How to use UTF-8 in C code?

My setup: gcc-4.9.2, UTF-8 environment.

The following C-program works in ASCII, but does not in UTF-8.

Create input file:

echo -n 'приветмир'> /tmp/вход

This is test.c:

#include <stdio.h>#include <stdlib.h>#include <string.h>#define SIZE 10int main(void){  char buf[SIZE+1];  char *pat = "приветмир";  char str[SIZE+2];  FILE *f1;  FILE *f2;  f1 = fopen("/tmp/вход","r");  f2 = fopen("/tmp/выход","w");  if (fread(buf, 1, SIZE, f1) > 0) {    buf[SIZE] = 0;    if (strncmp(buf, pat, SIZE) == 0) {      sprintf(str, "% 11s\n", buf);      fwrite(str, 1, SIZE+2, f2);    }  }  fclose(f1);  fclose(f2);  exit(0);}

Check the result:

./test; grep -q 'приветмир' /tmp/выход&& echo OK

What should be done to make UTF-8 code work as if it was ASCII code - not to bother how many bytes a symbol takes, etc. In other words: what to change in the example to treat any UTF-8 symbol as a single unit (that includes argv, STDIN, STDOUT, STDERR, file input, output and the program code)?