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I open the ibd file of MySQL InnoDB engine and confused by the number two's complement.

For unsigned numbers, InnoDB used different representations.

ibd binary data

the code '80 05' is the PAGE_N_HEAP field of innodb page header, which represent number of heap records, the real value is 5, explicitly is an unsigned number, but how the code '80 05' come from.

the code '00 03' is the PAGE_N_RECS field of the InnoDB page header, which represent the number of user records, the real value is 3, also an unsigned number, the code '00 03' can be understood as the two's complement of number 3.

the codes '00 00 00 01' and '00 00 00 02', is the user record id fields, which was defined as unsigned int, the codes also can be understanded as the tow's complement.

my question is why mysql use '80 05' represent the number 5.

Nitesh Sharma
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Li.xz
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  • Do you have a reference document on this file format? Just winging it is probably not going to work. There's two different InnoDB storage formats, so you'll need to know which one you're using. – tadman Jun 21 '20 at 07:46
  • Two's Complement only impacts negative numbers. `80 05` could be -32763, or it could be 5 with a bitflag set. Remember, in [Two's Complement](https://en.wikipedia.org/wiki/Two%27s_complement) it's not as simple as setting a sign bit. – tadman Jun 21 '20 at 07:49
  • In some places (perhaps not the one you are talking about, InnoDB uses a 15-bit number and uses the top bit to indicate something else. Perhaps 8005 is: flag bit ON, and "+5". (It is _not_ a 2's complement "-5".) – Rick James Jun 21 '20 at 18:06
  • I confirm that I have get the correct InnoDB storage formats(https://dev.mysql.com/doc/internals/en/innodb-page-overview.html), besides, I also find similar situation in other article – Li.xz Jun 22 '20 at 03:29

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