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# D. GDB Remote Serial Protocol

> D.1 Overview |  |
> D.2 Packets |  |
> D.3 Stop Reply Packets |  |
> D.4 General Query Packets |  |
> D.5 Register Packet Format |  |
> D.6 Tracepoint Packets |  |
> D.7 Host I/O Packets |  |
> D.8 Interrupts |  |
> D.9 Examples |  |
> D.10 File-I/O Remote Protocol Extension |  |
> D.11 Library List Format |  |
> D.12 Memory Map Format |  |

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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, respectively.

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).

Binary data in most packets is encoded either as two hexadecimal digits per byte of binary data. This allowed the traditional remote protocol to work over connections which were only seven-bit clean. Some packets designed more recently assume an eight-bit clean connection, and use a more efficient encoding to send and receive binary data.

The binary data representation uses `7d` (ASCII ``}'`) as an escape character. Any escaped byte is transmitted as the escape character followed by the original character XORed with `0x20`. For example, the byte `0x7d` would be transmitted as the two bytes `0x7d 0x5d`. The bytes `0x23` (ASCII ``#'`), `0x24` (ASCII ``$'`), and `0x7d` (ASCII ``}'`) must always be escaped. Responses sent by the stub must also escape `0x2a` (ASCII ``*'`), so that it is not interpreted as the start of a run-length encoded sequence (described next).

Response data can be run-length encoded to save space. Run-length encoding replaces runs of identical characters with one instance of the repeated character, followed by a ``*'` and a repeat count. The repeat count is itself sent encoded, to avoid binary characters in data: a value of n is sent as `n+29`. For a repeat count greater or equal to 3, this produces a printable ASCII character, e.g. a space (ASCII code 32) for a repeat count of 3. (This is because run-length encoding starts to win for counts 3 or more.) Thus, for example, ``0* '` is a run-length encoding of "0000": the space character after ``*'` means repeat the leading `0` `32 - 29 = 3` more times.

The printable characters ``#'` and ``$'` or with a numeric value greater than 126 must not be used. Runs of six repeats (``#'`) or seven repeats (``$'`) can be expanded using a repeat count of only five (``"'`). For example, ``00000000'` can be encoded as ``0*"00'`.

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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## D.2 Packets

The following table provides a complete list of all currently defined commands and their corresponding response data. See section D.10 File-I/O Remote Protocol Extension, for details about the File I/O extension of the remote protocol.

Each packet's description has a template showing the packet's overall syntax, followed by an explanation of the packet's meaning. We include spaces in some of the templates for clarity; these are not part of the packet's syntax. No GDB packet uses spaces to separate its components. For example, a template like ``foo bar baz'` describes a packet beginning with the three ASCII bytes ``foo'`, followed by a bar, followed directly by a baz. GDB does not transmit a space character between the ``foo'` and the bar, or between the bar and the baz.

Note that all packet forms beginning with an upper- or lower-case letter, other than those described here, are reserved for future use.

Here are the packet descriptions.

``!'`
  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.

``?'`
  Indicate the reason the target halted. The reply is the same as for step and continue.

  Reply: See section D.3 Stop Reply Packets, for the reply specifications.

``A arglen,argnum,arg,...'`
  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'`
    The arguments were set.
  ``E NN'`
    An error occurred.

``b baud'`
  (Don't use this packet; its behavior is not well-defined.) Change the serial line speed 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.

``B addr,mode'`
  Set (mode is ``S'`) or clear (mode is ``C'`) a breakpoint at addr.

  Don't use this packet. Use the ``Z'` and ``z'` packets instead (see insert breakpoint or watchpoint packet).

``c [addr]'`
  Continue. addr is address to resume. If addr is omitted, resume at current address.

  Reply: See section D.3 Stop Reply Packets, for the reply specifications.

``C sig[;addr]'`
  Continue with signal sig (hex signal number). If ``;addr'` is omitted, resume at same address.

  Reply: See section D.3 Stop Reply Packets, for the reply specifications.

``d'`
  Toggle debug flag.

  Don't use this packet; instead, define a general set packet (see section D.4 General Query Packets).

``D'`
  Detach GDB from the remote system. Sent to the remote target before GDB disconnects via the `detach` command.

  Reply:

  ``OK'`
    for success
  ``E NN'`
    for an error

``F RC,EE,CF;XX'`
  A reply from GDB to an ``F'` packet sent by the target. This is part of the File-I/O protocol extension. See section D.10 File-I/O Remote Protocol Extension, for the specification.

``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 gdbarch functions `DEPRECATED_REGISTER_RAW_SIZE` and `gdbarch_register_name`. The specification of several standard ``g'` packets is specified below.
  ``E NN'`
    for an error.

``G XX...'`
  Write general registers. See read registers packet, for a description of the XX... data.

  Reply:

  ``OK'`
    for success
  ``E NN'`
    for an error

``H c t'`
  Set thread for subsequent operations (``m'`, ``M'`, ``g'`, ``G'`, et.al.). c depends on the operation to be performed: it should be ``c'` for step and continue operations, ``g'` for other operations. The thread designator t may be ``-1'`, meaning all the threads, a thread number, or ``0'` which means pick any thread.

  Reply:

  ``OK'`
    for success
  ``E NN'`
    for an error

``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.

``I'`
  Signal, then cycle step. See step with signal packet. See cycle step packet.

``k'`
  Kill request.

  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?).

``m addr,length'`
  Read length bytes of memory starting at address addr. Note that addr may not be aligned to any particular boundary.

  The stub need not use any particular size or alignment when gathering data from memory for the response; even if addr is word-aligned and length is a multiple of the word size, the stub is free to use byte accesses, or not. For this reason, this packet may not be suitable for accessing memory-mapped I/O devices.

  Reply:

  ``XX...'`
    Memory contents; each byte is transmitted as a two-digit hexadecimal number. The reply may contain fewer bytes than requested if the server was able to read only part of the region of memory.
  ``E NN'`
    NN is errno

``M addr,length:XX...'`
  Write length bytes of memory starting at address addr. XX... is the data; each byte is transmitted as a two-digit hexadecimal number.

  Reply:

  ``OK'`
    for success
  ``E NN'`
    for an error (this includes the case where only part of the data was written).

``p n'`
  Read the value of register n; n is in hex. See read registers packet, for a description of how the returned register value is encoded.

  Reply:

  ``XX...'`
    the register's value
  ``E NN'`
    for an error
  ``'`
    Indicating an unrecognized query.

``P n...=r...'`
  Write register n... with value r.... The register number n is in hexadecimal, and r... contains two hex digits for each byte in the register (target byte order).

  Reply:

  ``OK'`
    for success
  ``E NN'`
    for an error

``q name params...'`
``Q name params...'`
  General query (``q'`) and set (``Q'`). These packets are described fully in D.4 General Query Packets.

``r'`
  Reset the entire system.

  Don't use this packet; use the ``R'` packet instead.

``R XX'`
  Restart the program being debugged. XX, while needed, is ignored. This packet is only available in extended mode (see extended mode).

  The ``R'` packet has no reply.

``s [addr]'`
  Single step. addr is the address at which to resume. If addr is omitted, resume at same address.

  Reply: See section D.3 Stop Reply Packets, for the reply specifications.

``S sig[;addr]'`
  Step with signal. This is analogous to the ``C'` packet, but requests a single-step, rather than a normal resumption of execution.

  Reply: See section D.3 Stop Reply Packets, for the reply specifications.

``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.

``T XX'`
  Find out if the thread XX is alive.

  Reply:

  ``OK'`
    thread is still alive
  ``E NN'`
    thread is dead

``v'`
  Packets starting with ``v'` are identified by a multi-letter name, up to the first ``;'` or ``?'` (or the end of the packet).

``vAttach;pid'`
  Attach to a new process with the specified process ID. pid is a hexadecimal integer identifying the process. The attached process is stopped.

  This packet is only available in extended mode (see extended mode).

  Reply:

  ``E nn'`
    for an error
  ``Any stop packet'`
    for success (see section D.3 Stop Reply Packets)

``vCont[;action[:tid]]...'`
  Resume the inferior, specifying different actions for each thread. If an action is specified with no tid, then it is applied to any threads that don't have a specific action specified; if no default action is specified then other threads should remain stopped. Specifying multiple default actions is an error; specifying no actions is also an error. Thread IDs are specified in hexadecimal. Currently supported actions are:

  ``c'`
    Continue.
  ``C sig'`
    Continue with signal sig. sig should be two hex digits.
  ``s'`
    Step.
  ``S sig'`
    Step with signal sig. sig should be two hex digits.

