Description of the .frm format

dbsake parses some undocumented / poorly documented formats for MySQL which were decoded by inspecting the MySQL source code. Here is a detailed document that attempts to describe the details of such formats for other who may hack on these parts of dbsake.

.frm fileinfo section

The fileinfo section consists of 64 bytes that encode information about the rest of the .frm file and various table level options.

The table here describes the information each byte encodes. Offsets are from the beginning of the file. All values are little-endian integer of the size noted in the Length column.

Offset Length (bytes) Description
Hex Dec
0000 0 2 bytes “Magic” identifier Always the byte sequence fe 01
0001 1
0002 2 1 byte .frm version [1]
0003 3 1 byte “legacy_db_type” [2]
0004 4 2-bytes “names_length” - always 3 and not used in recent MySQL. MySQL 3.23 set this to 1
0005 5
0006 6 2-bytes IO_SIZE; Always 4096 (0010)
0007 7
0008 8 2-bytes number of “forms” in the .frm Should always be 1, even back to 3.23
0009 9
000a 10 4-bytes Not really used except in .frm creation Purpose unclear, i guess for aligning sections in the ancient unireg format
000b 11
000c 12
000d 13
000e 14 2-bytes “tmp_key_length”; if equal to 0xffff then the key length is a 4-byte integer at offset 0x002f
000f 15
0010 16 2-bytes “rec_length” - this is the size of the byte string where default values are stored See Default Values
0011 17
0012 18 4-bytes Table MAX_ROWS=N opton
0013 19
0014 20
0015 21
0016 22 4-bytes Table MIN_ROWS=N option
0017 23
0018 24
0019 25
001a 26 1-byte Unused - always zero in 3.23 through 5.6
001b 27 1-byte Always 2 - “// Use long pack-fields”
001c 28 2-bytes key_info_length - size in bytes of the keyinfo section
001d 29
001e 30 2-bytes create_info->table_options See HA_OPTION_* values in include/my_base.h
001f 31
0020 32 1-byte Unused; comment “// No filename anymore”
0021 33 1-byte 5 in 5.0+ comment “// Mark for 5.0 frm file”
0022 34 4-bytes Table AVG_ROW_LENGTH option
0023 35
0024 36
0025 37
0026 38 1-byte Table DEFAULT CHARACTER SET option [3]
0027 39 1-byte Unused [4]
0028 40 1-byte Table ROW_FORMAT option
0029 41 1-byte Unused; formerly Table RAID_TYPE option
002a 42 1-byte Unused; formerly Table RAID_CHUNKS option
002b 43 4-bytes Unused; formerly Table RAID_CHUNKSIZE option
002c 44
002d 45
002e 46
002f 47 4-bytes Size in bytes of the keyinfo section where index metadata is defined
0030 48
0031 49
0032 50
0033 51 4-bytes MySQL version encoded as a 4-byte integer in little endian format. This is the value MYSQL_VERSION_ID from include/mysql_version.h in the mysql source tree. Example: ‘xb6xc5x00x00’ 0x0000c5b6 => 50614 => MySQL v5.6.14
0034 52
0035 53
0036 54
0037 55 4-bytes

Size in bytes of table “extra info”

  • CONNECTION=<string> (FEDERATED tables)
  • ENGINE=<string>
  • PARTITION BY clause + partitioning flags
  • WITH PARSER names (MySQL 5.1+)
  • Table COMMENT [5]
0038 56
0039 57
003a 58
003b 59 2-byte extra_rec_buf_length
003c 60
003d 61 1-byte Storage engine if table is partitioned [6]
003e 62 2-bytes Table KEY_BLOCK_SIZE option
003f 63
[1]This is defined as FRM_VER+3+ test(create_info->varchar) in 5.0+ Where FRM_VER is defined as 6, so the frm version will be either 9 or 10 depending on if the table has varchar columns
[2]Maps to an enum value from “enum legacy_db_type” in sql/handler.h
[3]Character set id maps to an id from INFORMATION_SCHEMA.COLLATIONS and encodes both the character set name and the collation
[4]In the source code, there is a comment indicating this byte will be used for TRANSACTIONAL and PAGE_CHECKSUM table options in the future
[5]The table comment is stored in one of two places in the .frm file If the comment size in bytes is < 255 this is stored in the forminfo Otherwise it will be estored in the extra info section after the fulltext parser names (if any)
[6]Numeric id that maps to a enum value from “enum legacy_db_type” in sql/handler.h, similar to legacy_db_type

Key info section

The key info section should always start at offset 0x1000 (4096); this is obtained from the 2-byte integer in fileinfo header at offset 6, but in any version of MySQL in the past decade will be 4096.

The size in bytes of this section is obtained from the key_length - typically this is 4-byte integer at offset 0x002f (47) in the header. Older versions of MySQL only allocated a 2-byte integer for this length, at offset 0x000e (14). This old location will have the value 0xffff if the key info length exceeds the capacity of a 2-byte integer.

The structure of this section consists of an initial header noting the total number of keys, total number of key components and the size of “extra” key data (namely index names and index comments). This is followed by a group for each index defined in the table and then the extra data - names for each index followed by an optional index comment strings.

The header is essentially three integers:

[key_count][key_parts_count][length of extra data]

Where key_count is the number of indexes this metdata describes,
key_parts_count is the number of components across all indexes and the length of extra data indicates how many bytes the index names and comments uses.

key_count and key_parts_count may be either 1 or 2 bytes. If the first byte is > 128 then key_count and key_parts_count use two bytes, otherwise they use one byte each. The extra length is always a 2 byte integer.

