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C语言中char*和char[]的区别

  • char *c : char型指针,只表示所指向的内存单元
  • char [] : 表示数组型的内存单元

结论char a[]char a[0]形式的定义在结构体中不占内存大小。

以下所有测试在64bit系统中进行,结果与32bit系统存在差别。

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#include <stdio.h>
#include <stdlib.h>

int main()
{
int int_a;
int* int_a_p;
char char_b;
char* char_b_p;
printf("\tSystem Data Width:\n");
printf("sizeof int(%ld), int*(%ld), char(%ld), char*(%ld)\n",
sizeof(int_a), sizeof(int_a_p), sizeof(char_b), sizeof(char_b_p));

char *c1 = "a b c d";
char c2[] = "a b c d";

printf("c1: %s\n", c1);
printf("c2: %s\n", c2);

printf("c1-c1[0]=%c\n", c1[0]);
printf("c2-c2[0]=%c\n", c2[0]);

//c1[0] = 'x'; /* Segmentation fault (core dumped) */
c2[0] = 'y';

printf("c1-c1[0]=%c\n", c1[0]);
printf("c2-c2[0]=%c\n", c2[0]);

struct sc1 {
char a;
char *b;
};
struct sc2 {
char a;
char b[];
};
struct sc2_1 {
char a;
char b[0];
};
printf("sc1: sizeof-char* = %ld\n", sizeof(struct sc1)); //8 + 8
printf("sc2: sizeof-char[] = %ld\n", sizeof(struct sc2)); //1
printf("sc2_1: packed sizeof-char[0] = %ld\n", sizeof(struct sc2_1)); //1

struct __attribute__ ((__packed__)) sc3 {
char a;
char *b;
};
struct __attribute__ ((__packed__)) sc4 {
char a;
char b[];
};
printf("sc3: packed sizeof-char* = %ld\n", sizeof(struct sc3)); //1 + 8
printf("sc4: packed sizeof-char[] = %ld\n", sizeof(struct sc4)); //1
return 0;
}

  • 运行:
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    System Data Width:
    sizeof int(4), int*(8), char(1), char*(8)
    c1: a b c d
    c2: a b c d
    c1-c1[0]=a
    c2-c2[0]=a
    c1-c1[0]=a
    c2-c2[0]=y
    sc1: sizeof-char* = 16
    sc2: sizeof-char[] = 1
    sc2_1: packed sizeof-char[0] = 1
    sc3: packed sizeof-char* = 9
    sc4: packed sizeof-char[] = 1

内存地址对比

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struct sc1 sc1_a;
printf("\tsc1_a addr: %p\n", &sc1_a);
printf("\tsc1_a.a addr: %p\n", &sc1_a.a);
printf("\tsc1_a.b addr: %p\n", &sc1_a.b);

struct sc2 sc2_a, sc2_b;
printf("\tsc2_a addr: %p\n", &sc2_a);
printf("\tsc2_a.a addr: %p\n", &sc2_a.a);
printf("\tsc2_a.b addr: %p\n", &sc2_a.b);
printf("\tsc2_b addr: %p\n", &sc2_b);
  • 运行结果:
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    sc1_a addr: 0x7ffd44ed7d60
    sc1_a.a addr: 0x7ffd44ed7d60
    sc1_a.b addr: 0x7ffd44ed7d68
    sc2_a addr: 0x7ffd44ed7d7e
    sc2_a.a addr: 0x7ffd44ed7d7e
    sc2_a.b addr: 0x7ffd44ed7d7f
    sc2_b addr: 0x7ffd44ed7d7f
  • 在结构体中使用char buf[]形式的定义,不占用内存空间

应用

内核部分结构体:如MMC中sdhci的结构体定义

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struct sdhci_host {
...
unsigned long private[0] ____cacheline_aligned;
};

利于将自定义结构体部分和公共结构体之间相关联。

自定义结构体:

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struct sdhci_custom {
int xxx;
};

struct sdhci_host *host;
struct sdhci_custom *custom;

//一次性申请内存看见
host = kmalloc(sizeof(struct sdhci_host) + sizeof(struct sdhci_custom));

//自定义结构体位置
custom = host->private;

参考

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  • 本文作者: Winddoing
  • 本文链接: https://winddoing.github.io/post/44009.html
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