Question

  struct {              /* Fileheader */
    uchar file_version[4];
    uchar options[2];
    uchar header_length[2];
    uchar state_info_length[2];
    uchar base_info_length[2];
    uchar base_pos[2];
    uchar key_parts[2];         /* Key parts */
    uchar unique_key_parts[2];      /* Key parts + unique parts */
    uchar keys;             /* number of keys in file */
    uchar uniques;          /* number of UNIQUE definitions */
    uchar language;         /* Language for indexes */
    uchar max_block_size_index;     /* max keyblock size */
    uchar fulltext_keys;
    uchar not_used;                     /* To align to 8 */
  } header;

以上来源于MySQL,

为什么两人都赞同<代码>8?

Answer 1

它是一种优化,能够更有效地利用万国邮联记忆中的结构。

Answer 2

<>Reason>: 地址计算速度更快,规模较小。

在X86和其他一些结构中,如果元素大小是“黑体,圆点数”,则使用阵列元素的效率更高。关于“种族、圆号”的定义,学习x86组。但是,你可以看到以下法典在组装上不同尺寸的阵列的效果:

struct s { char c[N]; };
int func(struct s *p, int i) { return p[i].c[0]; }

当N为23岁时(上述结构的大小不dding):

leaq    (%rsi,%rsi,2), %rax
salq    $3, %rax
subq    %rsi, %rax
movsbl  (%rax,%rdi),%eax

当N为24时(上述结构分为dding):

leaq    (%rsi,%rsi,2), %rax
movsbl  (%rdi,%rax,8),%eax

当N为32时(上述结构分为additional):

salq    $5, %rsi
movsbl  (%rsi,%rdi),%eax

发出通知,说明该守则对使用包含23个精细元素的阵列中的一个元素有何复杂性。

<>Reason2: 对于拆卸结构,它允许在档案中的其他部件与配装的负荷和库存相接触。

该结构在软盘上。在婚后,可以在结构之后直接出现一个32个字。这使查阅速度更快——编辑自动为记忆结构服务,但需要人工操作光盘的结构。如果你试图获取不结盟的数据,一些结构甚至会崩溃。

unsigned char *data = ...;
header *h = (header *) data;
do_something_with(h);
uint32_t x = *(uint32_t *) (data + sizeof(header));

The above code will crash a SPARC if sizeof(header) is not a multiple of 4, and on x86 it will be slower unless sizeof(header) is not a multiple of 4.

Answer 3

如果主角是阵列,那么阵列的所有组成部分都将以同样的方式加以统一。否则,情况并非如此。你可以研究以下法典/产出,以核实这一点。

如果 ,Dani 在其评论中正确无误,即用户打算将诸如file_version至,那么这种协调非常重要。

Some code

#include <stdio.h>

struct padded {
    unsigned char file_version[4];
    unsigned char options[2];
    unsigned char header_length[2];
    unsigned char state_info_length[2];
    unsigned char base_info_length[2];
    unsigned char base_pos[2];
    unsigned char key_parts[2];
    unsigned char unique_key_parts[2];
    unsigned char keys;
    unsigned char uniques;
    unsigned char language;
    unsigned char max_block_size_index;
    unsigned char fulltext_keys;
    unsigned char not_used;
};

struct unpadded {
    unsigned char file_version[4];
    unsigned char options[2];
    unsigned char header_length[2];
    unsigned char state_info_length[2];
    unsigned char base_info_length[2];
    unsigned char base_pos[2];
    unsigned char key_parts[2];
    unsigned char unique_key_parts[2];
    unsigned char keys;
    unsigned char uniques;
    unsigned char language;
    unsigned char max_block_size_index;
    unsigned char fulltext_keys;
};

int main() {
    printf("size padded:      %lu
", sizeof(struct padded));
    printf("size unpadded:    %lu
", sizeof(struct unpadded));

    printf("size padded[2]:   %lu
", sizeof(struct padded[2]));
    printf("size unpadded[2]: %lu
", sizeof(struct unpadded[2]));
}

The output

size padded:      24
size unpadded:    23
size padded[2]:   48
size unpadded[2]: 46

Some code

The output

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