[Dev-C++] strncpy output probs

[<Den...@ao...>]

Hi there, 
im having problems on a small program that I'm using for a course to display 
the properties of strncpy.
The trouble is having compiled the code I keep getting a strange output,
my code is

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

#define SIZE_OF 15

int main()
{
char FirstName[SIZE_OF], SecondName[SIZE_OF], 
FirstMix[SIZE_OF],SecondMix[SIZE_OF];
  // used SIZE_OF instead of declaring each array size individualy

printf("Please enter the name JACK\n");
scanf("%s", &FirstName);

printf("Now please enter the name ROBINSON\n");
scanf("%s", &SecondName);

strncpy(FirstMix,FirstName,3);
printf("FirstMix=%s\n", FirstMix);

strncat(FirstMix,SecondName,3);
printf("FirstMix=%s\n", FirstMix);


return 0 ;
}


and my output is 


Please enter the name JACK
JACK
Now please enter the name ROBINSON
ROBINSON
FirstMix=JAC?-?@
FirstMix=JAC?-?@ROB

has anybody an idea as to where the extra characters are coming from????
thanks in advance.