  The optional addr argument normally associated with these packets is not supported in ``vCont'`.

  Reply: See section D.3 Stop Reply Packets, for the reply specifications.

``vCont?'`
  Request a list of actions supported by the ``vCont'` packet.

  Reply:

  ``vCont[;action...]'`
    The ``vCont'` packet is supported. Each action is a supported command in the ``vCont'` packet.
  ``'`
    The ``vCont'` packet is not supported.

``vFile:operation:parameter...'`
  Perform a file operation on the target system. For details, see D.7 Host I/O Packets.

``vFlashErase:addr,length'`
  Direct the stub to erase length bytes of flash starting at addr. The region may enclose any number of flash blocks, but its start and end must fall on block boundaries, as indicated by the flash block size appearing in the memory map (see section D.12 Memory Map Format). GDB groups flash memory programming operations together, and sends a ``vFlashDone'` request after each group; the stub is allowed to delay erase operation until the ``vFlashDone'` packet is received.

  Reply:

  ``OK'`
    for success
  ``E NN'`
    for an error

``vFlashWrite:addr:XX...'`
  Direct the stub to write data to flash address addr. The data is passed in binary form using the same encoding as for the ``X'` packet (see Binary Data). The memory ranges specified by ``vFlashWrite'` packets preceding a ``vFlashDone'` packet must not overlap, and must appear in order of increasing addresses (although ``vFlashErase'` packets for higher addresses may already have been received; the ordering is guaranteed only between ``vFlashWrite'` packets). If a packet writes to an address that was neither erased by a preceding ``vFlashErase'` packet nor by some other target-specific method, the results are unpredictable.

  Reply:

  ``OK'`
    for success
  ``E.memtype'`
    for vFlashWrite addressing non-flash memory
  ``E NN'`
    for an error

``vFlashDone'`
  Indicate to the stub that flash programming operation is finished. The stub is permitted to delay or batch the effects of a group of ``vFlashErase'` and ``vFlashWrite'` packets until a ``vFlashDone'` packet is received. The contents of the affected regions of flash memory are unpredictable until the ``vFlashDone'` request is completed.

``vRun;filename[;argument]...'`
  Run the program filename, passing it each argument on its command line. The file and arguments are hex-encoded strings. If filename is an empty string, the stub may use a default program (e.g. the last program run). The program is created in the stopped state.

  This packet is only available in extended mode (see extended mode).

  Reply:

  ``E nn'`
    for an error
  ``Any stop packet'`
    for success (see section D.3 Stop Reply Packets)

``X addr,length:XX...'`
  Write data to memory, where the data is transmitted in binary. addr is address, length is number of bytes, ``XX...'` is binary data (see Binary Data).

  Reply:

  ``OK'`
    for success
  ``E NN'`
    for an error

``z type,addr,length'`
``Z type,addr,length'`
  Insert (``Z'`) or remove (``z'`) a type breakpoint or watchpoint starting at address address and covering the next length bytes.

  Each breakpoint and watchpoint packet type is documented separately.

  Implementation notes: A remote target shall return an empty string for an unrecognized breakpoint or watchpoint packet type. A remote target shall support either both or neither of a given ``Ztype...'` and ``ztype...'` packet pair. To avoid potential problems with duplicate packets, the operations should be implemented in an idempotent way.

``z0,addr,length'`
``Z0,addr,length'`
  Insert (``Z0'`) or remove (``z0'`) a memory breakpoint at address addr of size length.

  A memory breakpoint is implemented by replacing the instruction at addr with a software breakpoint or trap instruction. The length is used by targets that indicates the size of the breakpoint (in bytes) that should be inserted (e.g., the ARM and MIPS can insert either a 2 or 4 byte breakpoint).

  Implementation note: It is possible for a target to copy or move code that contains memory breakpoints (e.g., when implementing overlays). The behavior of this packet, in the presence of such a target, is not defined.

  Reply:

  ``OK'`
    success
  ``'`
    not supported
  ``E NN'`
    for an error

``z1,addr,length'`
``Z1,addr,length'`
  Insert (``Z1'`) or remove (``z1'`) a hardware breakpoint at address addr of size length.

  A hardware breakpoint is implemented using a mechanism that is not dependant on being able to modify the target's memory.

  Implementation note: A hardware breakpoint is not affected by code movement.

  Reply:

  ``OK'`
    success
  ``'`
    not supported
  ``E NN'`
    for an error

``z2,addr,length'`
``Z2,addr,length'`
  Insert (``Z2'`) or remove (``z2'`) a write watchpoint.

  Reply:

  ``OK'`
    success
  ``'`
    not supported
  ``E NN'`
    for an error

``z3,addr,length'`
``Z3,addr,length'`
  Insert (``Z3'`) or remove (``z3'`) a read watchpoint.

  Reply:

  ``OK'`
    success
  ``'`
    not supported
  ``E NN'`
    for an error

``z4,addr,length'`
``Z4,addr,length'`
  Insert (``Z4'`) or remove (``z4'`) an access watchpoint.

  Reply:

  ``OK'`
    success
  ``'`
    not supported
  ``E NN'`
    for an error

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## D.3 Stop Reply Packets

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 defined by the header ``include/gdb/signals.h'` in the GDB source code.

As in the description of request packets, we include spaces in the reply templates for clarity; these are not part of the reply packet's syntax. No GDB stop reply packet uses spaces to separate its components.

``S AA'`
  The program received signal number AA (a two-digit hexadecimal number). This is equivalent to a ``T'` response with no n:r pairs.

``T AA n1:r1;n2:r2;...'`
  The program received signal number AA (a two-digit hexadecimal number). This is equivalent to an ``S'` response, except that the ``n:r'` pairs can carry values of important registers and other information directly in the stop reply packet, reducing round-trip latency. Single-step and breakpoint traps are reported this way. Each ``n:r'` pair is interpreted as follows:

  - If n is a hexadecimal number, it is a register number, and the corresponding r gives that register's value. r is a series of bytes in target byte order, with each byte given by a two-digit hex number.

  - If n is ``thread'`, then r is the thread process ID, in hex.

  - If n is a recognized stop reason, it describes a more specific event that stopped the target. The currently defined stop reasons are listed below. aa should be ``05'`, the trap signal. At most one stop reason should be present.

  - Otherwise, GDB should ignore this ``n:r'` pair and go on to the next; this allows us to extend the protocol in the future.

  The currently defined stop reasons are:

  ``watch'`
  ``rwatch'`
  ``awatch'`
    The packet indicates a watchpoint hit, and r is the data address, in hex.

  ``library'`
    The packet indicates that the loaded libraries have changed. GDB should use ``qXfer:libraries:read'` to fetch a new list of loaded libraries. r is ignored.

``W AA'`
  The process exited, and AA is the exit status. This is only applicable to certain targets.

``X AA'`
  The process terminated with signal AA.

``O XX...'`
  ``XX...'` is hex encoding of ASCII data, to be written as the program's console output. This can happen at any time while the program is running and the debugger should continue to wait for ``W'`, ``T'`, etc.

``F call-id,parameter...'`
  call-id is the identifier which says which host system call should be called. This is just the name of the function. Translation into the correct system call is only applicable as it's defined in GDB. See section D.10 File-I/O Remote Protocol Extension, for a list of implemented system calls.

  ``parameter...'` is a list of parameters as defined for this very system call.

  The target replies with this packet when it expects GDB to call a host system call on behalf of the target. GDB replies with an appropriate ``F'` packet and keeps up waiting for the next reply packet from the target. The latest ``C'`, ``c'`, ``S'` or ``s'` action is expected to be continued. See section D.10 File-I/O Remote Protocol Extension, for more details.

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## D.4 General Query Packets

Packets starting with ``q'` are general query packets; packets starting with ``Q'` are general set packets. General query and set packets are a semi-unified form for retrieving and sending information to and from the stub.

The initial letter of a query or set packet is followed by a name indicating what sort of thing the packet applies to. For example, GDB may use a ``qSymbol'` packet to exchange symbol definitions with the stub. These packet names follow some conventions:

- The name must not contain commas, colons or semicolons.
- Most GDB query and set packets have a leading upper case letter.
- The names of custom vendor packets should use a company prefix, in lower case, followed by a period. For example, packets designed at the Acme Corporation might begin with ``qacme.foo'` (for querying foos) or ``Qacme.bar'` (for setting bars).

