c - 访问连续内存位置时出现打印错误

Question

这个问题已经有答案了:

Printing hexadecimal characters in C (8 个回答)

已关闭 9 年前。

下面的代码显示了不同数据类型如何存储在内存中。

#include <stdio.h>

void newline(void)
{
    putchar('\n');
}

void showbyte(char *string, int len)
{
    int i;
    for (i = 0; i < len; i++)
    printf("%p\t0x%.2x\n", string+i, *(string+i));
}

int main()
{
    int i = 12345;
    float f = 1234.5;
    double d = 1234.5;
    char name[] = "12345";
    showbyte((char *)&i, sizeof i);
    newline();
    showbyte((char *)&f, sizeof f);
    newline();
    showbyte((char *)&d, sizeof d);
    newline();
    showbyte((char *)&name, sizeof name);
    return 0;
}

输出

0x7fff8a9ab2cc  0x39
0x7fff8a9ab2cd  0x30
0x7fff8a9ab2ce  0x00
0x7fff8a9ab2cf  0x00

0x7fff8a9ab2c8  0x00
0x7fff8a9ab2c9  0x50
0x7fff8a9ab2ca  0xffffff9a
0x7fff8a9ab2cb  0x44

0x7fff8a9ab2c0  0x00
0x7fff8a9ab2c1  0x00
0x7fff8a9ab2c2  0x00
0x7fff8a9ab2c3  0x00
0x7fff8a9ab2c4  0x00
0x7fff8a9ab2c5  0x4a
0x7fff8a9ab2c6  0xffffff93
0x7fff8a9ab2c7  0x40

0x7fff8a9ab2b0  0x31
0x7fff8a9ab2b1  0x32
0x7fff8a9ab2b2  0x33
0x7fff8a9ab2b3  0x34
0x7fff8a9ab2b4  0x35
0x7fff8a9ab2b5  0x00

float 1234.5 的 IEEE-754 表示形式是 0x449a5000 ，以及 double 1234.5是 0x40934A0000000000 。当它打印float时和double变量内容，它显示 4-byte内容。

即， 0x7fff8a9ab2ca 0xffffff9a和 0x7fff8a9ab2c6 0xffffff93 。

但是每个内存位置只能存储1-byte数据，那么为什么会发生这种情况？

Answer 1

unsigned 在这种情况下是你的 friend ，因为 char is by default a signed type

如果将某些类型从 char 更改为 unsigned char，您将得到正确的结果:

#include <stdio.h>

void newline(void)
{
    putchar('\n');
}

void showbyte(unsigned char *string, int len)
{
    int i;
    for (i = 0; i < len; i++)
      printf("%p\t0x%.2x\n", string+i, *(string+i));
}

int main()
{
    int i = 12345;
    float f = 1234.5;
    double d = 1234.5;
    char name[] = "12345";
    showbyte((unsigned char *)&i, sizeof i);
    newline();
    showbyte((unsigned char *)&f, sizeof f);
    newline();
    showbyte((unsigned char *)&d, sizeof d);
    newline();
    showbyte((unsigned char *)&name, sizeof name);
    return 0;
}

在这种情况下，您将得到:

0x7fff5d5a98b8  0x39
0x7fff5d5a98b9  0x30
0x7fff5d5a98ba  0x00
0x7fff5d5a98bb  0x00

0x7fff5d5a98b4  0x00
0x7fff5d5a98b5  0x50
0x7fff5d5a98b6  0x9a
0x7fff5d5a98b7  0x44

0x7fff5d5a98a8  0x00
0x7fff5d5a98a9  0x00
0x7fff5d5a98aa  0x00
0x7fff5d5a98ab  0x00
0x7fff5d5a98ac  0x00
0x7fff5d5a98ad  0x4a
0x7fff5d5a98ae  0x93
0x7fff5d5a98af  0x40

0x7fff5d5a98a2  0x31
0x7fff5d5a98a3  0x32
0x7fff5d5a98a4  0x33
0x7fff5d5a98a5  0x34
0x7fff5d5a98a6  0x35
0x7fff5d5a98a7  0x00