unsigned int a = 2;
unsigned int b = 3;
unsigned int c = a- b;
printf("%u \r\n", c);
4294967295
a : 补码 0x00000002
b: 补码 0x00000003
a-b a补 + b反 + 1
0 0 ... 0 0 0 0 0 0 1 0
+1 1 ... 1 1 1 1 1 1 0 1
-----------------------------------
11111111 .... 11111111 (补码)