The name of a query or set packet should be separated from any parameters by a ``:'`; the parameters themselves should be separated by ``,'` or ``;'`. Stubs must be careful to match the full packet name, and check for a separator or the end of the packet, in case two packet names share a common prefix. New packets should not begin with ``qC'`, ``qP'`, or ``qL'`(9).

Like the descriptions of the other packets, each description here has a template showing the packet's overall syntax, followed by an explanation of the packet's meaning. We include spaces in some of the templates for clarity; these are not part of the packet's syntax. No GDB packet uses spaces to separate its components.

Here are the currently defined query and set packets:

``qC'`
  Return the current thread id.

  Reply:

  ``QC pid'`
    Where pid is an unsigned hexadecimal process id.
  ``(anything else)'`
    Any other reply implies the old pid.

``qCRC:addr,length'`
  Compute the CRC checksum of a block of memory. Reply:
  ``E NN'`
    An error (such as memory fault)
  ``C crc32'`
    The specified memory region's checksum is crc32.

``qfThreadInfo'`
``qsThreadInfo'`
  Obtain a list of all 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 ``qfThreadInfo'` query; subsequent queries in the sequence will be the ``qsThreadInfo'` query.

  NOTE: This packet replaces the ``qL'` query (see below).

  Reply:

  ``m id'`
    A single thread id
  ``m id,id...'`
    a comma-separated list of thread ids
  ``l'`
    (lower case letter ``L'`) 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 unsigned 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).

``qGetTLSAddr:thread-id,offset,lm'`
  Fetch the address associated with thread local storage specified by thread-id, offset, and lm.

  thread-id is the (big endian, hex encoded) thread id associated with the thread for which to fetch the TLS address.

  offset is the (big endian, hex encoded) offset associated with the thread local variable. (This offset is obtained from the debug information associated with the variable.)

  lm is the (big endian, hex encoded) OS/ABI-specific encoding of the the load module associated with the thread local storage. For example, a GNU/Linux system will pass the link map address of the shared object associated with the thread local storage under consideration. Other operating environments may choose to represent the load module differently, so the precise meaning of this parameter will vary.

  Reply:

  ``XX...'`
    Hex encoded (big endian) bytes representing the address of the thread local storage requested.

  ``E nn'`
    An error occurred. nn are hex digits.

  ``'`
    An empty reply indicates that ``qGetTLSAddr'` is not supported by the stub.

``qL startflag threadcount nextthread'`
  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.

  Don't use this packet; use the ``qfThreadInfo'` query instead (see above).

  Reply:

  ``qM count done argthread thread...'`
    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()`.

``qOffsets'`
  Get section offsets that the target used when relocating the downloaded image.

  Reply:

  ``Text=xxx;Data=yyy[;Bss=zzz]'`
    Relocate the `Text` section by xxx from its original address. Relocate the `Data` section by yyy from its original address. If the object file format provides segment information (e.g. ELF ``PT_LOAD'` program headers), GDB will relocate entire segments by the supplied offsets.

    Note: while a `Bss` offset may be included in the response, GDB ignores this and instead applies the `Data` offset to the `Bss` section.

  ``TextSeg=xxx[;DataSeg=yyy]'`
    Relocate the first segment of the object file, which conventionally contains program code, to a starting address of xxx. If ``DataSeg'` is specified, relocate the second segment, which conventionally contains modifiable data, to a starting address of yyy. GDB will report an error if the object file does not contain segment information, or does not contain at least as many segments as mentioned in the reply. Extra segments are kept at fixed offsets relative to the last relocated segment.

``qP mode threadid'`
  Returns information on threadid. Where: mode is a hex encoded 32 bit mode; threadid is a hex encoded 64 bit thread ID.

  Don't use this packet; use the ``qThreadExtraInfo'` query instead (see below).

  Reply: see `remote.c:remote_unpack_thread_info_response()`.

``QPassSignals: signal [;signal]...'`
  Each listed signal should be passed directly to the inferior process. Signals are numbered identically to continue packets and stop replies (see section D.3 Stop Reply Packets). Each signal list item should be strictly greater than the previous item. These signals do not need to stop the inferior, or be reported to GDB. All other signals should be reported to GDB. Multiple ``QPassSignals'` packets do not combine; any earlier ``QPassSignals'` list is completely replaced by the new list. This packet improves performance when using ``handle signal nostop noprint pass'`.

  Reply:

  ``OK'`
    The request succeeded.

  ``E nn'`
    An error occurred. nn are hex digits.

  ``'`
    An empty reply indicates that ``QPassSignals'` is not supported by the stub.

  Use of this packet is controlled by the `set remote pass-signals` command (see section set remote pass-signals). This packet is not probed by default; the remote stub must request it, by supplying an appropriate ``qSupported'` response (see qSupported).

``qRcmd,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 ``Ooutput'` 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.
  ``OUTPUT'`
    A command response with the hex encoded output string OUTPUT.
  ``E NN'`
    Indicate a badly formed request.
  ``'`
    An empty reply indicates that ``qRcmd'` is not recognized.

  (Note that the `qRcmd` packet's name is separated from the command by a ``,'`, not a ``:'`, contrary to the naming conventions above. Please don't use this packet as a model for new packets.)

``qSupported [:gdbfeature [;gdbfeature]... ]'`
  Tell the remote stub about features supported by GDB, and query the stub for features it supports. This packet allows GDB and the remote stub to take advantage of each others' features. ``qSupported'` also consolidates multiple feature probes at startup, to improve GDB performance--a single larger packet performs better than multiple smaller probe packets on high-latency links. Some features may enable behavior which must not be on by default, e.g. because it would confuse older clients or stubs. Other features may describe packets which could be automatically probed for, but are not. These features must be reported before GDB will use them. This "default unsupported" behavior is not appropriate for all packets, but it helps to keep the initial connection time under control with new versions of GDB which support increasing numbers of packets.

  Reply:

  ``stubfeature [;stubfeature]...'`
    The stub supports or does not support each returned stubfeature, depending on the form of each stubfeature (see below for the possible forms).
  ``'`
    An empty reply indicates that ``qSupported'` is not recognized, or that no features needed to be reported to GDB.

  The allowed forms for each feature (either a gdbfeature in the ``qSupported'` packet, or a stubfeature in the response) are:

  ``name=value'`
    The remote protocol feature name is supported, and associated with the specified value. The format of value depends on the feature, but it must not include a semicolon.
  ``name+'`
    The remote protocol feature name is supported, and does not need an associated value.
  ``name-'`
    The remote protocol feature name is not supported.
  ``name?'`
    The remote protocol feature name may be supported, and GDB should auto-detect support in some other way when it is needed. This form will not be used for gdbfeature notifications, but may be used for stubfeature responses.

  Whenever the stub receives a ``qSupported'` request, the supplied set of GDB features should override any previous request. This allows GDB to put the stub in a known state, even if the stub had previously been communicating with a different version of GDB.

  No values of gdbfeature (for the packet sent by GDB) are defined yet. Stubs should ignore any unknown values for gdbfeature. Any GDB which sends a ``qSupported'` packet supports receiving packets of unlimited length (earlier versions of GDB may reject overly long responses). Values for gdbfeature may be defined in the future to let the stub take advantage of new features in GDB, e.g. incompatible improvements in the remote protocol--support for unlimited length responses would be a gdbfeature example, if it were not implied by the ``qSupported'` query. The stub's reply should be independent of the gdbfeature entries sent by GDB; first GDB describes all the features it supports, and then the stub replies with all the features it supports.

  Similarly, GDB will silently ignore unrecognized stub feature responses, as long as each response uses one of the standard forms.

  Some features are flags. A stub which supports a flag feature should respond with a ``+'` form response. Other features require values, and the stub should respond with an ``='` form response.

  Each feature has a default value, which GDB will use if ``qSupported'` is not available or if the feature is not mentioned in the ``qSupported'` response. The default values are fixed; a stub is free to omit any feature responses that match the defaults.

  Not all features can be probed, but for those which can, the probing mechanism is useful: in some cases, a stub's internal architecture may not allow the protocol layer to know some information about the underlying target in advance. This is especially common in stubs which may be configured for multiple targets.

