c - What does 'u' mean after a number?

Question

Can you tell me what exactly does the u after a number, for example:

#define NAME_DEFINE 1u 

question from:https://stackoverflow.com/questions/9029974/what-does-u-mean-after-a-number

Answer 1

Integer literals like 1 in C code are always of the type int. int is the same thing as signed int. One adds u or U (equivalent) to the literal to ensure it is unsigned int, to prevent various unexpected bugs and strange behavior.

One example of such a bug:

On a 16-bit machine where int is 16 bits, this expression will result in a negative value:

long x = 30000 + 30000;

Both 30000 literals are int, and since both operands are int, the result will be int. A 16-bit signed int can only contain values up to 32767, so it will overflow. x will get a strange, negative value because of this, rather than 60000 as expected.

The code

long x = 30000u + 30000u;

will however behave as expected.