(gdb.info) Protocol
Info Catalog
(gdb.info) Debug Session
(gdb.info) Remote Serial
(gdb.info) Server
Communication protocol
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The stub files provided with GDB implement the target side of the
communication protocol, and the GDB side is implemented in the GDB
source file `remote.c'. Normally, you can simply allow these
subroutines to communicate, and ignore the details. (If you're
implementing your own stub file, you can still ignore the details: start
with one of the existing stub files. `sparc-stub.c' is the best
organized, and therefore the easiest to read.)
However, 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.
Below is a complete list of all currently defined COMMANDs and their
corresponding response DATA:
Packet Request Description
extended ops `!' Use the extended remote
protocol. Sticky--only
needs to be set once. The
extended remote protocol
supports the `R' packet.
reply `' Stubs that support the
extended remote protocol
return `' which,
unfortunately, is identical
to the response returned by
stubs that do not support
protocol extensions.
last signal `?' Indicate the reason the
target halted. The reply is
the same as for step and
continue.
reply see below
reserved `a' Reserved for future use
set program arguments `A'ARGLEN`,'ARGNUM`,'ARG`,...'
*(reserved)*
Initialized `argv[]' array
passed into program. ARGLEN
specifies the number of
bytes in the hex encoded
byte stream ARG. See
`gdbserver' for more details.
reply `OK'
reply `E'NN
set baud `b'BAUD Change the serial line speed
*(deprecated)* to BAUD. JTC: _When does the
transport layer state
change? When it's received,
or after the ACK is
transmitted. In either
case, there are problems if
the command or the
acknowledgment packet is
dropped._ Stan: _If people
really wanted to add
something like this, and get
it working for the first
time, they ought to modify
ser-unix.c to send some kind
of out-of-band message to a
specially-setup stub and
have the switch happen "in
between" packets, so that
from remote protocol's point
of view, nothing actually
happened._
set breakpoint `B'ADDR,MODE Set (MODE is `S') or clear
*(deprecated)* (MODE is `C') a breakpoint
at ADDR. _This has been
replaced by the `Z' and `z'
packets._
continue `c'ADDR ADDR is address to resume.
If ADDR is omitted, resume at
current address.
reply see below
continue with signal `C'SIG`;'ADDR Continue with signal SIG
(hex signal number). If
`;'ADDR is omitted, resume
at same address.
reply see below
toggle debug `d' toggle debug flag.
*(deprecated)*
detach `D' Detach GDB from the remote
system. Sent to the remote
target before GDB
disconnects.
reply _no response_ GDB does not check for any
response after sending this
packet.
reserved `e' Reserved for future use
reserved `E' Reserved for future use
reserved `f' Reserved for future use
reserved `F' Reserved for future use
read registers `g' Read general registers.
reply XX... Each byte of register data
is described by two hex
digits. The bytes with the
register are transmitted in
target byte order. The size
of each register and their
position within the `g'
PACKET are determined by the
GDB internal macros
REGISTER_RAW_SIZE and
REGISTER_NAME macros. The
specification of several
standard `g' packets is
specified below.
`E'NN for an error.
write regs `G'XX... See `g' for a description of
the XX... data.
reply `OK' for success
reply `E'NN for an error
reserved `h' Reserved for future use
set thread `H'CT... Set thread for subsequent
operations (`m', `M', `g',
`G', et.al.). C = `c' for
thread used in step and
continue; T... can be -1 for
all threads. C = `g' for
thread used in other
operations. If zero, pick a
thread, any thread.
reply `OK' for success
reply `E'NN for an error
cycle step *(draft)* `i'ADDR`,'NNN Step the remote target by a
single clock cycle. If
`,'NNN is present, cycle
step NNN cycles. If ADDR is
present, cycle step starting
at that address.
signal then cycle `I' See `i' and `S' for likely
step *(reserved)* syntax and semantics.
reserved `j' Reserved for future use
reserved `J' Reserved for future use
kill request `k' FIXME: _There is no
description of how operate
when a specific thread
context has been selected
(ie. does 'k' kill only that
thread?)_.
reserved `l' Reserved for future use
reserved `L' Reserved for future use
read memory `m'ADDR`,'LENGTH Read LENGTH bytes of memory
starting at address ADDR.
Neither GDB nor the stub
assume that sized memory
transfers are assumed using
word alligned accesses.
FIXME: _A word aligned memory
transfer mechanism is
needed._
reply XX... XX... is mem contents. Can
be fewer bytes than
requested if able to read
only part of the data.
Neither GDB nor the stub
assume that sized memory
transfers are assumed using
word alligned accesses.
FIXME: _A word aligned
memory transfer mechanism is
needed._
reply `E'NN NN is errno
write mem `M'ADDR,LENGTH`:'XX... Write LENGTH bytes of memory
starting at address ADDR.