  These are the currently defined stub features and their properties:

  Feature Name | Value Required | Default | Probe Allowed
  ``PacketSize'` | Yes | ``-'` | No
  ``qXfer:auxv:read'` | No | ``-'` | Yes
  ``qXfer:features:read'` | No | ``-'` | Yes
  ``qXfer:libraries:read'` | No | ``-'` | Yes
  ``qXfer:memory-map:read'` | No | ``-'` | Yes
  ``qXfer:spu:read'` | No | ``-'` | Yes
  ``qXfer:spu:write'` | No | ``-'` | Yes
  ``QPassSignals'` | No | ``-'` | Yes

  These are the currently defined stub features, in more detail:

  ``PacketSize=bytes'`
    The remote stub can accept packets up to at least bytes in length. GDB will send packets up to this size for bulk transfers, and will never send larger packets. This is a limit on the data characters in the packet, including the frame and checksum. There is no trailing NUL byte in a remote protocol packet; if the stub stores packets in a NUL-terminated format, it should allow an extra byte in its buffer for the NUL. If this stub feature is not supported, GDB guesses based on the size of the ``g'` packet response.

  ``qXfer:auxv:read'`
    The remote stub understands the ``qXfer:auxv:read'` packet (see qXfer auxiliary vector read).

  ``qXfer:features:read'`
    The remote stub understands the ``qXfer:features:read'` packet (see qXfer target description read).

  ``qXfer:libraries:read'`
    The remote stub understands the ``qXfer:libraries:read'` packet (see qXfer library list read).

  ``qXfer:memory-map:read'`
    The remote stub understands the ``qXfer:memory-map:read'` packet (see qXfer memory map read).

  ``qXfer:spu:read'`
    The remote stub understands the ``qXfer:spu:read'` packet (see qXfer spu read).

  ``qXfer:spu:write'`
    The remote stub understands the ``qXfer:spu:write'` packet (see qXfer spu write).

  ``QPassSignals'`
    The remote stub understands the ``QPassSignals'` packet (see QPassSignals).

``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.
  ``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.

``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.
  ``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.

``QTDP'`
``QTFrame'`
  See section D.6 Tracepoint Packets.

``qThreadExtraInfo,id'`
  Obtain a printable string description of a thread's attributes from the target OS. id is a thread-id in big-endian hex. 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.

  (Note that the `qThreadExtraInfo` packet's name is separated from the command by a ``,'`, not a ``:'`, contrary to the naming conventions above. Please don't use this packet as a model for new packets.)

``QTStart'`
``QTStop'`
``QTinit'`
``QTro'`
``qTStatus'`
  See section D.6 Tracepoint Packets.

``qXfer:object:read:annex:offset,length'`
  Read uninterpreted bytes from the target's special data area identified by the keyword object. Request length bytes starting at offset bytes into the data. The content and encoding of annex is specific to object; it can supply additional details about what data to access.

  Here are the specific requests of this form defined so far. All ``qXfer:object:read:...'` requests use the same reply formats, listed below.

  ``qXfer:auxv:read::offset,length'`
    Access the target's auxiliary vector. See section auxiliary vector. Note annex must be empty.

    This packet is not probed by default; the remote stub must request it, by supplying an appropriate ``qSupported'` response (see qSupported).

  ``qXfer:features:read:annex:offset,length'`
    Access the target description. See section F. Target Descriptions. The annex specifies which XML document to access. The main description is always loaded from the ``target.xml'` annex.

    This packet is not probed by default; the remote stub must request it, by supplying an appropriate ``qSupported'` response (see qSupported).

  ``qXfer:libraries:read:annex:offset,length'`
    Access the target's list of loaded libraries. See section D.11 Library List Format. The annex part of the generic ``qXfer'` packet must be empty (see qXfer read).

    Targets which maintain a list of libraries in the program's memory do not need to implement this packet; it is designed for platforms where the operating system manages the list of loaded libraries.

    This packet is not probed by default; the remote stub must request it, by supplying an appropriate ``qSupported'` response (see qSupported).

  ``qXfer:memory-map:read::offset,length'`
    Access the target's memory-map. See section D.12 Memory Map Format. The annex part of the generic ``qXfer'` packet must be empty (see qXfer read).

    This packet is not probed by default; the remote stub must request it, by supplying an appropriate ``qSupported'` response (see qSupported).

  ``qXfer:spu:read:annex:offset,length'`
    Read contents of an `spufs` file on the target system. The annex specifies which file to read; it must be of the form ``id/name'`, where id specifies an SPU context ID in the target process, and name identifes the `spufs` file in that context to be accessed.

    This packet is not probed by default; the remote stub must request it, by supplying an appropriate ``qSupported'` response (see qSupported).

  Reply:

  ``m data'`
    Data data (see Binary Data) has been read from the target. There may be more data at a higher address (although it is permitted to return ``m'` even for the last valid block of data, as long as at least one byte of data was read). data may have fewer bytes than the length in the request.

  ``l data'`
    Data data (see Binary Data) has been read from the target. There is no more data to be read. data may have fewer bytes than the length in the request.

  ``l'`
    The offset in the request is at the end of the data. There is no more data to be read.

  ``E00'`
    The request was malformed, or annex was invalid.

  ``E nn'`
    The offset was invalid, or there was an error encountered reading the data. nn is a hex-encoded `errno` value.

  ``'`
    An empty reply indicates the object string was not recognized by the stub, or that the object does not support reading.

``qXfer:object:write:annex:offset:data...'`
  Write uninterpreted bytes into the target's special data area identified by the keyword object, starting at offset bytes into the data. data... is the binary-encoded data (see Binary Data) to be written. The content and encoding of annex is specific to object; it can supply additional details about what data to access.

  Here are the specific requests of this form defined so far. All ``qXfer:object:write:...'` requests use the same reply formats, listed below.

  ``qXfer:spu:write:annex:offset:data...'`
    Write data to an `spufs` file on the target system. The annex specifies which file to write; it must be of the form ``id/name'`, where id specifies an SPU context ID in the target process, and name identifes the `spufs` file in that context to be accessed.

    This packet is not probed by default; the remote stub must request it, by supplying an appropriate ``qSupported'` response (see qSupported).

  Reply:

  ``nn'`
    nn (hex encoded) is the number of bytes written. This may be fewer bytes than supplied in the request.

  ``E00'`
    The request was malformed, or annex was invalid.

  ``E nn'`
    The offset was invalid, or there was an error encountered writing the data. nn is a hex-encoded `errno` value.

  ``'`
    An empty reply indicates the object string was not recognized by the stub, or that the object does not support writing.

``qXfer:object:operation:...'`
  Requests of this form may be added in the future. When a stub does not recognize the object keyword, or its support for object does not recognize the operation keyword, the stub must respond with an empty packet.

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## D.5 Register Packet Format

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 transferred in target byte order. The two nibbles within a register byte are transferred most-significant - least-significant.

MIPS32

  All registers are transferred 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 transferred as sixty-four bit quantities (including thirty-two bit registers such as `sr`). The ordering is the same as `MIPS32`.

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## D.6 Tracepoint Packets

Here we describe the packets GDB uses to implement tracepoints (see section 10. Tracepoints).

``QTDP:n:addr:ena:step:pass[-]'`
  Create a new tracepoint, number n, at addr. If ena is ``E'`, then the tracepoint is enabled; if it is ``D'`, then the tracepoint is disabled. step is the tracepoint's step count, and pass is its pass count. If the trailing ``-'` is present, further ``QTDP'` packets will follow to specify this tracepoint's actions.

  Replies:

  ``OK'`
    The packet was understood and carried out.
  ``'`
    The packet was not recognized.

``QTDP:-n:addr:[S]action...[-]'`
  Define actions to be taken when a tracepoint is hit. n and addr must be the same as in the initial ``QTDP'` packet for this tracepoint. This packet may only be sent immediately after another ``QTDP'` packet that ended with a ``-'`. If the trailing ``-'` is present, further ``QTDP'` packets will follow, specifying more actions for this tracepoint.

  In the series of action packets for a given tracepoint, at most one can have an ``S'` before its first action. If such a packet is sent, it and the following packets define "while-stepping" actions. Any prior packets define ordinary actions -- that is, those taken when the tracepoint is first hit. If no action packet has an ``S'`, then all the packets in the series specify ordinary tracepoint actions.

  The ``action...'` portion of the packet is a series of actions, concatenated without separators. Each action has one of the following forms:

  ``R mask'`
    Collect the registers whose bits are set in mask. mask is a hexadecimal number whose i'th bit is set if register number i should be collected. (The least significant bit is numbered zero.) Note that mask may be any number of digits long; it may not fit in a 32-bit word.

