Run the following program on a system with a fr_FR.UTF-8 locale.
===================================================================
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <wchar.h>

int main ()
{
  int locale_found = setlocale (LC_ALL, "fr_FR.UTF-8") != NULL;
  printf ("%d\n", locale_found);

  printf ("%d\n", mbtowc (NULL, NULL, 0));

  static const char input[4] = { 195, 188, 195, 159 };
  wchar_t wc;

  int res2a = mbtowc (&wc, input, 4);
  printf ("%d\n", res2a);

  int res1 = mbtowc (&wc, input, 1);
  printf ("%d\n", res1);

  int res2b = mbtowc (&wc, input, 4);
  printf ("%d\n", res2b);

  return 0;
}
===================================================================
$ gcc -O -Wall foo.c
$ ./a.out 
1
0
2
-1
-1

Expected output:

1
0
2
-1
2

Rationale:
The first line shows that the locale was correctly set. So the locale encoding
is UTF-8.
The second line that the UTF-8 encoding is non state dependent.
The third and fifth line show that the same call has different results, that
is, it must depend on a hidden state.
But the mbtowc specification says that "For a state-dependent encoding ...
Subsequent calls with s as other than a null pointer shall cause the internal 
state of the function to be altered as necessary."
However, the encoding in use here is not state dependent. Hence the function's
results should not depend on hidden state.

Reference:
POSIX:2008 specification of mbtowc:
<http://www.opengroup.org/onlinepubs/9699919799/functions/mbtowc.html>