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Debugging with GDB

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D.1 Overview

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:

 
$packet-data#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:

 
$sequence-id:packet-data#checksum

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 retransmission):

 
-> $packet-data#checksum
<- +

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 exceptions).

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.

So:
 
"0* "
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 optional.


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