  ``M basereg,offset,len'`
    Collect len bytes of memory starting at the address in register number basereg, plus offset. If basereg is ``-1'`, then the range has a fixed address: offset is the address of the lowest byte to collect. The basereg, offset, and len parameters are all unsigned hexadecimal values (the ``-1'` value for basereg is a special case).

  ``X len,expr'`
    Evaluate expr, whose length is len, and collect memory as it directs. expr is an agent expression, as described in E. The GDB Agent Expression Mechanism. Each byte of the expression is encoded as a two-digit hex number in the packet; len is the number of bytes in the expression (and thus one-half the number of hex digits in the packet).

  Any number of actions may be packed together in a single ``QTDP'` packet, as long as the packet does not exceed the maximum packet length (400 bytes, for many stubs). There may be only one ``R'` action per tracepoint, and it must precede any ``M'` or ``X'` actions. Any registers referred to by ``M'` and ``X'` actions must be collected by a preceding ``R'` action. (The "while-stepping" actions are treated as if they were attached to a separate tracepoint, as far as these restrictions are concerned.)

  Replies:

  ``OK'`
    The packet was understood and carried out.
  ``'`
    The packet was not recognized.

``QTFrame:n'`
  Select the n'th tracepoint frame from the buffer, and use the register and memory contents recorded there to answer subsequent request packets from GDB.

  A successful reply from the stub indicates that the stub has found the requested frame. The response is a series of parts, concatenated without separators, describing the frame we selected. Each part has one of the following forms:

  ``F f'`
    The selected frame is number n in the trace frame buffer; f is a hexadecimal number. If f is ``-1'`, then there was no frame matching the criteria in the request packet.

  ``T t'`
    The selected trace frame records a hit of tracepoint number t; t is a hexadecimal number.

``QTFrame:pc:addr'`
  Like ``QTFrame:n'`, but select the first tracepoint frame after the currently selected frame whose PC is addr; addr is a hexadecimal number.

``QTFrame:tdp:t'`
  Like ``QTFrame:n'`, but select the first tracepoint frame after the currently selected frame that is a hit of tracepoint t; t is a hexadecimal number.

``QTFrame:range:start:end'`
  Like ``QTFrame:n'`, but select the first tracepoint frame after the currently selected frame whose PC is between start (inclusive) and end (exclusive); start and end are hexadecimal numbers.

``QTFrame:outside:start:end'`
  Like ``QTFrame:range:start:end'`, but select the first frame outside the given range of addresses.

``QTStart'`
  Begin the tracepoint experiment. Begin collecting data from tracepoint hits in the trace frame buffer.

``QTStop'`
  End the tracepoint experiment. Stop collecting trace frames.

``QTinit'`
  Clear the table of tracepoints, and empty the trace frame buffer.

``QTro:start1,end1:start2,end2:...'`
  Establish the given ranges of memory as "transparent". The stub will answer requests for these ranges from memory's current contents, if they were not collected as part of the tracepoint hit.

  GDB uses this to mark read-only regions of memory, like those containing program code. Since these areas never change, they should still have the same contents they did when the tracepoint was hit, so there's no reason for the stub to refuse to provide their contents.

``qTStatus'`
  Ask the stub if there is a trace experiment running right now.

  Replies:

  ``T0'`
    There is no trace experiment running.
  ``T1'`
    There is a trace experiment running.

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## D.7 Host I/O Packets

The Host I/O packets allow GDB to perform I/O operations on the far side of a remote link. For example, Host I/O is used to upload and download files to a remote target with its own filesystem. Host I/O uses the same constant values and data structure layout as the target-initiated File-I/O protocol. However, the Host I/O packets are structured differently. The target-initiated protocol relies on target memory to store parameters and buffers. Host I/O requests are initiated by GDB, and the target's memory is not involved. See section D.10 File-I/O Remote Protocol Extension, for more details on the target-initiated protocol.

The Host I/O request packets all encode a single operation along with its arguments. They have this format:

``vFile:operation: parameter...'`
  operation is the name of the particular request; the target should compare the entire packet name up to the second colon when checking for a supported operation. The format of parameter depends on the operation. Numbers are always passed in hexadecimal. Negative numbers have an explicit minus sign (i.e. two's complement is not used). Strings (e.g. filenames) are encoded as a series of hexadecimal bytes. The last argument to a system call may be a buffer of escaped binary data (see Binary Data).

The valid responses to Host I/O packets are:

``F result [, errno] [; attachment]'`
  result is the integer value returned by this operation, usually non-negative for success and -1 for errors. If an error has occured, errno will be included in the result. errno will have a value defined by the File-I/O protocol (see section Errno Values). For operations which return data, attachment supplies the data as a binary buffer. Binary buffers in response packets are escaped in the normal way (see Binary Data). See the individual packet documentation for the interpretation of result and attachment.

``'`
  An empty response indicates that this operation is not recognized.

These are the supported Host I/O operations:

``vFile:open: pathname, flags, mode'`
  Open a file at pathname and return a file descriptor for it, or return -1 if an error occurs. pathname is a string, flags is an integer indicating a mask of open flags (see section Open Flags), and mode is an integer indicating a mask of mode bits to use if the file is created (see section mode_t Values). See section open, for details of the open flags and mode values.

``vFile:close: fd'`
  Close the open file corresponding to fd and return 0, or -1 if an error occurs.

``vFile:pread: fd, count, offset'`
  Read data from the open file corresponding to fd. Up to count bytes will be read from the file, starting at offset relative to the start of the file. The target may read fewer bytes; common reasons include packet size limits and an end-of-file condition. The number of bytes read is returned. Zero should only be returned for a successful read at the end of the file, or if count was zero.

  The data read should be returned as a binary attachment on success. If zero bytes were read, the response should include an empty binary attachment (i.e. a trailing semicolon). The return value is the number of target bytes read; the binary attachment may be longer if some characters were escaped.

``vFile:pwrite: fd, offset, data'`
  Write data (a binary buffer) to the open file corresponding to fd. Start the write at offset from the start of the file. Unlike many `write` system calls, there is no separate count argument; the length of data in the packet is used. ``vFile:write'` returns the number of bytes written, which may be shorter than the length of data, or -1 if an error occurred.

``vFile:unlink: pathname'`
  Delete the file at pathname on the target. Return 0, or -1 if an error occurs. pathname is a string.

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## D.8 Interrupts

When a program on the remote target is running, GDB may attempt to interrupt it by sending a ``Ctrl-C'` or a `BREAK`, control of which is specified via GDB's ``remotebreak'` setting (see set remotebreak).

The precise meaning of `BREAK` is defined by the transport mechanism and may, in fact, be undefined. GDB does not currently define a `BREAK` mechanism for any of the network interfaces.

``Ctrl-C'`, on the other hand, is defined and implemented for all transport mechanisms. It is represented by sending the single byte `0x03` without any of the usual packet overhead described in the Overview section (see section D.1 Overview). When a `0x03` byte is transmitted as part of a packet, it is considered to be packet data and does not represent an interrupt. E.g., an ``X'` packet (see X packet), used for binary downloads, may include an unescaped `0x03` as part of its packet.

Stubs are not required to recognize these interrupt mechanisms and the precise meaning associated with receipt of the interrupt is implementation defined. If the stub is successful at interrupting the running program, it is expected that it will send one of the Stop Reply Packets (see section D.3 Stop Reply Packets) to GDB as a result of successfully stopping the program. Interrupts received while the program is stopped will be discarded.

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## D.9 Examples

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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## D.10 File-I/O Remote Protocol Extension

> D.10.1 File-I/O Overview |  |
> D.10.2 Protocol Basics |  |
> D.10.3 The `F` Request Packet |  |
> D.10.4 The `F` Reply Packet |  |
> D.10.5 The ``Ctrl-C'` Message |  |
> D.10.6 Console I/O |  |
> D.10.7 List of Supported Calls |  |
> D.10.8 Protocol-specific Representation of Datatypes |  |
> D.10.9 Constants |  |
> D.10.10 File-I/O Examples |  |

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### D.10.1 File-I/O Overview

The File I/O remote protocol extension (short: File-I/O) allows the target to use the host's file system and console I/O to perform various system calls. System calls on the target system are translated into a remote protocol packet to the host system, which then performs the needed actions and returns a response packet to the target system. This simulates file system operations even on targets that lack file systems.