XX... is the data.
reply `OK' for success
reply `E'NN for an error (this includes
the case where only part of
the data was written).
reserved `n' Reserved for future use
reserved `N' Reserved for future use
reserved `o' Reserved for future use
reserved `O' Reserved for future use
read reg *(reserved)* `p'N... See write register.
return R.... The hex encoded value of the
register in target byte
order.
write reg `P'N...`='R... Write register N... with
value R..., which contains
two hex digits for each byte
in the register (target byte
order).
reply `OK' for success
reply `E'NN for an error
general query `q'QUERY Request info about QUERY.
In general GDB queries have
a leading upper case letter.
Custom vendor queries
should use a company prefix
(in lower case) ex:
`qfsf.var'. QUERY may
optionally be followed by a
`,' or `;' separated list.
Stubs must ensure that they
match the full QUERY name.
reply `XX...' Hex encoded data from query.
The reply can not be empty.
reply `E'NN error reply
reply `' Indicating an unrecognized
QUERY.
general set `Q'VAR`='VAL Set value of VAR to VAL.
See `q' for a discussing of
naming conventions.
reset *(deprecated)* `r' Reset the entire system.
remote restart `R'XX Restart the remote server.
XX while needed has no clear
definition. FIXME: _An
example interaction
explaining how this packet
is used in extended-remote
mode is needed_.
step `s'ADDR ADDR is address to resume.
If ADDR is omitted, resume at
same address.
reply see below
step with signal `S'SIG`;'ADDR Like `C' but step not
continue.
reply see below
search `t'ADDR`:'PP`,'MM Search backwards starting at
address ADDR for a match
with pattern PP and mask MM.
PP and MM are 4 bytes.
ADDR must be at least 3
digits.
thread alive `T'XX Find out if the thread XX is
alive.
reply `OK' thread is still alive
reply `E'NN thread is dead
reserved `u' Reserved for future use
reserved `U' Reserved for future use
reserved `v' Reserved for future use
reserved `V' Reserved for future use
reserved `w' Reserved for future use
reserved `W' Reserved for future use
reserved `x' Reserved for future use
write mem (binary) `X'ADDR`,'LENGTH:XX... ADDR is address, LENGTH is
number of bytes, XX... is
binary data. The characters
`$', `#', and `0x7d' are
escaped using `0x7d'.
reply `OK' for success
reply `E'NN for an error
reserved `y' Reserved for future use
reserved `Y' Reserved for future use
remove break or `z'T`,'ADDR`,'LENGTH See `Z'.
watchpoint *(draft)*
insert break or `Z'T`,'ADDR`,'LENGTH T is type: `0' - software
watchpoint *(draft)* breakpoint, `1' - hardware
breakpoint, `2' - write
watchpoint, `3' - read
watchpoint, `4' - access
watchpoint; ADDR is address;
LENGTH is in bytes. For a
software breakpoint, LENGTH
specifies the size of the
instruction to be patched.
For hardware breakpoints and
watchpoints LENGTH specifies
the memory region to be
monitored. To avoid
potential problems with
duplicate packets, the
operations should be
implemented in an idempotent
way.
reply `E'NN for an error
reply `OK' for success
`' If not supported.
reserved <other> Reserved for future use
The `C', `c', `S', `s' and `?' packets can receive any of the below
as a reply. In the case of the `C', `c', `S' and `s' packets, that
reply is only returned when the target halts. In the below the exact
meaning of `signal number' is poorly defined. In general one of the
UNIX signal numbering conventions is used.
`S'AA AA is the signal number
`T'AAN...`:'R...`;'N...`:'R...`;'N...`:'R...`;'AA = two hex digit signal number; N... =
register number (hex), R... = target byte
ordered register contents, size defined by
`REGISTER_RAW_SIZE'; N... = `thread', R...
= thread process ID, this is a hex
integer; N... = other string not starting
with valid hex digit. GDB should ignore
this N..., R... pair and go on to the
next. This way we can extend the protocol.
`W'AA The process exited, and AA is the exit
status. This is only applicable for
certains sorts of targets.
`X'AA The process terminated with signal AA.
`N'AA`;'T...`;'D...`;'B... AA = signal number; T... = address of
*(obsolete)* symbol "_start"; D... = base of data
section; B... = base of bss section.
_Note: only used by Cisco Systems targets.
The difference between this reply and the
"qOffsets" query is that the 'N' packet
may arrive spontaneously whereas the
'qOffsets' is a query initiated by the host
debugger._
`O'XX... XX... is hex encoding of ASCII data. This
can happen at any time while the program
is running and the debugger should
continue to wait for 'W', 'T', etc.
