Analysis of 0xffffff in hexadecimal notation in C language!

Analysis of 0xffffff in hexadecimal notation in C language!
In fact, similar problems do not occur only in network programs. See the sample code: Copy code 1 # include <stdio. h> 2 int main () 3 {4 char c = 0xc9; 5 printf ("A: c = % 2x \ n", (unsigned char) c ); 6 printf ("B: c = % 2x \ n", c & 0xff); 7 printf ("C: c = % 2x \ n", c); 8 return 0; 9} copy the code program output as follows: A: c = c9B: c = c9C: c = ffffffc9 you can see that converting c to unsigned char is correct. As case. Print c and 0xff correctly. B. If c is not processed, the problem is reproduced and ffffc9 is printed. C. Case a B is A solution from Baidu to solve the phenomenon C. Now, let's analyze and explain the three situations of ABC one by one. First, we must know that the % x (X) output by the printf () function is in hexadecimal format of the Int type. Therefore, c variables of the char type are converted to the Int type. Second, we know that computers use supplementary codes to represent data. For the original code, anti-code, and supplemental code knowledge, please recharge your own. Case C: c completion code: 11001001 (0xc9 ). C: 11001000 (0xc9 ). Source code of c: 10110111 (0xc9 ). Because the char type is signed, the first of the highest bits is regarded as a negative number. Convert c to the source code of Int type char -----> Int Int_c: 10000000 00000000 00000000 00110111 (the highest bit 1 of the c source code is raised to the highest bit. Add 0 for other high positions ). Int_c anti-code: 11111111 11111111 11111111 11001000 11111111 Int_c complement: 11111111 11111111 11001001 (0xffffc9 ). Therefore, it is reasonable to print out seemingly strange values. How to avoid it? Check the situation of AB. Case B: We will operate C and 0xff on the basis of case c. Int_c complement: 11111111 11111111 11111111 11001001 (0xffffffc9 ). & Amp; 00000000 00000000 00000000 11111111 final result: 00000000 00000000 00000000 11001001 (0xc9 ). Case A: I think situation A is the most formal solution, but it is said that the Linux kernel is used (& 0xff ). C complement: 11001001 (0xc9 ). C: 11001001 (0xc9 ). Source code of c: 11001001 (0xc9 ). Here, the type of c is forcibly converted to unsigned char. Therefore, the first digit of the highest digit is not the plus or minus sign to convert c to the Int type char -----> Int Int_c original code: 00000000 00000000 00000000 11001001 (the highest digit of the c original code 1 is mentioned to the highest digit. Add 0 for other high positions ). Int_c anti-code: 00000000 00000000 00000000 11001001 00000000 Int_c complement: 00000000 00000000 11001001 (0xc9 ). Therefore, the print is normal. If any of the above analyses is incorrect, please correct them.