In this case, note that the "%s" is superfluous - you just need::
// print szInterface to auxBuf
sprintf( auxBuf, par->INPS->szInterface );


Andy had said as above.

But you see he only included a "destination" and a "format" string. For many normal cases this would simply copy the format string to the destination. But if the "format" string (acting as source string in Andy's "tricky example") happens to have an embedded % character with a format field specification in it then the sprintf() function will look for non-existant [argument] and thus pull some bogus data out of the stack causing the undefined behavior as pointed out by Mr Catto.

Michael Karas