.frm文件开头的64字节是fileinfo
在fileinfo[6]处记录着IO_SIZE=0x1000
fileinfo[47]处记录着key_buff_length
fileinfo[28]处记录着key_info_length, 两者的主要区别是:buff_length是计算出来的值,info_length是实际长度
在.frm的IO_SIZE处,写入keybuff
在.frm的IO_SIZE + key_buff_length处,make_empty_rec()
再后边紧跟着connect_string,db_type,6个0,format_section_buff
在.frm的0x2000处记录着forminfo, screen_buff,pack_fields

.frm 64-65字节处的两个"//"是在make_new_entry()里写入的, 再后边的4个字节是fileinfo[10],一般为00 00 20 00

// fileinfo[64]
0: 0xfe
1: 1
2: FRM_VER+3+varchar
3: engine type
4-5: 1 -> 3 (make_new_entry)
6-7: IO_SIZE = 0x1000
8-9: 0 -> 1 (make_new_entry)
10-13: next_io_size(IO_SIZE + key_length + reclength + extra_size) = 0x2000
       -> 0x3000 (after make_new_entry)
14-15: key_length   // key_length怎么计算?
                    // for each key: 8(heaer) + 9*N(key_part) + 1(name_sep_char) + strlen(name)
                    // for all keys: 6(header) + 1(name_sep_char) + 9(extra bytes)
                    // keys * (8 + key_parts * 9 + name_len + 1) + (6+1+9) + key_comment_total_bytes.
16-17: reclength
18-21: max_rows
22-25: min_rows
27: 2
28-29: key_info_length, 实际key定义占的字节数
30-31: table_options
32: 0
33: 5
34-37: agv_row_length
38: default_charset low byte
39: 0
40: row_type
41: default_charset high byte
42-46: 0
47-48: key_length
51-54: mysql_version_id
55-58: extra_size
62-63: key_block_size

// 14-15和47-48的key_length其实是key_buff_length,程序根据这个大小分配出内存,然后在这块内存里pack_keys()
// pack_keys()最终会返回实际占有的字节数,并保存在fileinfo[28]处.
// 在.frm文件里,key定义是占有key_buff_length字节的,只不过没用上的部分为0而已

0x1000:
keybuff:
0: key_count
1: key_parts
2: 0
3: 0
4-5: __pos__ 到keybuff结尾的长度

// for each key
0-1: key->flags ^ HA_NOSAME
2-3: key->key_length
4: key_parts
5: key_alg
6-7: block_size
    // for each part
    0-1: key_part->fieldnr + 1 + FIELD_NAME_USED
    2-3: key_part->offset + data_offset + 1
    4: 0
    5-6: key_part->key_type
    7-8: key_part->length

__pos__: NAME_SEP_CHAR

// for each key
key_name
NAME_SEP_CHAR
0

0

// for each key
key comment

// empty record, reclength 在 forminfo[266]处
// 2字节的connect_string length + connect_string
// 2字节的engine name length + engine name
// 6字节的0
// 如果key有parser name,写入,不清楚什么是parser name


// Format section
// format_section_length = format_section_header_size + tablespace_length + 1 + create_fields.elements
0-1: format_section_length
2-5: format_section_flags
6-7: format_section_unused
tablespace
// for each field
one byte field_flags

0x3000:

// forminfo [288]
0-1: info_length + create_fields.elements*FCOMP + 288 +nlength + int_length + com_length
2-3: 0 -> 0x1000 (after make_new_entry)
46: comment.length + comment
256: screens
258: create_fields.elements
260: info_length
262: totlength
264: no_empty
266: reclength
268: n_length
270: int_count
272: int_parts
274: int_length
276: time_stamp_pos
278: 80
280: 22
282: null_fields
284: com_length

// screen_buff

// pack_fields

open_binary_frm()

/sql/table.cc?v=mysql-5.5.53#0730

// 在写read-frm.c时,有一个纠结的问题就是forminfo的位置.
// open_binary_frm()里一开始就通过 get_form_pos() 取得了forminfo的位置
// 这个位置在fileinfo的后边

table.frm

open_binary_frm()