官网说明：http://dev.mysql.com/doc/internals/en/frm-file-format.html
frm是MySQL表结构定义文件，通常frm文件是不会损坏的，但是如果出现特殊情况出现frm文件损坏也不要放弃希望，例如下面报错：
150821 16:31:27 [ERROR] /usr/local/mysql51/libexec/mysqld: Incorrect information in file: './t/test1.frm'
当修复MyISAM和InnoDB表时，MySQL服务会首先去调用frm文件，所以我们只能通过修复frm文件进行后面的数据恢复。
MySQL通过sql/table.cc的create_frm()函数创建frm文件，创建出来的frm文件是二进制文件，需要通过hexdump解析成16进制来分析。
create_frm()函数对frm文件头部定义的代码
/* Create a .frm file */ File create_frm(THD *thd, const char *name, const char *db, const char *table, uint reclength, uchar *fileinfo, HA_CREATE_INFO *create_info, uint keys, KEY *key_info) { register File file; ulong length; uchar fill[IO_SIZE]; int create_flags= O_RDWR | O_TRUNC; ulong key_comment_total_bytes= 0; uint i; if (create_info->options & HA_LEX_CREATE_TMP_TABLE) create_flags|= O_EXCL | O_NOFOLLOW; /* Fix this when we have new .frm files; Current limit is 4G rows (QQ) */ if (create_info->max_rows > UINT_MAX32) create_info->max_rows= UINT_MAX32; if (create_info->min_rows > UINT_MAX32) create_info->min_rows= UINT_MAX32; if ((file= mysql_file_create(key_file_frm, name, CREATE_MODE, create_flags, MYF(0))) >= 0) { uint key_length, tmp_key_length, tmp, csid; bzero((char*) fileinfo,64); /* header */ fileinfo[0]=(uchar) 254; fileinfo[1]= 1; fileinfo[2]= FRM_VER+3+ test(create_info->varchar); fileinfo[3]= (uchar) ha_legacy_type( ha_checktype(thd,ha_legacy_type(create_info->db_type),0,0)); fileinfo[4]=1; int2store(fileinfo+6,IO_SIZE); /* Next block starts here */ /* Keep in sync with pack_keys() in unireg.cc For each key: 8 bytes for the key header 9 bytes for each key-part (MAX_REF_PARTS) NAME_LEN bytes for the name 1 byte for the NAMES_SEP_CHAR (before the name) For all keys: 6 bytes for the header 1 byte for the NAMES_SEP_CHAR (after the last name) 9 extra bytes (padding for safety? alignment?) */ for (i= 0; i < keys; i++) { DBUG_ASSERT(test(key_info[i].flags & HA_USES_COMMENT) == (key_info[i].comment.length > 0)); if (key_info[i].flags & HA_USES_COMMENT) key_comment_total_bytes += 2 + key_info[i].comment.length; } key_length= keys * (8 + MAX_REF_PARTS * 9 + NAME_LEN + 1) + 16 + key_comment_total_bytes; length= next_io_size((ulong) (IO_SIZE+key_length+reclength+ create_info->extra_size)); int4store(fileinfo+10,length); tmp_key_length= (key_length < 0xffff) ? key_length : 0xffff; int2store(fileinfo+14,tmp_key_length); int2store(fileinfo+16,reclength); int4store(fileinfo+18,create_info->max_rows); int4store(fileinfo+22,create_info->min_rows); /* fileinfo[26] is set in mysql_create_frm() */ fileinfo[27]=2; // Use long pack-fields /* fileinfo[28 & 29] is set to key_info_length in mysql_create_frm() */ create_info->table_options|=HA_OPTION_LONG_BLOB_PTR; // Use portable blob pointers int2store(fileinfo+30,create_info->table_options); fileinfo[32]=0; // No filename anymore fileinfo[33]=5; // Mark for 5.0 frm file