打印值0x89（-119）时发生奇怪的输出 [英] Strange output when printing the value 0x89 (-119)

问题描述

正如标题所示，运行以下代码时出现怪异结果：

  #include  $ b int main（）
 {
 char buff [4] = {0x17，0x89，0x39，0x40}; 
 unsigned int * ptr =（unsigned int *）buff; （3 * 8））; 
 char a =（char）（（* ptr <<<（0 * 8））> （b *（char *）（（* ptr <（（1 * 8））>（3 * 8））; （char *）（char *（char *）（char *（2 * 8））>（3 * 8））; （char *）（（* ptr <<（3 * 8））>>（3 * 8））; 
 
 printf（0x％x \ n，* ptr）; 
 printf（0x％x \，a）; 
 printf（0x％x \ n，b）; 
 printf（0x％x \，c）; 
 printf（0x％x \ n，d）; 
 
返回0; 
} 
  

输出：

  0x40398917 
 0x40 
 0x39 
 0xffffff89 
 0x17 
  

为什么我没有得到 0x89 ？

解决方案这是因为你的 char 变量是被签名的，并且它们在被升级（在这种情况下被升级到更宽的类型）时正在进行符号扩展。签名扩展是在进行此促销时保留标志的一种方式，因此 -119 保持为 -119 ，无论它是8位，16位或更宽的类型。

您可以通过明确使用 unsigned char 来修复它，因为，至少在C中， char 是有符号还是无符号是特定于实现的。从 C11 6.2.5类型/ 15 ：

与signed char或unsigned char具有相同的范围，表示形式和行为。

签名扩展名无法用于无符号类型因为他们，...好，没有签名： - ）

As the title says, I get a "weird" result when running the following code:

#include <stdio.h>

int main()
{
    char buff[4] = {0x17, 0x89, 0x39, 0x40};
    unsigned int* ptr = (unsigned int*)buff;
    char a = (char)((*ptr << (0*8)) >> (3*8));
    char b = (char)((*ptr << (1*8)) >> (3*8));
    char c = (char)((*ptr << (2*8)) >> (3*8));
    char d = (char)((*ptr << (3*8)) >> (3*8));

    printf("0x%x\n", *ptr);
    printf("0x%x\n", a);
    printf("0x%x\n", b);
    printf("0x%x\n", c);
    printf("0x%x\n", d);

    return 0;
}

Output:

0x40398917
0x40
0x39
0xffffff89
0x17

Why am I not getting 0x89 ?

解决方案

It's because your char variables are signed and they're undergoing sign extension when being promoted (upgraded to a wider type in this case). Sign extension is a way of preserving the sign when doing this promotion, so that -119 stays as -119 whether it's 8-bit, 16-bit or a wider type.

You can fix it by explicitly using unsigned char since, in C at least, whether char is signed or unsigned is implementation-specific. From C11 6.2.5 Types /15:

The implementation shall define char to have the same range, representation, and behavior as either signed char or unsigned char.

Sign extension does not come into play for unsigned types because they're, ... well, unsigned :-)

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