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 Examples
D.7 File-I/O remote protocol extension

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 respectfully.

All GDB commands and responses (other than acknowledgments) are sent as a packet. A packet is introduced with the character `$', the actual packet-data, and the terminating character `#' followed by a two-digit checksum:

$packet-data#checksum

The two-digit checksum is computed as the modulo 256 sum of all characters between the leading `$' and the trailing `#' (an eight bit unsigned checksum).

Implementors should note that prior to GDB 5.0 the protocol specification also included an optional two-digit sequence-id:

$sequence-id:packet-data#checksum

That sequence-id was appended to the acknowledgment. GDB has never output sequence-ids. Stubs that handle packets added since GDB 5.0 must not accept sequence-id.

When either the host or the target machine receives a packet, the first response expected is an acknowledgment: either `+' (to indicate the package was received correctly) or `-' (to request retransmission):

-> $packet-data#checksum <- +

The host (GDB) sends commands, and the target (the debugging stub incorporated in your program) sends a response. In the case of step and continue commands, the response is only sent when the operation has completed (the target has again stopped).

packet-data consists of a sequence of characters with the exception of `#' and `$' (see `X' packet for additional exceptions).

Fields within the packet should be separated using `,' `;' or `:'. Except where otherwise noted all numbers are represented in HEX with leading zeros suppressed.

Implementors should note that prior to GDB 5.0, the character `:' could not appear as the third character in a packet (as it would potentially conflict with the sequence-id).

Response data can be run-length encoded to save space. A `*' means that the next character is an ASCII encoding giving a repeat count which stands for that many repetitions of the character preceding the `*'. The encoding is n+29, yielding a printable character where n >=3 (which is where rle starts to win). The printable characters `$', `#', `+' and `-' or with a numeric value greater than 126 should not be used.

Some remote systems have used a different run-length encoding mechanism loosely refered to as the cisco encoding. Following the `*' character are two hex digits that indicate the size of the packet.

So:

"0* "

means the same as "0000".

The error response returned for some packets includes a two character error number. That number is not well defined.

For any command not supported by the stub, an empty response (`$#00') should be returned. That way it is possible to extend the protocol. A newer GDB can tell if a packet is supported based on that response.

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

D.2 Packets

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

! -- extended mode

Enable extended mode. In extended mode, the remote server is made persistent. The `R' packet is used to restart the program being debugged.

Reply:

`OK'

The remote target both supports and has enabled extended mode.

? -- last signal

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

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

a -- reserved

Reserved for future use.

Aarglen,argnum,arg,... -- set program arguments (reserved)

Initialized `argv[]' array passed into program. arglen specifies the number of bytes in the hex encoded byte stream arg. See gdbserver for more details.

Reply:

`OK'

`ENN'

bbaud -- set baud (deprecated)

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.

Baddr,mode -- set breakpoint (deprecated)

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

This packet has been replaced by the `Z' and `z' packets (see insert breakpoint or watchpoint packet).

caddr -- 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.

Csig;addr -- continue with signal

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 (deprecated)

Toggle debug flag.

D -- detach

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

Reply:

`no response'

GDB does not check for any response after sending this packet.

e -- reserved

Reserved for future use.

E -- reserved

Reserved for future use.

f -- reserved

Reserved for future use.

FRC,EE,CF;XX -- Reply to target's F packet.

This packet is send by GDB as reply to a F request packet sent by the target. This is part of the File-I/O protocol extension. See section D.7 File-I/O remote protocol extension, for the specification.

g -- read registers

Read general registers.

Reply:

`XX...'

Each byte of register data is described by two hex digits. The bytes with the register are transmitted in target byte order. The size of each register and their position within the `g' packet are determined by the GDB internal macros REGISTER_RAW_SIZE and REGISTER_NAME macros. The specification of several standard g packets is specified below.

`ENN'

for an error.

GXX... -- write regs

See read registers packet, for a description of the XX... data.

Reply:

`OK'

for success

`ENN'

for an error

h -- reserved

Reserved for future use.

Hct... -- set thread

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 zero which means pick any thread.