int4store(fileinfo+34,create_info->avg_row_length); csid= (create_info->default_table_charset ? create_info->default_table_charset->number : 0); fileinfo[38]= (uchar) csid; /* In future versions, we will store in fileinfo[39] the values of the TRANSACTIONAL and PAGE_CHECKSUM clauses of CREATE TABLE. */ fileinfo[39]= 0; fileinfo[40]= (uchar) create_info->row_type; /* Next few bytes where for RAID support */ fileinfo[41]= (uchar) (csid >> 8); fileinfo[42]= 0; fileinfo[43]= 0; fileinfo[44]= 0; fileinfo[45]= 0; fileinfo[46]= 0; int4store(fileinfo+47, key_length); tmp= MYSQL_VERSION_ID; // Store to avoid warning from int4store int4store(fileinfo+51, tmp); int4store(fileinfo+55, create_info->extra_size); /* 59-60 is reserved for extra_rec_buf_length, 61 for default_part_db_type */ int2store(fileinfo+62, create_info->key_block_size); bzero(fill,IO_SIZE); for (; length > IO_SIZE ; length-= IO_SIZE) { if (mysql_file_write(file, fill, IO_SIZE, MYF(MY_WME | MY_NABP))) { (void) mysql_file_close(file, MYF(0)); (void) mysql_file_delete(key_file_frm, name, MYF(0)); return(-1); } } } else { if (my_errno == ENOENT) my_error(ER_BAD_DB_ERROR,MYF(0),db); else my_error(ER_CANT_CREATE_TABLE,MYF(0),table,my_errno); } return (file); } /* create_frm */
open_binary_frm()函数对对frm索引部分定义的代码
for (i=0 ; i < keys ; i++, keyinfo++) { keyinfo->table= 0; // Updated in open_frm if (new_frm_ver >= 3) { keyinfo->flags= (uint) uint2korr(strpos) ^ HA_NOSAME; keyinfo->key_length= (uint) uint2korr(strpos+2); keyinfo->key_parts= (uint) strpos[4]; keyinfo->algorithm= (enum ha_key_alg) strpos[5]; keyinfo->block_size= uint2korr(strpos+6); strpos+=8; } else { keyinfo->flags= ((uint) strpos[0]) ^ HA_NOSAME; keyinfo->key_length= (uint) uint2korr(strpos+1); keyinfo->key_parts= (uint) strpos[3]; keyinfo->algorithm= HA_KEY_ALG_UNDEF; strpos+=4; } keyinfo->key_part= key_part; keyinfo->rec_per_key= rec_per_key; for (j=keyinfo->key_parts ; j-- ; key_part++) { *rec_per_key++=0; key_part->fieldnr= (uint16) (uint2korr(strpos) & FIELD_NR_MASK); key_part->offset= (uint) uint2korr(strpos+2)-1; key_part->key_type= (uint) uint2korr(strpos+5); // key_part->field= (Field*) 0; // Will be fixed later if (new_frm_ver >= 1) { key_part->key_part_flag= *(strpos+4); key_part->length= (uint) uint2korr(strpos+7); strpos+=9; } else { key_part->length= *(strpos+4); key_part->key_part_flag=0; if (key_part->length > 128) { key_part->length&=127; /* purecov: inspected */ key_part->key_part_flag=HA_REVERSE_SORT; /* purecov: inspected */ } strpos+=7; } key_part->store_length=key_part->length; } } keynames=(char*) key_part; strpos+= (strmov(keynames, (char *) strpos) - keynames)+1; //reading index comments for (keyinfo= share->key_info, i=0; i < keys; i++, keyinfo++) { if (keyinfo->flags & HA_USES_COMMENT) { keyinfo->comment.length= uint2korr(strpos); keyinfo->comment.str= strmake_root(&share->mem_root, (char*) strpos+2, keyinfo->comment.length); strpos+= 2 + keyinfo->comment.length; } DBUG_ASSERT(test(keyinfo->flags & HA_USES_COMMENT) == (keyinfo->comment.length > 0)); }