The protocol is defined to be independent of both the host and target systems. It uses its own internal representation of datatypes and values. Both GDB and the target's GDB stub are responsible for translating the system-dependent value representations into the internal protocol representations when data is transmitted.

The communication is synchronous. A system call is possible only when GDB is waiting for a response from the ``C'`, ``c'`, ``S'` or ``s'` packets. While GDB handles the request for a system call, the target is stopped to allow deterministic access to the target's memory. Therefore File-I/O is not interruptible by target signals. On the other hand, it is possible to interrupt File-I/O by a user interrupt (``Ctrl-C'`) within GDB.

The target's request to perform a host system call does not finish the latest ``C'`, ``c'`, ``S'` or ``s'` action. That means, after finishing the system call, the target returns to continuing the previous activity (continue, step). No additional continue or step request from GDB is required.

    (gdb) continue
      <- target requests 'system call X'
      target is stopped, GDB executes system call
      -> GDB returns result
      ... target continues, GDB returns to wait for the target
      <- target hits breakpoint and sends a Txx packet

The protocol only supports I/O on the console and to regular files on the host file system. Character or block special devices, pipes, named pipes, sockets or any other communication method on the host system are not supported by this protocol.

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### D.10.2 Protocol Basics

The File-I/O protocol uses the `F` packet as the request as well as reply packet. Since a File-I/O system call can only occur when GDB is waiting for a response from the continuing or stepping target, the File-I/O request is a reply that GDB has to expect as a result of a previous ``C'`, ``c'`, ``S'` or ``s'` packet. This `F` packet contains all information needed to allow GDB to call the appropriate host system call:

- A unique identifier for the requested system call.

- All parameters to the system call. Pointers are given as addresses in the target memory address space. Pointers to strings are given as pointer/length pair. Numerical values are given as they are. Numerical control flags are given in a protocol-specific representation.

At this point, GDB has to perform the following actions.

- If the parameters include pointer values to data needed as input to a system call, GDB requests this data from the target with a standard `m` packet request. This additional communication has to be expected by the target implementation and is handled as any other `m` packet.

- GDB translates all value from protocol representation to host representation as needed. Datatypes are coerced into the host types.

- GDB calls the system call.

- It then coerces datatypes back to protocol representation.

- If the system call is expected to return data in buffer space specified by pointer parameters to the call, the data is transmitted to the target using a `M` or `X` packet. This packet has to be expected by the target implementation and is handled as any other `M` or `X` packet.

Eventually GDB replies with another `F` packet which contains all necessary information for the target to continue. This at least contains

- Return value.

- `errno`, if has been changed by the system call.

- "Ctrl-C" flag.

After having done the needed type and value coercion, the target continues the latest continue or step action.

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### D.10.3 The `F` Request Packet

The `F` request packet has the following format:

``Fcall-id,parameter...'`

  call-id is the identifier to indicate the host system call to be called. This is just the name of the function.

  parameter... are the parameters to the system call. Parameters are hexadecimal integer values, either the actual values in case of scalar datatypes, pointers to target buffer space in case of compound datatypes and unspecified memory areas, or pointer/length pairs in case of string parameters. These are appended to the call-id as a comma-delimited list. All values are transmitted in ASCII string representation, pointer/length pairs separated by a slash.

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

### D.10.4 The `F` Reply Packet

The `F` reply packet has the following format:

``Fretcode,errno,Ctrl-C flag;call-specific attachment'`

  retcode is the return code of the system call as hexadecimal value.

  errno is the `errno` set by the call, in protocol-specific representation. This parameter can be omitted if the call was successful.

  Ctrl-C flag is only sent if the user requested a break. In this case, errno must be sent as well, even if the call was successful. The Ctrl-C flag itself consists of the character ``C'`:

      F0,0,C

  or, if the call was interrupted before the host call has been performed:

      F-1,4,C

  assuming 4 is the protocol-specific representation of `EINTR`.

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### D.10.5 The ``Ctrl-C'` Message

If the ``Ctrl-C'` flag is set in the GDB reply packet (see section D.10.4 The `F` Reply Packet), the target should behave as if it had gotten a break message. The meaning for the target is "system call interrupted by `SIGINT`". Consequentially, the target should actually stop (as with a break message) and return to GDB with a `T02` packet.

It's important for the target to know in which state the system call was interrupted. There are two possible cases:

- The system call hasn't been performed on the host yet.

- The system call on the host has been finished.

These two states can be distinguished by the target by the value of the returned `errno`. If it's the protocol representation of `EINTR`, the system call hasn't been performed. This is equivalent to the `EINTR` handling on POSIX systems. In any other case, the target may presume that the system call has been finished -- successfully or not -- and should behave as if the break message arrived right after the system call.

GDB must behave reliably. If the system call has not been called yet, GDB may send the `F` reply immediately, setting `EINTR` as `errno` in the packet. If the system call on the host has been finished before the user requests a break, the full action must be finished by GDB. This requires sending `M` or `X` packets as necessary. The `F` packet may only be sent when either nothing has happened or the full action has been completed.

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### D.10.6 Console I/O

By default and if not explicitly closed by the target system, the file descriptors 0, 1 and 2 are connected to the GDB console. Output on the GDB console is handled as any other file output operation (`write(1, ...)` or `write(2, ...)`). Console input is handled by GDB so that after the target read request from file descriptor 0 all following typing is buffered until either one of the following conditions is met:

- The user types `Ctrl-c`. The behaviour is as explained above, and the `read` system call is treated as finished.

- The user presses `RET`. This is treated as end of input with a trailing newline.

- The user types `Ctrl-d`. This is treated as end of input. No trailing character (neither newline nor ``Ctrl-D'`) is appended to the input.

If the user has typed more characters than fit in the buffer given to the `read` call, the trailing characters are buffered in GDB until either another `read(0, ...)` is requested by the target, or debugging is stopped at the user's request.

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### D.10.7 List of Supported Calls

> open |  |
> close |  |
> read |  |
> write |  |
> lseek |  |
> rename |  |
> unlink |  |
> stat/fstat |  |
> gettimeofday |  |
> isatty |  |
> system |  |

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### open

Synopsis:

      int open(const char *pathname, int flags);
      int open(const char *pathname, int flags, mode_t mode);

Request:
  ``Fopen,pathptr/len,flags,mode'`

  flags is the bitwise `OR` of the following values:

  `O_CREAT`
    If the file does not exist it will be created. The host rules apply as far as file ownership and time stamps are concerned.

  `O_EXCL`
    When used with `O_CREAT`, if the file already exists it is an error and open() fails.

  `O_TRUNC`
    If the file already exists and the open mode allows writing (`O_RDWR` or `O_WRONLY` is given) it will be truncated to zero length.

  `O_APPEND`
    The file is opened in append mode.

  `O_RDONLY`
    The file is opened for reading only.

  `O_WRONLY`
    The file is opened for writing only.

  `O_RDWR`
    The file is opened for reading and writing.

  Other bits are silently ignored.

  mode is the bitwise `OR` of the following values:

  `S_IRUSR`
    User has read permission.

  `S_IWUSR`
    User has write permission.

  `S_IRGRP`
    Group has read permission.

  `S_IWGRP`
    Group has write permission.

  `S_IROTH`
    Others have read permission.

  `S_IWOTH`
    Others have write permission.

  Other bits are silently ignored.

Return value:
  `open` returns the new file descriptor or -1 if an error occurred.

Errors:

  `EEXIST`
    pathname already exists and `O_CREAT` and `O_EXCL` were used.

  `EISDIR`
    pathname refers to a directory.

  `EACCES`
    The requested access is not allowed.

  `ENAMETOOLONG`
    pathname was too long.

  `ENOENT`
    A directory component in pathname does not exist.

  `ENODEV`
    pathname refers to a device, pipe, named pipe or socket.

  `EROFS`
    pathname refers to a file on a read-only filesystem and write access was requested.

  `EFAULT`
    pathname is an invalid pointer value.

  `ENOSPC`
    No space on device to create the file.

  `EMFILE`
    The process already has the maximum number of files open.

  `ENFILE`
    The limit on the total number of files open on the system has been reached.

  `EINTR`
    The call was interrupted by the user.

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#### close

Synopsis:

      int close(int fd);

Request:
  ``Fclose,fd'`

Return value:
  `close` returns zero on success, or -1 if an error occurred.

