Question

int main()
{
  unsigned int b;
  signed int a;
  char z=-1;
  b=z;
  a=z;
  printf("%d %d",a,b);
}

gives -1 -1. why does no sign extension occur, ALSO, when does it occur?

Answer 1

Sign extension DID occur, but you are printing the results incorrectly. In your printf you specified %d for b, but b is unsigned, you should have used %u to print b.

printf does not know the type of its arguments and uses the format specifies to interpret them.

printf("%d %u",a,b);