hexdump是Linux下的一个二进制文件查看工具，可以将二进制文件转换为ASCII、10进制、16进制或8进制进行查看。
hexdump 参数
-C 每一字节以16进制显示，一行共16个字节，显示十六进制存储的文本内容
-b 每一字节以八进制显示，一行共16个字节，一行开始以十六进制显示偏移值；
0000000 177 105 114 106 002 001 001 000 000 000 000 000 000 000 000 000
-c 每一字节以ASCII字符显示，其余同上；
0000000 177 E L F 002 001 001 \0 \0 \0 \0 \0 \0 \0 \0 \0
-n 只解释指定长度字节
单位：默认十进制，0x或0X开头则为16进制，0开头则为8进制。默认为字节，b则为512字节，k则为1024字节，m则为1048576字节
-d 双字节十进制显示
-o 双字节八进制显示
-v 去除中间显示的“*”字符
-x 双字节十六进制显示
-e 格式化参数
实例版本与表字符集：
参考：https://www.percona.com/blog/2015/07/09/obtain-mysql-version-frm-file/
建表的实例版本0x033 语句hexdump -s 0x33 -n 2 -v -d table.frm
[root@test1 ~]# hexdump -s 0x33 -n 2 -v -d /data/3308/test/test1.frm
0000033 50153
0000035 所以版本为5.1.53，因为5.1/5.5和5.6在字段类型定义上有不同，所以确定好建表实例版本很重要，字段类型定义见下面
表字符集0x026
21=utf8
08=latin1
1c=GBK
语句hexdump -s 0x26 -n 1 table.frm
frm列属性：
1、列序号（初始列序号为4）
2、字段长度，整形长度
3、字段长度，latin1字符集字符类型长度，GBK字符集字符类型varchar长度*2，varchar(30)相当于就是60字节长度，换成16进制是3c，utf8字符集字符类型varchar长度*3，varchar(30)相当于就是90字节长度，换成16进制是5a
4、
5、
6、
7、
8、Flags for zerofill, unsigned, etc.(int 1b)
9、Additional flags,and scale if decimal/numeric(DEFAULT NULL 80,NOT NULL 40,DEFAULT 'VALUE' 00)
10、代码定义unireg_type，AUTO_INCREMENT of
11、
12、代码定义interval_nr
13、字段类型
14、字符集
15、备注长度
16、备注长度
字段类型（注意5.6版本字段类型有不同，会影响数据恢复）：
Data type for v5.1&v5.5 (v5.6)
fe=char
fa=mediumtext
f6=decimal
fc=text
of=varchar
01=tinyint
02=smallint
03=int
04=float
05=real
07=timestamp (v5.6 11=timestamp)
08=bigint
09=mediumint
10=bit
ob=time (v5.6 13=time)
oc=datetime (v5.6 12=datetime)
0d=year
0e=date
表中所含索引：
偏移量在0x1000之后的一段是frm索引部分，用hexdump -C打开后很容易找到
0x1000：有几个索引
0x1001：全部索引包含几个字段
索引名是明文，具体索引结构见示例。
表：
CREATE TABLE `test3` ( `a` int(11) NOT NULL, `b` varchar(10) DEFAULT NULL, `c` int(11) NOT NULL, PRIMARY KEY (`a`), UNIQUE KEY `uniq_1` (`b`,`c`), KEY `idx_1` (`c`,`b`), KEY `idx_2` (`c`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8
十六进制文件打开：
[root@test1 ~]# hexdump -C /data/3308/test/test3.frm
00000000 fe 01 0a 0c 03 00 00 10 01 00 00 30 00 00 74 05 |...........0..t.|
00000010 28 00 00 00 00 00 00 00 00 00 00 02 79 00 09 00 |(...........y...|
00000020 00 05 00 00 00 00 21 00 00 00 00 00 00 00 00 74 |......!........t| #表字符集
00000030 05 00 00 e9 c3 00 00 10 00 00 00 00 00 00 00 00 |................| #标红的是建表实例版本号
00000040 2f 2f 00 00 20 00 00 00 00 00 00 00 00 00 00 00 |//.. ...........|
00000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00001000 04 06 00 00 1d 00 00 00 04 00 01 00 00 00 01 80 |................|
00001010 02 00 00 1b 40 04 00 68 00 22 00 02 00 00 00 02 |....@..h."......|
00001020 80 06 00 00 00 80 1e 00 03 80 25 00 00 1b 40 04 |..........%...@.|
00001030 00 69 00 22 00 02 00 00 00 03 80 25 00 00 1b 40 |.i.".......%...@|
00001040 04 00 02 80 06 00 00 00 80 1e 00 01 00 04 00 01 |................|