Errors:

  `EBADF`
    fd isn't a valid open file descriptor.

  `EINTR`
    The call was interrupted by the user.

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#### read

Synopsis:

      int read(int fd, void *buf, unsigned int count);

Request:
  ``Fread,fd,bufptr,count'`

Return value:
  On success, the number of bytes read is returned. Zero indicates end of file. If count is zero, read returns zero as well. On error, -1 is returned.

Errors:

  `EBADF`
    fd is not a valid file descriptor or is not open for reading.

  `EFAULT`
    bufptr is an invalid pointer value.

  `EINTR`
    The call was interrupted by the user.

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#### write

Synopsis:

      int write(int fd, const void *buf, unsigned int count);

Request:
  ``Fwrite,fd,bufptr,count'`

Return value:
  On success, the number of bytes written are returned. Zero indicates nothing was written. On error, -1 is returned.

Errors:

  `EBADF`
    fd is not a valid file descriptor or is not open for writing.

  `EFAULT`
    bufptr is an invalid pointer value.

  `EFBIG`
    An attempt was made to write a file that exceeds the host-specific maximum file size allowed.

  `ENOSPC`
    No space on device to write the data.

  `EINTR`
    The call was interrupted by the user.

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### lseek

Synopsis:

      long lseek (int fd, long offset, int flag);

Request:
  ``Flseek,fd,offset,flag'`

  flag is one of:

  `SEEK_SET`
    The offset is set to offset bytes.

  `SEEK_CUR`
    The offset is set to its current location plus offset bytes.

  `SEEK_END`
    The offset is set to the size of the file plus offset bytes.

Return value:
  On success, the resulting unsigned offset in bytes from the beginning of the file is returned. Otherwise, a value of -1 is returned.

Errors:

  `EBADF`
    fd is not a valid open file descriptor.

  `ESPIPE`
    fd is associated with the GDB console.

  `EINVAL`
    flag is not a proper value.

  `EINTR`
    The call was interrupted by the user.

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### rename

Synopsis:

      int rename(const char *oldpath, const char *newpath);

Request:
  ``Frename,oldpathptr/len,newpathptr/len'`

Return value:
  On success, zero is returned. On error, -1 is returned.

Errors:

  `EISDIR`
    newpath is an existing directory, but oldpath is not a directory.

  `EEXIST`
    newpath is a non-empty directory.

  `EBUSY`
    oldpath or newpath is a directory that is in use by some process.

  `EINVAL`
    An attempt was made to make a directory a subdirectory of itself.

  `ENOTDIR`
    A component used as a directory in oldpath or new path is not a directory. Or oldpath is a directory and newpath exists but is not a directory.

  `EFAULT`
    oldpathptr or newpathptr are invalid pointer values.

  `EACCES`
    No access to the file or the path of the file.

  `ENAMETOOLONG`

    oldpath or newpath was too long.

  `ENOENT`
    A directory component in oldpath or newpath does not exist.

  `EROFS`
    The file is on a read-only filesystem.

  `ENOSPC`
    The device containing the file has no room for the new directory entry.

  `EINTR`
    The call was interrupted by the user.

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### unlink

Synopsis:

      int unlink(const char *pathname);

Request:
  ``Funlink,pathnameptr/len'`

Return value:
  On success, zero is returned. On error, -1 is returned.

Errors:

  `EACCES`
    No access to the file or the path of the file.

  `EPERM`
    The system does not allow unlinking of directories.

  `EBUSY`
    The file pathname cannot be unlinked because it's being used by another process.

  `EFAULT`
    pathnameptr is an invalid pointer value.

  `ENAMETOOLONG`
    pathname was too long.

  `ENOENT`
    A directory component in pathname does not exist.

  `ENOTDIR`
    A component of the path is not a directory.

  `EROFS`
    The file is on a read-only filesystem.

  `EINTR`
    The call was interrupted by the user.

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### stat/fstat

Synopsis:

      int stat(const char *pathname, struct stat *buf);
      int fstat(int fd, struct stat *buf);

Request:
  ``Fstat,pathnameptr/len,bufptr'`
  ``Ffstat,fd,bufptr'`

Return value:
  On success, zero is returned. On error, -1 is returned.

Errors:

  `EBADF`
    fd is not a valid open file.

  `ENOENT`
    A directory component in pathname does not exist or the path is an empty string.

  `ENOTDIR`
    A component of the path is not a directory.

  `EFAULT`
    pathnameptr is an invalid pointer value.

  `EACCES`
    No access to the file or the path of the file.

  `ENAMETOOLONG`
    pathname was too long.

  `EINTR`
    The call was interrupted by the user.

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### gettimeofday

Synopsis:

      int gettimeofday(struct timeval *tv, void *tz);

Request:
  ``Fgettimeofday,tvptr,tzptr'`

Return value:
  On success, 0 is returned, -1 otherwise.

Errors:

  `EINVAL`
    tz is a non-NULL pointer.

  `EFAULT`
    tvptr and/or tzptr is an invalid pointer value.

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### isatty

Synopsis:

      int isatty(int fd);

Request:
  ``Fisatty,fd'`

Return value:
  Returns 1 if fd refers to the GDB console, 0 otherwise.

Errors:

  `EINTR`
    The call was interrupted by the user.

Note that the `isatty` call is treated as a special case: it returns 1 to the target if the file descriptor is attached to the GDB console, 0 otherwise. Implementing through system calls would require implementing `ioctl` and would be more complex than needed.

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#### system

Synopsis:

      int system(const char *command);

Request:
  ``Fsystem,commandptr/len'`

Return value:
  If len is zero, the return value indicates whether a shell is available. A zero return value indicates a shell is not available. For non-zero len, the value returned is -1 on error and the return status of the command otherwise. Only the exit status of the command is returned, which is extracted from the host's `system` return value by calling `WEXITSTATUS(retval)`. In case ``/bin/sh'` could not be executed, 127 is returned.

Errors:

  `EINTR`
    The call was interrupted by the user.

GDB takes over the full task of calling the necessary host calls to perform the `system` call. The return value of `system` on the host is simplified before it's returned to the target. Any termination signal information from the child process is discarded, and the return value consists entirely of the exit status of the called command.

Due to security concerns, the `system` call is by default refused by GDB. The user has to allow this call explicitly with the `set remote system-call-allowed 1` command.

`set remote system-call-allowed`
  Control whether to allow the `system` calls in the File I/O protocol for the remote target. The default is zero (disabled).

`show remote system-call-allowed`
  Show whether the `system` calls are allowed in the File I/O protocol.

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### D.10.8 Protocol-specific Representation of Datatypes

> Integral Datatypes |  |
> Pointer Values |  |
> Memory Transfer |  |
> struct stat |  |
> struct timeval |  |

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#### Integral Datatypes

The integral datatypes used in the system calls are `int`, `unsigned int`, `long`, `unsigned long`, `mode_t`, and `time_t`.

`int`, `unsigned int`, `mode_t` and `time_t` are implemented as 32 bit values in this protocol.

`long` and `unsigned long` are implemented as 64 bit types.

See section Limits, for corresponding MIN and MAX values (similar to those in ``limits.h'`) to allow range checking on host and target.

`time_t` datatypes are defined as seconds since the Epoch.

All integral datatypes transferred as part of a memory read or write of a structured datatype e.g. a `struct stat` have to be given in big endian byte order.

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### Pointer Values

Pointers to target data are transmitted as they are. An exception is made for pointers to buffers for which the length isn't transmitted as part of the function call, namely strings. Strings are transmitted as a pointer/length pair, both as hex values, e.g.

    1aaf/12

which is a pointer to data of length 18 bytes at position 0x1aaf. The length is defined as the full string length in bytes, including the trailing null byte. For example, the string `"hello world"` at address 0x123456 is transmitted as

    123456/d

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#### Memory Transfer

Structured data which is transferred using a memory read or write (for example, a `struct stat`) is expected to be in a protocol-specific format with all scalar multibyte datatypes being big endian. Translation to this representation needs to be done both by the target before the `F` packet is sent, and by GDB before it transfers memory to the target. Transferred pointers to structured data should point to the already-coerced data at any time.

