#include <stdio.h>

int main(void) {
	const char *file_path = "D:/MyFolder/YetAnotherFolder/test.txt";
const char *bk_string = ".bk";
char *file_bk_path = malloc((strlen(file_path) + strlen(bk_string) + 1)*sizeof(char));
if (!file_bk_path) { exit(1); }
strcpy(file_bk_path, file_path);
strcat(file_bk_path, bk_string);
printf("%s\n", file_bk_path);
free(file_bk_path);
	return 0;
}