00001050 00 00 00 03 80 25 00 00 1b 40 04 00 ff 50 52 49 |.....%...@...PRI|
00001060 4d 41 52 59 ff 75 6e 69 71 5f 31 ff 69 64 78 5f |MARY.uniq_1.idx_|
00001070 31 ff 69 64 78 5f 32 ff 00 00 00 00 00 00 00 00 |1.idx_2.........|
00001080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00001570 00 00 00 00 ff 00 00 00 00 00 00 00 00 00 00 00 |................|
00001580 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00001590 00 00 00 00 00 00 00 00 00 00 00 00 00 00 06 00 |................|
000015a0 49 6e 6e 6f 44 42 00 00 00 00 00 00 00 00 00 00 |InnoDB..........|
000015b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00002000 9a 01 00 10 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00002010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00002100 01 00 03 00 3f 00 34 00 00 00 28 00 08 00 00 00 |....?.4...(.....|
00002110 00 00 00 00 00 00 50 00 16 00 01 00 00 00 00 00 |......P.........|
00002120 3f 00 04 03 02 14 29 20 20 20 20 20 20 20 20 20 |?.....) |
00002130 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
00002140 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 00 | .|
00002150 04 00 02 61 00 05 00 02 62 00 06 00 02 63 00 04 |...a....b....c..|
00002160 02 0b 0b 00 02 00 00 1b 40 00 00 00 03 3f 00 00 |........@....?..|
00002170 05 02 1e 1e 00 06 00 00 00 80 00 00 00 0f 21 00 |..............!.|
00002180 00 06 02 0b 0b 00 25 00 00 1b 40 00 00 00 03 3f |......%...@....?|
00002190 00 00 ff 61 ff 62 ff 63 ff 00 |...a.b.c..|
通过上面的颜色区分，圈出的黄色部分是索引属性，下面红蓝绿三色是三列属性。
列属性结构：
红色部分：字段序号（4开始，4、5、6就是字段第一第二第三）
蓝色部分：字段长度
棕色部分：是否为空
绿色部分：字段类型
黄色部分：字符集
索引属性结构：
索引头部：
淡蓝色部分：索引统计数
粉色部分：索引总共有多少列
索引主体：
棕色部分：是否唯一索引
红色部分：表中列的序号
绿色部分：表中对应列的属性
字段默认值：
字段默认值不保存在字段属性中，而是保存在描述表引擎的那段中 int类型默认值保存为十六进制需转换十进制，char类型默认值保存为十六进制文本可通过hexdump -C直接看到 如果没有索引段则默认值在，0x1011后，如果有索引段，则位置顺延 例如表 CREATE TABLE `test1` ( `a` int(11) NOT NULL DEFAULT '2010', `b` varchar(10) NOT NULL DEFAULT '2011' , `c` int(11) default '30', `d` varchar(10) NOT NULL DEFAULT 'Yes' )engine=innodb default charset=utf8; * 00001000 00 00 00 00 02 00 ff 00 00 00 00 00 00 00 00 00 |................| 00001010 fe da 07 00 00 04 32 30 31 31 00 00 00 00 00 00 |......2011......| 00001020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00001030 00 00 00 00 1e 00 00 00 03 59 65 73 00 00 00 00 |.........Yes....| 00001040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00001050 00 00 00 00 00 00 00 00 00 06 00 49 6e 6e 6f 44 |...........InnoD| 00001060 42 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |B...............| 00001070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * column a:da 07 00 00 column b:04 32 30 31 31 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 column c:1e 00 00 00 column d:03 59 65 73 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
需要注意char字段的默认值是根据字段长度和字符集相关的，如上表varchar(10)，utf8是3bit，就是30个十六进制长度。
