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INFO-DIR-SECTION Programming & development tools.
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* Gdb: (gdb).                     The GNU debugger.
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   This file documents the GNU debugger GDB.

   This is the Ninth Edition, December 2001, of `Debugging with GDB:
the GNU Source-Level Debugger' for GDB Version 5.2.1.

   Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1998,
1999, 2000, 2001, 2002 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.

   (a) The Free Software Foundation's Back-Cover Text is: "You have
freedom to copy and modify this GNU Manual, like GNU software.  Copies
published by the Free Software Foundation raise funds for GNU
development."


File: gdb.info,  Node: Remote Protocol,  Next: Copying,  Prev: Maintenance Commands,  Up: Top

GDB Remote Serial Protocol
**************************

   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 mode          `!'                    Enable extended mode.  In
                                              extended mode, the remote
                                              server is made persistent.
                                              The `R' packet is used to
                                              restart the program being
                                              debugged.
                       reply `OK'             The remote target both
                                              supports and has enabled
                                              extended mode.
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 to
                                              operate when a specific
                                              thread context has been
                                              selected (i.e. 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 program being
                                              debugged.  XX, while needed,
                                              is ignored.  This packet is
                                              only available in extended
                                              mode.
                       no reply               The `R' packet has no reply.
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.
symbol lookup  `qSymbol::'    Notify the target that GDB is prepared to
                              serve symbol lookup requests.  Accept
                              requests from the target for the values of
                              symbols.

               reply `OK'     The target does not need to look up any
                              (more) symbols.
               reply          
               `qSymbol:'SYM_NAME
                              The target requests the value of symbol
                              SYM_NAME (hex encoded).  GDB may provide
                              the value by using the
                              `qSymbol:'SYM_VALUE:SYM_NAME message,
                              described below.
symbol value   `qSymbol:'SYM_VALUE:SYM_NAME
                              Set the value of SYM_NAME to SYM_VALUE.
                              SYM_NAME (hex encoded) is the name of a
                              symbol whose value the target has
                              previously requested.
                              SYM_VALUE (hex) is the value for symbol
                              SYM_NAME.  If GDB cannot supply a value
                              for SYM_NAME, then this field will be
                              empty.
               reply `OK'     The target does not need to look up any
                              (more) symbols.
               reply          
               `qSymbol:'SYM_NAME
                              The target requests the value of a new
                              symbol SYM_NAME (hex encoded).  GDB will
                              continue to supply the values of symbols
                              (if available), until the target ceases to
                              request them.

   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...'
     <- `+'

