There may be occasions when you need to know something about the
protocol--for example, if there is only one serial port to your target
machine, you might want your program to do something special if it
recognizes a packet meant for gdb.
In the examples below, -> and <- are used to indicate
transmitted and received data respectfully.
All gdb commands and responses (other than acknowledgments) are
sent as a packet. A packet is introduced with the character
$, the actual packet-data, and the terminating character
# followed by a two-digit checksum:
The two-digit checksum is computed as the modulo 256 sum of all
characters between the leading $ and the trailing # (an
eight bit unsigned checksum).
Implementors should note that prior to gdb 5.0 the protocol
specification also included an optional two-digit sequence-id:
That sequence-id was appended to the acknowledgment. gdb
has never output sequence-ids. Stubs that handle packets added
since gdb 5.0 must not accept sequence-id.
When either the host or the target machine receives a packet, the first
response expected is an acknowledgment: either + (to indicate
the package was received correctly) or - (to request
The host (gdb) sends commands, and the target (the
debugging stub incorporated in your program) sends a response. In
the case of step and continue commands, the response is only sent
when the operation has completed (the target has again stopped).
packet-data consists of a sequence of characters with the
exception of # and $ (see X packet for additional
Fields within the packet should be separated using , ; or
:. Except where otherwise noted all numbers are represented in
hex with leading zeros suppressed.
Implementors should note that prior to gdb 5.0, the character
: could not appear as the third character in a packet (as it
would potentially conflict with the sequence-id).
Response data can be run-length encoded to save space. A *
means that the next character is an ascii encoding giving a repeat count
which stands for that many repetitions of the character preceding the
*. The encoding is n+29, yielding a printable character
where n >=3 (which is where rle starts to win). The printable
characters $, #, + and - or with a numeric
value greater than 126 should not be used.
Some remote systems have used a different run-length encoding mechanism
loosely refered to as the cisco encoding. Following the *
character are two hex digits that indicate the size of the packet.
means the same as "0000".
The error response returned for some packets includes a two character
error number. That number is not well defined.
For any command not supported by the stub, an empty response
($#00) should be returned. That way it is possible to extend the
protocol. A newer gdb can tell if a packet is supported based
on that response.
A stub is required to support the g, G, m, M,
c, and s commands. All other commands are