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

int main() {
	FILE *fp = fopen("test.txt", "a+");
	if (fp == NULL) {
		puts("failed to open test file in append mode");
		exit(1);
	}

	size_t written = fwrite("1234567890", 10, 1, fp);
	if (written != 1) {
		puts("failed to write");
		exit(1);
	}

	int ret = fseek(fp, -8, SEEK_CUR);
	if (ret == -1) {
		puts("failed to seek");
		exit(1);
	}

	char read_buf[100];
	memset(read_buf, 0, sizeof(read_buf));

	size_t read = fread(read_buf, 2, 1, fp);
	if (read != 1) {
		printf("failed to read %d\n", errno);
		exit(1);
	}

	if (strcmp(read_buf, "34") != 0) {
		printf("unexpected read value %s\n", read_buf);
		exit(1);
	}

	ret = ungetc('a', fp);
	if (ret != 'a') {
		puts("failed to ungetc");
		exit(1);
	}

	ret = fseek(fp, 3, SEEK_CUR);
	if (ret == -1) {
		puts("failed to seek");
		exit(1);
	}

	read = fread(read_buf, 2, 1, fp);
	if (read != 1) {
		printf("failed to read %d\n", errno);
		exit(1);
	}

	if (strcmp(read_buf, "78") != 0) {
		printf("unexpected read value %s\n", read_buf);
		exit(1);
	}

	written = fwrite("0987654321", 10, 1, fp);
	if (written != 1) {
		puts("failed to write");
		exit(1);
	}

	ret = fseek(fp, -20, SEEK_CUR);
	if (ret == -1) {
		puts("failed to seek");
		exit(1);
	}

	read = fread(read_buf, 20, 1, fp);
	if (read != 1) {
		printf("failed to read %d\n", errno);
		exit(1);
	}

	if (strcmp(read_buf, "12345678900987654321") != 0) {
		printf("unexpected read value %s\n", read_buf);
		exit(1);
	}

	fclose(fp);

	puts("success");

	return 0;
}