General Query Packets - Debugging with GDB

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E.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'¹.

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:

`QAgent:1'

`QAgent:0'

Turn on or off the agent as a helper to perform some debugging operations delegated from gdb (see Control Agent).

`QAllow:op:val...'

Specify which operations gdb expects to request of the target, as a semicolon-separated list of operation name and value pairs. Possible values for op include `WriteReg', `WriteMem', `InsertBreak', `InsertTrace', `InsertFastTrace', and `Stop'. val is either 0, indicating that gdb will not request the operation, or 1, indicating that it may. (The target can then use this to set up its own internals optimally, for instance if the debugger never expects to insert breakpoints, it may not need to install its own trap handler.)

`qC'

Return the current thread ID.

Reply:

`QC thread-id': Where thread-id is a thread ID as documented in thread-id syntax.
`(anything else)': Any other reply implies the old thread ID.

`qCRC:addr,length'

Compute the CRC checksum of a block of memory using CRC-32 defined in IEEE 802.3. The CRC is computed byte at a time, taking the most significant bit of each byte first. The initial pattern code 0xffffffff is used to ensure leading zeros affect the CRC.

Note: This is the same CRC used in validating separate debug files (see Debugging Information in Separate Files). However the algorithm is slightly different. When validating separate debug files, the CRC is computed taking the least significant bit of each byte first, and the final result is inverted to detect trailing zeros.

Reply:

`E NN': An error (such as memory fault)
`C crc32': The specified memory region's checksum is crc32.

`QDisableRandomization:value'

Some target operating systems will randomize the virtual address space of the inferior process as a security feature, but provide a feature to disable such randomization, e.g. to allow for a more deterministic debugging experience. On such systems, this packet with a value of 1 directs the target to disable address space randomization for processes subsequently started via `vRun' packets, while a packet with a value of 0 tells the target to enable address space randomization.

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

Reply:

`OK': The request succeeded.
`E nn': An error occurred. nn are hex digits.
`': An empty reply indicates that `QDisableRandomization' is not supported by the stub.

This packet is not probed by default; the remote stub must request it, by supplying an appropriate `qSupported' response (see qSupported). This should only be done on targets that actually support disabling address space randomization.

`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 thread-id': A single thread ID
`m thread-id,thread-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, 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 ell, for last). Refer to thread-id syntax, for the format of the thread-id fields.

`qGetTLSAddr:thread-id,offset,lm'

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

thread-id is the thread ID associated with the thread for which to fetch the TLS address. See thread-id syntax.

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 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.

`qGetTIBAddr:thread-id'

Fetch address of the Windows OS specific Thread Information Block.

thread-id is the thread ID associated with the thread.

Reply:

`XX...': Hex encoded (big endian) bytes representing the linear address of the thread information block.
`E nn': An error occured. This means that either the thread was not found, or the address could not be retrieved.
`': An empty reply indicates that `qGetTIBAddr' 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 thread-id'

Returns information on thread-id. Where: mode is a hex encoded 32 bit mode; thread-id is a thread ID (see thread-id syntax).

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

Reply: see remote.c:remote_unpack_thread_info_response().

`QNonStop:1'

`QNonStop:0'

Enter non-stop (`QNonStop:1') or all-stop (`QNonStop:0') mode. See Remote Non-Stop, for more information.

Reply:

`OK': The request succeeded.
`E nn': An error occurred. nn are hex digits.
`': An empty reply indicates that `QNonStop' is not supported by the stub.

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

`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 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 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).

`QProgramSignals: signal [;signal]...'

Each listed signal may be delivered to the inferior process. Others should be silently discarded.

In some cases, the remote stub may need to decide whether to deliver a signal to the program or not without gdb involvement. One example of that is while detaching — the program's threads may have stopped for signals that haven't yet had a chance of being reported to gdb, and so the remote stub can use the signal list specified by this packet to know whether to deliver or ignore those pending signals.

This does not influence whether to deliver a signal as requested by a resumption packet (see vCont packet).

Signals are numbered identically to continue packets and stop replies (see Stop Reply Packets). Each signal list item should be strictly greater than the previous item. Multiple `QProgramSignals' packets do not combine; any earlier `QProgramSignals' list is completely replaced by the new list.

Reply:

`OK': The request succeeded.
`E nn': An error occurred. nn are hex digits.
`': An empty reply indicates that `QProgramSignals' is not supported by the stub.

Use of this packet is controlled by the set remote program-signals command (see set remote program-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.)

`qSearch:memory:address;length;search-pattern'

Search length bytes at address for search-pattern. address and length are encoded in hex. search-pattern is a sequence of bytes, hex encoded.

Reply:

`0': The pattern was not found.
`1,address': The pattern was found at address.
`E NN': A badly formed request or an error was encountered while searching memory.
`': An empty reply indicates that `qSearch:memory' is not recognized.

`QStartNoAckMode'

Request that the remote stub disable the normal `+'/`-' protocol acknowledgments (see Packet Acknowledgment).

Reply:

`OK': The stub has switched to no-acknowledgment mode. gdb acknowledges this reponse, but neither the stub nor gdb shall send or expect further `+'/`-' acknowledgments in the current connection.
`': An empty reply indicates that the stub does not support no-acknowledgment mode.

`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.