The logic in dbsake is:

key_count = keyinfo.uint8()
if key_count < 128:
    key_parts_count = keyinfo.uint8()
    key_count = (key_count & 0x7f) | (keyinfo.uint8() << 7)
    key_parts_count = keyinfo.uint16()
key_extra_length = keyinfo.uint16()

Each key metadata consists of 8 bytes and each key part consists of 9 bytes. So the total length of the index metadata is calculated by the formula:

key_count * 8 + key_parts_count * 9

And this is the offset, relative to the start of keyinfo section, where the index names and comments are found.

Each index group consists of 8 bytes of key metadata followed by 9 bytes of metadata for each indexed column.

Index metadata (8 bytes)
flags 2 bytes key flags from include/my_base.h.
length 2 bytes length of the index
key parts 1 byte number of columns covered by this index
algorithm 1 byte Key algorithm - maps to enum value “enum ha_key_alg”
unused 2 bytes  

Followed by 1 or more column index metadata:

Column index metadata (9 bytes)
field number 2 bytes Which column is indexed
offset 2 bytes Offset into a MySQL datastructure (internal use)
unused 1 byte  
key_type 2 bytes maps to enum ha_base_keytype
length 2 bytes length of this index component

The names and comments follow this data with names being separated by the byte value 255 (‘\xff’) and the names and comments sections being separated by a null byte. So this essentially looks like this sort of python bytestring:

b'\xffPRIMARY\xffix_column1\xff\x00<index comments>'

Index comments are length-prefixed strings. So there is a 2 byte integer (little-endian) followed by the specified number of bytes for each comment.

Index comments are not terribly common so this will often be empty.

Defaults Section

Immediately after the keyinfo section there is a byte string that details the defaults for each column. So this starts at IO_SIZE + key_length, which can be derived from the .frm header.

The format of this buffer is essentially:

[null_map][encoded column data]

Where the null_map is 1 or more bytes, with a bit per-column that can be nullable. The total number of bytes will be:

(null_column_count + 7 ) // 8

The first bit is always set and column bits start a 1 offset in the null map. If a bit is set for the current column then this indicates the the default is null (ie. DEFAULT NULL).

If a column’s default is not null, then its default data will be recorded at some offset noted in the Column metadata (described elsewhere in this document). The actual data format depends on the column type. This basically breaks down into the following cases:

  • integer-types - little-endian integers of 1, 2, 3, 4 or 8 bytes
  • float/double - little endian IEEE 745 values
  • decimal - either ascii strings (“3.14”) < MySQL 5.0, or a binary
    encoding of 9 decimal digits per 4-byte big-endian integer
  • timestamp - little endian integer representing seconds relative to epoch (< 5.6)
  • timestamp2 - big-endian integer representing seconds relative to epoch (5.6+);
    additionally packed fractional digits, similar to the decimal format
  • date/time - encodes the various components into various bits of a 3 - 8 byte integer
  • char - just a string with length bytes (space padded)
  • varchar - length-prefix string, with the prefix being a little endian integer of
    1 to 2 bytes.

See dbsake/mysqlfrm/ unpack_type_<name> method for how each datatype is actually decoded.

Extra data section

The “extra” section encodes some basic table properties. These include:

  • CONNECTION=<name> string (used by FEDERATED)
  • ENGINE=<name> strings
  • PARTITION BY string
  • “auto partitioned flag” (used by NDB, at least)
  • WITH PARSER - fulltext parser plugin names
  • Table COMMENT ‘…’ - only if > 254 bytes

Except for the fulltext parser plugin names (which are null terminated), all of these properties are length-prefix strings. This essentially has the format:

[2-byte length][<connection string>]
[2-byte length][<engine name string>]
[4-byte length][<partition by clause>][null byte]
[1-byte is_autopartitioned flag]
[parser name][null_byte] for each fulltext parser plugin used
[2-byte length][<table comment string>]

These strings should be decoded per the table’s default character set.


The .frm form info is a section consisting of 288 bytes with integers noting the length or count of elements in the table.

The start of this section can be found at offset 64 + names_len from the .frm header and the offset is a 4 byte integer. In python this would be found via

>> f = open('/var/lib/mysql/mysql/user.frm', 'rb')
>> # "names_length" documented in the .frm fileinfo header
>> names_len, = struct.unpack("<H", # always 3 in modern mysql
>> + names_len)
>> forminfo_offset, = struct.unpack("<I",

Here is a description of some of the more interesting fields available in the forminfo section. This is not meant to be exhaustive but merely to document the fields necessary for interpreting pertinent column metadata.

All offsets are relative to the start of the forminfo section


2 byte integer at offset 258

The number of coumns defined on this table


2 byte integer at offset 260

How many bytes follow the forminfo section prior to the start of the column metadata


2 byte integer at offset 282

How many nullable columns are defined in this table


2 byte integer at offset 268

Length in bytes (including delimiters) of column names


2 byte integer at offset 274

Length in bytes (including delimiters) of the set/enum labels


2 byte integer at offset 284

Length in bytes of the column comments

Column Metadata

17 bytes per column

Followed by \xff separated column names

Followed by a null byte

Followed by null terminated interval groups with each interval group consisting of interval names \xff separated.

Followed by a single string of column comments.