Quoting straight up from Effective C:

Integer conversion rank

No two signed integer types have the same rank, even if they have the same representation. The rank of a signed integer type is greater than the rank of any signed integer type with less precision. The rank of long long int is greater than the rank of long int , which is greater than the rank of int, which is greater than the rank of short int, which is greater than the rank of signed char The rank of any unsigned integer type equals the rank of the corresponding signed integer type, if any. The rank of char equals the rank of signed char and unsigned char . The rank of _Bool is less than the rank of all other standard integer types. The rank of any enumerated type equals the rank of the compatible integer type. Each enumerated type is compatible with char, a signed integer type, or an unsigned integer type. The rank of any extended signed integer type relative to another extended signed integer type with the same precision is implementation-defined but still subject to the other rules for determining the integer conversion rank.

Integer Promotions

A small type is an integer with a lower conversion rank than int or unsigned int . Integer promotion is the process of converting values of small types to an int or unsigned int . Integer promotions allow you to use an expression of a small type in any expression where an int or unsigned int may be used. For example, you could use the lower-ranked integer type—typically, char or short —on the right-hand side of an assignment or as an argument to a function.

Now consider this snippet:

unsigned ui = UINT_MAX;
signed char c = -1;
if (c == ui)
    printf("%d equals %u", c, ui);
else
    printf("Not equal");

What does it print? If you answered -1 equals 4294967295 or some variant of that (depending on your platform) - Even though I’m still puzzled by the fact that int is 32 bits while I am on a 64 bit platform - you are correct. But jeez what’s going on? Remember? First thing is integer promotion of c because signed char has a lower integer conversion rank. Then because of the == operand c is converted to an unsigned and with the overflow rules it’s now UINT_MAX. Fortunately gcc and clang generates warning for that so if you compile using the -Wall flag (thing that any sane person must do) you must be preserved of that kind of mistakes.