The following set and query packets have already been defined.
current thread `q'`C' Return the current thread id.
reply `QC'PID Where PID is a HEX encoded 16 bit process
id.
reply * Any other reply implies the old pid.
all thread ids `q'`fThreadInfo'
`q'`sThreadInfo'Obtain a list of active thread ids from
the target (OS). Since there may be too
many active threads to fit into one reply
packet, this query works iteratively: it
may require more than one query/reply
sequence to obtain the entire list of
threads. The first query of the sequence
will be the `qf'`ThreadInfo' query;
subsequent queries in the sequence will be
the `qs'`ThreadInfo' query.
NOTE: replaces the `qL' query (see below).
reply `m'<ID> A single thread id
reply a comma-separated list of thread ids
`m'<ID>,<ID>...
reply `l' (lower case 'el') denotes end of list.
In response to each query, the target will
reply with a list of one or more thread
ids, in big-endian hex, separated by
commas. GDB will respond to each reply
with a request for more thread ids (using
the `qs' form of the query), until the
target responds with `l' (lower-case el,
for `'last'').
extra thread `q'`ThreadExtraInfo'`,'ID
info
Where <ID> is a thread-id in big-endian
hex. Obtain a printable string
description of a thread's attributes from
the target OS. This string may contain
anything that the target OS thinks is
interesting for GDB to tell the user about
the thread. The string is displayed in
GDB's `info threads' display. Some
examples of possible thread extra info
strings are "Runnable", or "Blocked on
Mutex".
reply XX... Where XX... is a hex encoding of ASCII
data, comprising the printable string
containing the extra information about the
thread's attributes.
query LIST or `q'`L'STARTFLAGTHREADCOUNTNEXTTHREAD
THREADLIST
*(deprecated)*
Obtain thread information from RTOS.
Where: STARTFLAG (one hex digit) is one to
indicate the first query and zero to
indicate a subsequent query; THREADCOUNT
(two hex digits) is the maximum number of
threads the response packet can contain;
and NEXTTHREAD (eight hex digits), for
subsequent queries (STARTFLAG is zero), is
returned in the response as ARGTHREAD.
NOTE: this query is replaced by the
`q'`fThreadInfo' query (see above).
reply
`q'`M'COUNTDONEARGTHREADTHREAD...
Where: COUNT (two hex digits) is the
number of threads being returned; DONE
(one hex digit) is zero to indicate more
threads and one indicates no further
threads; ARGTHREADID (eight hex digits) is
NEXTTHREAD from the request packet;
THREAD... is a sequence of thread IDs from
the target. THREADID (eight hex digits).
See `remote.c:parse_threadlist_response()'.
compute CRC `q'`CRC:'ADDR`,'LENGTH
of memory
block
reply `E'NN An error (such as memory fault)
reply `C'CRC32 A 32 bit cyclic redundancy check of the
specified memory region.
query sect `q'`Offsets' Get section offsets that the target used
offs when re-locating the downloaded image.
_Note: while a `Bss' offset is included in
the response, GDB ignores this and instead
applies the `Data' offset to the `Bss'
section._
reply
`Text='XXX`;Data='YYY`;Bss='ZZZ
thread info `q'`P'MODETHREADID
request
Returns information on THREADID. Where:
MODE is a hex encoded 32 bit mode;
THREADID is a hex encoded 64 bit thread ID.
reply * See
`remote.c:remote_unpack_thread_info_response()'.
remote command `q'`Rcmd,'COMMAND
COMMAND (hex encoded) is passed to the
local interpreter for execution. Invalid
commands should be reported using the
output string. Before the final result
packet, the target may also respond with a
number of intermediate `O'OUTPUT console
output packets. _Implementors should note
that providing access to a stubs's
interpreter may have security
implications_.
reply `OK' A command response with no output.
reply OUTPUT A command response with the hex encoded
output string OUTPUT.
reply `E'NN Indicate a badly formed request.
reply `' When `q'`Rcmd' is not recognized.
The following `g'/`G' packets have previously been defined. In the
below, some thirty-two bit registers are transferred as sixty-four
bits. Those registers should be zero/sign extended (which?) to fill the
space allocated. Register bytes are transfered in target byte order.
The two nibbles within a register byte are transfered most-significant -
least-significant.
MIPS32 All registers are transfered as
thirty-two bit quantities in the
order: 32 general-purpose; sr; lo;
hi; bad; cause; pc; 32
floating-point registers; fsr; fir;
fp.
MIPS64 All registers are transfered as
sixty-four bit quantities (including
thirty-two bit registers such as
`sr'). The ordering is the same as
`MIPS32'.
Example sequence of a target being re-started. Notice how the
restart does not get any direct output:
<- `R00'
-> `+'
_target restarts_
<- `?'
-> `+'
-> `T001:1234123412341234'
<- `+'
Example sequence of a target being stepped by a single instruction:
<- `G1445...'
-> `+'
<- `s'
-> `+'
_time passes_
-> `T001:1234123412341234'
<- `+'
<- `g'
-> `+'
-> `1455...'
<- `+'
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