The following values of gdbfeature (for the packet sent by gdb) are defined:

`multiprocess': This feature indicates whether gdb supports multiprocess extensions to the remote protocol. gdb does not use such extensions unless the stub also reports that it supports them by including `multiprocess+' in its `qSupported' reply. See multiprocess extensions, for details.
`xmlRegisters': This feature indicates that gdb supports the XML target description. If the stub sees `xmlRegisters=' with target specific strings separated by a comma, it will report register description.
`qRelocInsn': This feature indicates whether gdb supports the `qRelocInsn' packet (see Relocate instruction reply packet).

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). Additional 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—the `multiprocess' feature is an example of such a feature. 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:sdata:read' No `-' Yes

`qXfer:spu:read' No `-' Yes

`qXfer:spu:write' No `-' Yes

`qXfer:siginfo:read' No `-' Yes

`qXfer:siginfo:write' No `-' Yes

`qXfer:threads:read' No `-' Yes

`qXfer:traceframe-info:read' No `-' Yes

`qXfer:uib:read' No `-' Yes

`qXfer:fdpic:read' No `-' Yes

`QNonStop' No `-' Yes

`QPassSignals' No `-' Yes

`QStartNoAckMode' No `-' Yes

`multiprocess' No `-' No

`ConditionalBreakpoints' No `-' No

`ConditionalTracepoints' No `-' No

`ReverseContinue' No `-' No

`ReverseStep' No `-' No

`TracepointSource' No `-' No

`QAgent' No `-' No

`QAllow' No `-' No

`QDisableRandomization' No `-' No

`EnableDisableTracepoints' No `-' No

`tracenz' No `-' No

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:libraries-svr4:read': The remote stub understands the `qXfer:libraries-svr4:read' packet (see qXfer svr4 library list read).
`qXfer:memory-map:read': The remote stub understands the `qXfer:memory-map:read' packet (see qXfer memory map read).
`qXfer:sdata:read': The remote stub understands the `qXfer:sdata:read' packet (see qXfer sdata 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).
`qXfer:siginfo:read': The remote stub understands the `qXfer:siginfo:read' packet (see qXfer siginfo read).
`qXfer:siginfo:write': The remote stub understands the `qXfer:siginfo:write' packet (see qXfer siginfo write).
`qXfer:threads:read': The remote stub understands the `qXfer:threads:read' packet (see qXfer threads read).
`qXfer:traceframe-info:read': The remote stub understands the `qXfer:traceframe-info:read' packet (see qXfer traceframe info read).
`qXfer:uib:read': The remote stub understands the `qXfer:uib:read' packet (see qXfer unwind info block).
`qXfer:fdpic:read': The remote stub understands the `qXfer:fdpic:read' packet (see qXfer fdpic loadmap read).
`QNonStop': The remote stub understands the `QNonStop' packet (see QNonStop).
`QPassSignals': The remote stub understands the `QPassSignals' packet (see QPassSignals).
`QStartNoAckMode': The remote stub understands the `QStartNoAckMode' packet and prefers to operate in no-acknowledgment mode. See Packet Acknowledgment.
`multiprocess': The remote stub understands the multiprocess extensions to the remote protocol syntax. The multiprocess extensions affect the syntax of thread IDs in both packets and replies (see thread-id syntax), and add process IDs to the `D' packet and `W' and `X' replies. Note that reporting this feature indicates support for the syntactic extensions only, not that the stub necessarily supports debugging of more than one process at a time. The stub must not use multiprocess extensions in packet replies unless gdb has also indicated it supports them in its `qSupported' request.
`qXfer:osdata:read': The remote stub understands the `qXfer:osdata:read' packet ((see qXfer osdata read).
`ConditionalBreakpoints': The target accepts and implements evaluation of conditional expressions defined for breakpoints. The target will only report breakpoint triggers when such conditions are true (see Break Conditions).
`ConditionalTracepoints': The remote stub accepts and implements conditional expressions defined for tracepoints (see Tracepoint Conditions).
`ReverseContinue': The remote stub accepts and implements the reverse continue packet (see bc).
`ReverseStep': The remote stub accepts and implements the reverse step packet (see bs).
`TracepointSource': The remote stub understands the `QTDPsrc' packet that supplies the source form of tracepoint definitions.
`QAgent': The remote stub understands the `QAgent' packet.
`QAllow': The remote stub understands the `QAllow' packet.
`QDisableRandomization': The remote stub understands the `QDisableRandomization' packet.
`StaticTracepoint': The remote stub supports static tracepoints.
`InstallInTrace': The remote stub supports installing tracepoint in tracing.
`EnableDisableTracepoints': The remote stub supports the `QTEnable' (see QTEnable) and `QTDisable' (see QTDisable) packets that allow tracepoints to be enabled and disabled while a trace experiment is running.
`tracenz': The remote stub supports the `tracenz' bytecode for collecting strings. See Bytecode Descriptions for details about the bytecode.

`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.

`qTBuffer'

`QTBuffer'

`QTDisconnected'

`QTDP'

`QTDPsrc'

`QTDV'

`qTfP'

`qTfV'

`QTFrame'

`qTMinFTPILen'

See Tracepoint Packets.

`qThreadExtraInfo,thread-id'

Obtain a printable string description of a thread's attributes from the target OS. thread-id is a thread ID; see thread-id syntax. 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.)

`QTNotes'

`qTP'

`QTSave'

`qTsP'

`qTsV'

`QTStart'

`QTStop'

`QTEnable'

`QTDisable'

`QTinit'

`QTro'

`qTStatus'

`qTV'

`qTfSTM'

`qTsSTM'

`qTSTMat'

See 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 auxiliary vector. Note annex must be empty.