# C language Printing 16 binary 0xffffff phenomenon problem analysis!

Source: Internet
Author: User

Today in Bo asked to see a friend's problem, roughly in the network program, printed out of the 16 number, inexplicable appearance ffffff. For example, a byte is really worth 0xc9, and printing out is 0xffffffc9. The original Bo asked the connection as follows: http://q.cnblogs.com/q/71073/

In fact, a similar problem is not only in the network program will appear, see the Sample code:

`  1#include <stdio.h>2 intMain ()3 {  4     Charc =0xc9; 5printf"a:c =%2x\n", (unsignedChar) c); 6printf"b:c =%2x\n", C &0xFF); 7printf"c:c =%2x\n", c); 8     return 0; 9}`

The program output is as follows:

`A:C == = Ffffffc9`

can see:

Converting c to unsigned char printing is correct. As a condition.

C and 0xFF do & operation after printing correctly. As a condition B.

c do not do any processing, the problem is reproduced, print out FFFFFFC9. As a situation c.

Situation a B is my Baidu to solve the C phenomenon of the method. So let's now analyze and explain the ABC three cases one by one.

First we have to know that the printf () function's%x (x) outputs the 16 binary format of the int type. So c variables of char type are converted to int type.

Secondly, we know that the computer uses the complement to represent the data. For the original code, anti-code, complementary knowledge, please charge yourself.

Case C:

C's Complement: 11001001 (0XC9).

C's anti-code: 11001000 (0XC9).

The original code of C: 10110111 (0XC9).

Because the char type is signed, the highest bit of 1 is considered a minus.

Convert c to int type char-----> int

Int_c The original code: 10000000 00000000 00000000 00110111 (the highest digit of the C source code is referred to the highest bit.) The remaining highs are 0).

Anti-code for Int_c: 11111111 11111111 11111111 11001000

Int_c's complement: 11111111 11111111 11111111 11001001 (0XFFFFFFC9).

So it's reasonable to print out a seemingly bizarre value. How to avoid it? See AB case.

Case B:

int_c's complement: 11111111 11111111 11111111 11001001 ( 0XFFFFFFC9).

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00000000 00000000 00000000 11111111

The end result is:00000000 00000000 00000000 11001001 (0xc9).

case A:

/span>

C's Complement: 11001001 (0XC9).

C's anti-code:11001001(0xc9).

The original code of C:11001001(0xc9).

Here the cast c is unsigned char type. So the highest bit of 1 is not the plus sign

Convert c to int type char-----> int

The original code of Int_c: 00000000 00000000 00000000 11001001(the highest bit of the C original code is referred to the highest bit 1. The remaining highs are 0).

Int_c Anti-code:00000000 00000000 00000000 11001001

Int_c's complement:00000000 00000000 00000000 11001001(0xc9).

So the print is normal.

The above analysis, if there is an incorrect place please correct me.

C language Printing 16 binary 0xffffff phenomenon problem analysis!

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