

	CloseHandle(ol.hEvent);
	return 0;
}

int main()
{
	const char *command = "loadfile test.mp3\n";
	const wchar_t *pipe_name = L"\\\\.\\pipe\\mpvsocket";

	// If more than one program tries to open the pipe at the same time, only
	// one program will be successful and the rest will get ERROR_PIPE_BUSY.
	// Hence, a retry loop must be used to reliably connect to the pipe.
	HANDLE pipe;
	for (;;) {
		pipe = CreateFileW(pipe_name, GENERIC_READ | GENERIC_WRITE, 0, NULL,
			OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);
		if (pipe != INVALID_HANDLE_VALUE)
			break;

		// Abort if there was an error other than ERROR_PIPE_BUSY
		if (GetLastError() != ERROR_PIPE_BUSY)
			return 1;

		// Abort if it takes too long for a pipe instance to become available.
		// In practice, this will block for up to 50ms when waiting for a pipe
		// instance, then fail if none were available.
		if (!WaitNamedPipeW(pipe_name, NMPWAIT_USE_DEFAULT_WAIT))
			return 1;
	}

	// Start a thread to read and discard pipe input, otherwise the pipe's
	// input buffer will fill up and mpv will block when writing events
	HANDLE thread = CreateThread(NULL, 0, discard_thread, pipe, 0, NULL);

	// Write a command to the pipe every 300 milliseconds
	OVERLAPPED ol = { 0 };
	ol.hEvent = CreateEventW(NULL, TRUE, TRUE, NULL);
	for (;;) {
		DWORD r;
		DWORD err = WriteFile(pipe, command, strlen(command), NULL, &ol)
			? 0 : GetLastError();

		// ERROR_IO_PENDING means GetOverlappedResult must block and wait for
		// the pipe to be writable before the write can be completed
		if (err && err != ERROR_IO_PENDING)
			break;
		if (!GetOverlappedResult(pipe, &ol, &r, TRUE))
			break;
		Sleep(300);
	}

	// Wait for the read thread to finish. When mpv closes, both the read and
	// write loops will exit at approximately the same time.
	WaitForSingleObject(thread, INFINITE);
	CloseHandle(thread);

	CloseHandle(pipe);
	CloseHandle(ol.hEvent);
	return 0;
}