Thread scheduling test. The low priority thread join is not successful, but the low priority thread has a chance to run during one of the printf calls (due to svcSendSyncRequest).

thread-scheduling.cpp

#include <3ds.h>
#include <stdio.h>

#include <cstdint>
#include <string>
#include <string_view>
#include <utility>
#include <vector>

static LightEvent test_event;
static s64 base_tick;
static std::vector<std::pair<std::string_view, s64>> thread_order;

int main(int argc, char **argv)
{
	gfxInitDefault();

	//Initialize console on top screen. Using NULL as the second argument tells the console library to use the internal console structure as current one
	consoleInit(GFX_TOP, NULL);

	LightEvent_Init(&test_event, ResetType::RESET_ONESHOT);

	base_tick = svcGetSystemTick();

	auto highPrio = threadCreate([](void *arg) {
		thread_order.emplace_back("High priority thread start!", svcGetSystemTick() - base_tick);
		LightEvent_Wait(&test_event);
		thread_order.emplace_back("High priority thread done!", svcGetSystemTick() - base_tick);
	}, nullptr, 0x1000, 0x20, 0, false);
	thread_order.emplace_back("Created high priority thread", svcGetSystemTick() - base_tick);

	auto lowPrio = threadCreate([](void *arg) {
		thread_order.emplace_back("Low priority thread start!", svcGetSystemTick() - base_tick);
		LightEvent_Wait(&test_event);
		thread_order.emplace_back("Low priority thread done!", svcGetSystemTick() - base_tick);
	}, nullptr, 0x1000, 0x30, 0, false);
	thread_order.emplace_back("Created low priority thread", svcGetSystemTick() - base_tick);

	thread_order.emplace_back("Send signal 1", svcGetSystemTick() - base_tick);
	LightEvent_Signal(&test_event);
	svcSleepThread(500000ULL);

	thread_order.emplace_back("Send signal 2", svcGetSystemTick() - base_tick);
	LightEvent_Signal(&test_event);

	thread_order.emplace_back("Join high priority thread", svcGetSystemTick() - base_tick);
	threadJoin(highPrio, 0);

	thread_order.emplace_back("Join low priority thread", svcGetSystemTick() - base_tick);
	threadJoin(lowPrio, 0);

	thread_order.emplace_back("Test done", svcGetSystemTick() - base_tick);

	printf("Hello World!\n");

	std::string str;
	for (auto &sv : thread_order) {
		str = sv.first;
		printf("%s %lld\n", str.c_str(), sv.second);
	}

	printf("Press Start to exit.\n");

	// Main loop
	while (aptMainLoop())
	{
		//Scan all the inputs. This should be done once for each frame
		hidScanInput();

		//hidKeysDown returns information about which buttons have been just pressed (and they weren't in the previous frame)
		u32 kDown = hidKeysDown();

		if (kDown & KEY_START) break; // break in order to return to hbmenu

		// Flush and swap framebuffers
		gfxFlushBuffers();
		gfxSwapBuffers();

		//Wait for VBlank
		gspWaitForVBlank();
	}

	gfxExit();
	return 0;
}