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#### struct stat

The buffer of type `struct stat` used by the target and GDB is defined as follows:

    struct stat {
        unsigned int  st_dev;      /* device */
        unsigned int  st_ino;      /* inode */
        mode_t        st_mode;     /* protection */
        unsigned int  st_nlink;    /* number of hard links */
        unsigned int  st_uid;      /* user ID of owner */
        unsigned int  st_gid;      /* group ID of owner */
        unsigned int  st_rdev;     /* device type (if inode device) */
        unsigned long st_size;     /* total size, in bytes */
        unsigned long st_blksize;  /* blocksize for filesystem I/O */
        unsigned long st_blocks;   /* number of blocks allocated */
        time_t        st_atime;    /* time of last access */
        time_t        st_mtime;    /* time of last modification */
        time_t        st_ctime;    /* time of last change */
    };

The integral datatypes conform to the definitions given in the appropriate section (see Integral Datatypes, for details) so this structure is of size 64 bytes.

The values of several fields have a restricted meaning and/or range of values.

`st_dev`
  A value of 0 represents a file, 1 the console.

`st_ino`
  No valid meaning for the target. Transmitted unchanged.

`st_mode`
  Valid mode bits are described in D.10.9 Constants. Any other bits have currently no meaning for the target.

`st_uid`
`st_gid`
`st_rdev`
  No valid meaning for the target. Transmitted unchanged.

`st_atime`
`st_mtime`
`st_ctime`
  These values have a host and file system dependent accuracy. Especially on Windows hosts, the file system may not support exact timing values.

The target gets a `struct stat` of the above representation and is responsible for coercing it to the target representation before continuing.

Note that due to size differences between the host, target, and protocol representations of `struct stat` members, these members could eventually get truncated on the target.

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#### struct timeval

The buffer of type `struct timeval` used by the File-I/O protocol is defined as follows:

    struct timeval {
        time_t tv_sec;  /* second */
        long   tv_usec; /* microsecond */
    };

The integral datatypes conform to the definitions given in the appropriate section (see Integral Datatypes, for details) so this structure is of size 8 bytes.

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### D.10.9 Constants

The following values are used for the constants inside of the protocol. GDB and target are responsible for translating these values before and after the call as needed.

> Open Flags |  |
> mode_t Values |  |
> Errno Values |  |
> Lseek Flags |  |
> Limits |  |

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#### Open Flags

All values are given in hexadecimal representation.

      O_RDONLY        0x0
      O_WRONLY        0x1
      O_RDWR          0x2
      O_APPEND        0x8
      O_CREAT       0x200
      O_TRUNC       0x400
      O_EXCL        0x800

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### mode_t Values

All values are given in octal representation.

      S_IFREG       0100000
      S_IFDIR        040000
      S_IRUSR          0400
      S_IWUSR          0200
      S_IXUSR          0100
      S_IRGRP           040
      S_IWGRP           020
      S_IXGRP           010
      S_IROTH            04
      S_IWOTH            02
      S_IXOTH            01

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#### Errno Values

All values are given in decimal representation.

      EPERM           1
      ENOENT          2
      EINTR           4
      EBADF           9
      EACCES         13
      EFAULT         14
      EBUSY          16
      EEXIST         17
      ENODEV         19
      ENOTDIR        20
      EISDIR         21
      EINVAL         22
      ENFILE         23
      EMFILE         24
      EFBIG          27
      ENOSPC         28
      ESPIPE         29
      EROFS          30
      ENAMETOOLONG   91
      EUNKNOWN       9999

`EUNKNOWN` is used as a fallback error value if a host system returns any error value not in the list of supported error numbers.

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#### Lseek Flags

      SEEK_SET      0
      SEEK_CUR      1
      SEEK_END      2

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

#### Limits

All values are given in decimal representation.

      INT_MIN       -2147483648
      INT_MAX        2147483647
      UINT_MAX       4294967295
      LONG_MIN      -9223372036854775808
      LONG_MAX       9223372036854775807
      ULONG_MAX      18446744073709551615

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

### D.10.10 File-I/O Examples

Example sequence of a write call, file descriptor 3, buffer is at target address 0x1234, 6 bytes should be written:

    <- Fwrite,3,1234,6
    request memory read from target
    -> m1234,6
    <- XXXXXX
    return "6 bytes written"
    -> F6

Example sequence of a read call, file descriptor 3, buffer is at target address 0x1234, 6 bytes should be read:

    <- Fread,3,1234,6
    request memory write to target
    -> X1234,6:XXXXXX
    return "6 bytes read"
    -> F6

Example sequence of a read call, call fails on the host due to invalid file descriptor (`EBADF`):

    <- Fread,3,1234,6
    -> F-1,9

Example sequence of a read call, user presses `Ctrl-c` before syscall on host is called:

    <- Fread,3,1234,6
    -> F-1,4,C
    <- T02

Example sequence of a read call, user presses `Ctrl-c` after syscall on host is called:

    <- Fread,3,1234,6
    -> X1234,6:XXXXXX
    <- T02

[ < ] | [ > ] |  | [ << ] | [ Up ] | [ >> ] |  |  |  |  | [Top] | [Contents] | [Index] | [ ? ]

## D.11 Library List Format

On some platforms, a dynamic loader (e.g. ``ld.so'`) runs in the same process as your application to manage libraries. In this case, GDB can use the loader's symbol table and normal memory operations to maintain a list of shared libraries. On other platforms, the operating system manages loaded libraries. GDB can not retrieve the list of currently loaded libraries through memory operations, so it uses the ``qXfer:libraries:read'` packet (see qXfer library list read) instead. The remote stub queries the target's operating system and reports which libraries are loaded.

The ``qXfer:libraries:read'` packet returns an XML document which lists loaded libraries and their offsets. Each library has an associated name and one or more segment base addresses, which report where the library was loaded in memory. The segment bases are start addresses, not relocation offsets; they do not depend on the library's link-time base addresses.

GDB must be linked with the Expat library to support XML library lists. See Expat.

A simple memory map, with one loaded library relocated by a single offset, looks like this:

    <library-list>
      <library name="/lib/libc.so.6">
        <segment address="0x10000000"/>
      </library>
    </library-list>

The format of a library list is described by this DTD:

    <!-- library-list: Root element with versioning -->
    <!ELEMENT library-list  (library)*>
    <!ATTLIST library-list  version CDATA   #FIXED  "1.0">
    <!ELEMENT library       (segment)*>
    <!ATTLIST library       name    CDATA   #REQUIRED>
    <!ELEMENT segment       EMPTY>
    <!ATTLIST segment       address CDATA   #REQUIRED>

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## D.12 Memory Map Format

To be able to write into flash memory, GDB needs to obtain a memory map from the target. This section describes the format of the memory map.

The memory map is obtained using the ``qXfer:memory-map:read'` (see qXfer memory map read) packet and is an XML document that lists memory regions.

GDB must be linked with the Expat library to support XML memory maps. See Expat.

The top-level structure of the document is shown below:

    <?xml version="1.0"?>
    <!DOCTYPE memory-map
              PUBLIC "+//IDN gnu.org//DTD GDB Memory Map V1.0//EN"
                     "http://sourceware.org/gdb/gdb-memory-map.dtd">
    <memory-map>
        region...
    </memory-map>

Each region can be either:

- A region of RAM starting at addr and extending for length bytes from there:

      <memory type="ram" start="addr" length="length"/>

- A region of read-only memory:

      <memory type="rom" start="addr" length="length"/>

- A region of flash memory, with erasure blocks blocksize bytes in length:

      <memory type="flash" start="addr" length="length">
        <property name="blocksize">blocksize</property>
      </memory>

Regions must not overlap. GDB assumes that areas of memory not covered by the memory map are RAM, and uses the ordinary ``M'` and ``X'` packets to write to addresses in such ranges.

The formal DTD for memory map format is given below:

    <!-- ................................................... -->
    <!-- Memory Map XML DTD ................................ -->
    <!-- File: memory-map.dtd .............................. -->
    <!-- .................................... .............. -->
    <!-- memory-map.dtd -->
    <!-- memory-map: Root element with versioning -->
    <!ELEMENT memory-map (memory | property)>
    <!ATTLIST memory-map    version CDATA   #FIXED  "1.0.0">
    <!ELEMENT memory (property)>
    <!-- memory: Specifies a memory region,
                 and its type, or device. -->
    <!ATTLIST memory        type    CDATA   #REQUIRED
                            start   CDATA   #REQUIRED
                            length  CDATA   #REQUIRED
                            device  CDATA   #IMPLIED>
    <!-- property: Generic attribute tag -->
    <!ELEMENT property (#PCDATA | property)*>
    <!ATTLIST property      name    CDATA   #REQUIRED>

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