A lightweight tester to check if your UTF-8 -> UTF-16 conversion function works correctly. Further test cases can be added in the test_battery().

AKO-UTF8-UTF16-Tester.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <inttypes.h>

// How many tests passed?
int tests = 0;
int passed = 0;

struct test_case {
	int id;
	unsigned char utf8_input[5];
	unsigned char utf16_test[5];
};

void test_battery();
void run_test(struct test_case tcase);
unsigned char * utf8_to_utf16(unsigned char * utf8);

// Run a single test case - call the converter, compare
// the output against the expected utf16_test.
void run_test(struct test_case tcase) {
	printf("Test %d... ", tcase.id);
	tests++;

	unsigned char * output = utf8_to_utf16(tcase.utf8_input);

	size_t test_length = strlen(tcase.utf16_test);
	size_t output_length = strlen(output);

	if (test_length != output_length) {
		printf("\033[1;31mFAILED!\033[0m\n");
		printf("Expected %d bytes, got %d bytes.\n", strlen(tcase.utf16_test), strlen(output));

		free(output);
		return;
	} else if (memcmp(tcase.utf16_test, output, output_length) != 0) {
		printf("\033[1;31mFAILED!\033[0m\n");

		printf("Output mismatch, got: ");
		for (int i = 0; i < output_length; i++) {
			printf("0x%02X ", (unsigned char)output[i]);
		}

		printf("\n");
		free(output);
		return;
	}

	passed++;
	free(output);
	printf("\033[1;32mOK!\033[0m\n");
}

int main() {
	test_battery();

	printf("Passed %d/%d tests. ", passed, tests);

	if (passed == tests) {
		printf("See you next mission!\n");
	} else {
		printf("Good luck next time!\n");
	}
}

// --- Interesting stuff below... ---

void test_battery() {
	run_test((struct test_case){0,  { 0x30, 0 },                   { 0x30, 0x00, 0 }}); // 1-byte ASCII
	run_test((struct test_case){1,  { 0x31, 0 },                   { 0x31, 0x00, 0 }});
	run_test((struct test_case){2,  { 0x32, 0 },                   { 0x32, 0x00, 0 }});
	run_test((struct test_case){3,  { 0xCE, 0xA9, 0 },             { 0xA9, 0x03, 0 }}); // 2-byte UTF-8
	run_test((struct test_case){4,  { 0xCA, 0x98, 0 },             { 0x98, 0x02, 0 }});
	run_test((struct test_case){5,  { 0xD6, 0x8D, 0 },             { 0x8D, 0x05, 0 }});
	run_test((struct test_case){6,  { 0xD1, 0xAC, 0 },             { 0x6C, 0x04, 0 }});
	run_test((struct test_case){7,  { 0xE0, 0xAE, 0x94, 0 },       { 0x94, 0x0B, 0 }}); // 3-byte UTF-8
	run_test((struct test_case){8,  { 0xE0, 0xAE, 0x87, 0 },       { 0x87, 0x0B, 0 }});
	run_test((struct test_case){9,  { 0xE1, 0xAE, 0xBF, 0 },       { 0xBF, 0x1B, 0 }});
	run_test((struct test_case){10, { 0xE1, 0xAF, 0xBC, 0 },       { 0xFC, 0x1B, 0 }});
	run_test((struct test_case){11, { 0xE3, 0xBA, 0xBA, 0 },       { 0xBA, 0x3E, 0 }});
	run_test((struct test_case){12, { 0xF0, 0x90, 0x90, 0xB2, 0 }, { 0x41, 0xD8, 0x32, 0xDC, 0 }}); // Have fun!
	run_test((struct test_case){1337, { 0xF0, 0x92, 0x88, 0xA2, 0 }, { 0x48, 0xD8, 0x22, 0xDE, 0 }});
}

// *********************************
// * PASTE YOUR utf8_to_utf16 HERE *
// *********************************