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D.3 Packets

The following table provides a complete list of all currently defined commands and their corresponding response data. See 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.

Several packets and replies include a thread-id field to identify a thread. Normally these are positive numbers with a target-specific interpretation, formatted as big-endian hex strings. A thread-id can also be a literal ‘-1’ to indicate all threads, or ‘0’ to pick any thread.

In addition, the remote protocol supports a multiprocess feature in which the thread-id syntax is extended to optionally include both process and thread ID fields, as ‘ppid.tid’. The pid (process) and tid (thread) components each have the format described above: a positive number with target-specific interpretation formatted as a big-endian hex string, literal ‘-1’ to indicate all processes or threads (respectively), or ‘0’ to indicate an arbitrary process or thread. Specifying just a process, as ‘ppid’, is equivalent to ‘ppid.-1’. It is an error to specify all processes but a specific thread, such as ‘p-1.tid’. Note that the ‘p’ prefix is not used for those packets and replies explicitly documented to include a process ID, rather than a thread-id.

The multiprocess thread-id syntax extensions are only used if both gdb and the stub report support for the ‘multiprocess’ feature using ‘qSupported’. See multiprocess extensions, for more information.

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

A stub is required to support the ‘g’, ‘G’, ‘m’, ‘M’, ‘c’, and ‘s’ commands. All other commands are optional.

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. This packet has a special interpretation when the target is in non-stop mode; see Remote Non-Stop.

Reply: See 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.

‘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).


‘bc’
Backward continue. Execute the target system in reverse. No parameter. See Reverse Execution, for more information.

Reply: See Stop Reply Packets, for the reply specifications.


‘bs’
Backward single step. Execute one instruction in reverse. No parameter. See Reverse Execution, for more information.

Reply: See Stop Reply Packets, for the reply specifications.

‘c [addr]’
Continue. addr is address to resume. If addr is omitted, resume at current address.

Reply: See 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 Stop Reply Packets, for the reply specifications.

‘d’
Toggle debug flag.

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

‘D’
‘D;pid’
The first form of the packet is used to detach gdb from the remote system. It is sent to the remote target before gdb disconnects via the detach command.

The second form, including a process ID, is used when multiprocess protocol extensions are enabled (see multiprocess extensions), to detach only a specific process. The pid is specified as a big-endian hex string.

Reply:

‘OK’
for success

‘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 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.

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

Reply:

‘OK’
for success

‘H c thread-id’
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 thread-id has the format and interpretation described in thread-id syntax.

Reply:

‘OK’
for success

‘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.

‘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’
All the data was written successfully. (If only part of the data was written, this command returns an error.)

‘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

‘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

‘q name params...’
‘Q name params...’
General query (‘q’) and set (‘Q’). These packets are described fully in 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 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 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 thread-id’
Find out if the thread thread-id is alive. See thread-id syntax.

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. The process ID is a hexadecimal integer identifying the process. In all-stop mode, all threads in the attached process are stopped; in non-stop mode, it may be attached without being stopped if that is supported by the target.

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

Reply:

‘E nn’
for an error
‘Any stop packet’
for success in all-stop mode (see Stop Reply Packets)
‘OK’
for success in non-stop mode (see Remote Non-Stop)

‘vCont[;action[:thread-id]]...’
Resume the inferior, specifying different actions for each thread. If an action is specified with no thread-id, 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 in all-stop mode and in their current state in non-stop mode. Specifying multiple default actions is an error; specifying no actions is also an error. Thread IDs are specified using the syntax described in thread-id syntax.

Currently supported actions are:

‘c’
Continue.
‘C sig’
Continue with signal sig. The signal sig should be two hex digits.
‘s’
Step.
‘S sig’
Step with signal sig. The signal sig should be two hex digits.
‘t’
Stop.

The optional argument addr normally associated with the ‘c’, ‘C’, ‘s’, and ‘S’ packets is not supported in ‘vCont’.

The ‘t’ action is only relevant in non-stop mode (see Remote Non-Stop) and may be ignored by the stub otherwise. A stop reply should be generated for any affected thread not already stopped. When a thread is stopped by means of a ‘t’ action, the corresponding stop reply should indicate that the thread has stopped with signal ‘0’, regardless of whether the target uses some other signal as an implementation detail.

Reply: See 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 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 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.

The stub must support ‘vCont’ if it reports support for multiprocess extensions (see multiprocess extensions). Note that in this case ‘vCont’ actions can be specified to apply to all threads in a process by using the ‘ppid.-1’ form of the thread-id.

Reply:

‘OK’
for success
‘E.memtype’
for vFlashErase addressing non-flash memory

‘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

‘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.
‘vKill;pid’
Kill the process with the specified process ID. pid is a hexadecimal integer identifying the process. This packet is used in preference to ‘k’ when multiprocess protocol extensions are supported; see multiprocess extensions.

Reply:

‘E nn’
for an error
‘OK’
for success

‘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 Stop Reply Packets)

‘vStopped’
In non-stop mode (see Remote Non-Stop), acknowledge a previous stop reply and prompt for the stub to report another one.

Reply:

‘Any stop packet’
if there is another unreported stop event (see Stop Reply Packets)
‘OK’
if there are no unreported stop events

‘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

‘z type,addr,kind’
‘Z type,addr,kind’
Insert (‘Z’) or remove (‘z’) a type breakpoint or watchpoint starting at address address of kind kind.

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,kind’
‘Z0,addr,kind’
Insert (‘Z0’) or remove (‘z0’) a memory breakpoint at address addr of type kind.

A memory breakpoint is implemented by replacing the instruction at addr with a software breakpoint or trap instruction. The kind is target-specific and typically 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. Some architectures have additional meanings for kind; see Architecture-Specific Protocol Details.

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

‘z1,addr,kind’
‘Z1,addr,kind’
Insert (‘Z1’) or remove (‘z1’) a hardware breakpoint at address addr.

A hardware breakpoint is implemented using a mechanism that is not dependant on being able to modify the target's memory. kind has the same meaning as in ‘Z0’ packets.

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

Reply:

‘OK’
success

‘z2,addr,kind’
‘Z2,addr,kind’
Insert (‘Z2’) or remove (‘z2’) a write watchpoint at addr. kind is interpreted as the number of bytes to watch.

Reply:

‘OK’
success

‘z3,addr,kind’
‘Z3,addr,kind’
Insert (‘Z3’) or remove (‘z3’) a read watchpoint at addr. kind is interpreted as the number of bytes to watch.

Reply:

‘OK’
success

‘z4,addr,kind’
‘Z4,addr,kind’
Insert (‘Z4’) or remove (‘z4’) an access watchpoint at addr. kind is interpreted as the number of bytes to watch.

Reply:

‘OK’
success

Error responses use one of the following formats:

‘Enn’
‘E.text’
‘Enn :text’

Error numbers are not well defined, any non-zero error number can be used to indicate an error. nn is a 2 character hex encoded value. The text string may provide additional information about the error.