Question

I am a student, going through the book by Kerningham and Ritchie for C.

A line in the book says that -1l is less than 1u because in that case unsigned int is promoted to signed long. But -1l > 1ul because in this case -1l is promoted to unsigned long.

I can’t really understand the promotion properly. What will be the value of -1l when it is promoted to unsigned long? It’ll be great if anyone can help.
Thanks.

Answer 1

When you’re learning C, if you have a question, just write yourself a simple program:

#include <stdio.h>
main() {
  int si = -1;
  unsigned int ui = 1;
  if ( si > ui ) printf("-1l > 1u\n");
  else           printf("-1l <= 1u\n");
}

You’ll see that -1l > 1u is shown for the output.

Because both si and ui have the same rank (they’re both ints), the rule says that the negative value will be promoted to unsigned at set to UINT_MAX which is the largest possible unsigned value.