Reply:

`OK'

for success

`ENN'

for an error

iaddr,nnn -- cycle step (draft)

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 (reserved)

See step with signal packet. See cycle step packet.

j -- reserved

Reserved for future use.

J -- reserved

Reserved for future use.

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?).

K -- reserved

Reserved for future use.

l -- reserved

Reserved for future use.

L -- reserved

Reserved for future use.

maddr,length -- read memory

Read length bytes of memory starting at address addr. Neither GDB nor the stub assume that sized memory transfers are assumed using word aligned accesses. FIXME: A word aligned memory transfer mechanism is needed.

Reply:

`XX...'

XX... is mem contents. Can be fewer bytes than requested if able to read only part of the data. Neither GDB nor the stub assume that sized memory transfers are assumed using word aligned accesses. FIXME: A word aligned memory transfer mechanism is needed.

`ENN'

NN is errno

Maddr,length:XX... -- write mem

Write length bytes of memory starting at address addr. XX... is the data.

Reply:

`OK'

for success

`ENN'

for an error (this includes the case where only part of the data was written).

n -- reserved

Reserved for future use.

N -- reserved

Reserved for future use.

o -- reserved

Reserved for future use.

O -- reserved

Reserved for future use.

pn... -- read reg (reserved)

See write register packet.

Reply:

`r....'

The hex encoded value of the register in target byte order.

Pn...=r... -- write register

Write register n... with value r..., which contains two hex digits for each byte in the register (target byte order).

Reply:

`OK'

for success

`ENN'

for an error

qquery -- general query

Request info about query. In general GDB queries have a leading upper case letter. Custom vendor queries should use a company prefix (in lower case) ex: `qfsf.var'. query may optionally be followed by a `,' or `;' separated list. Stubs must ensure that they match the full query name.

Reply:

`XX...'

Hex encoded data from query. The reply can not be empty.

`ENN'

error reply

`'

Indicating an unrecognized query.

Qvar=val -- general set

Set value of var to val.

See general query packet, for a discussion of naming conventions.

r -- reset (deprecated)

Reset the entire system.

RXX -- remote restart

Restart the program being debugged. XX, while needed, is ignored. This packet is only available in extended mode.

Reply:

`no reply'

The `R' packet has no reply.

saddr -- step

addr is address to resume. If addr is omitted, resume at same address.

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

Ssig;addr -- step with signal

Like `C' but step not continue.

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

taddr:PP,MM -- search

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.

TXX -- thread alive

Find out if the thread XX is alive.

Reply:

`OK'

thread is still alive

`ENN'

thread is dead

u -- reserved

Reserved for future use.

U -- reserved

Reserved for future use.

v -- reserved

Reserved for future use.

V -- reserved

Reserved for future use.

w -- reserved

Reserved for future use.

W -- reserved

Reserved for future use.

x -- reserved

Reserved for future use.

Xaddr,length:XX... -- write mem (binary)

addr is address, length is number of bytes, XX... is binary data. The characters $, #, and 0x7d are escaped using 0x7d.

Reply:

`OK'

for success

`ENN'

for an error

y -- reserved

Reserved for future use.

Y reserved

Reserved for future use.

ztype,addr,length -- remove breakpoint or watchpoint (draft)

Ztype,addr,length -- insert breakpoint or watchpoint (draft)

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 -- remove memory breakpoint (draft)

Z0,addr,length -- insert memory breakpoint (draft)

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

`ENN'

for an error

z1,addr,length -- remove hardware breakpoint (draft)

Z1,addr,length -- insert hardware breakpoint (draft)

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

`ENN'

for an error

z2,addr,length -- remove write watchpoint (draft)

Z2,addr,length -- insert write watchpoint (draft)

Insert (Z2) or remove (z2) a write watchpoint.

Reply:

`OK'

success

`'

not supported

`ENN'

for an error

z3,addr,length -- remove read watchpoint (draft)

Z3,addr,length -- insert read watchpoint (draft)

Insert (Z3) or remove (z3) a read watchpoint.

Reply:

`OK'

success

`'

not supported

`ENN'

for an error

z4,addr,length -- remove access watchpoint (draft)

Z4,addr,length -- insert access watchpoint (draft)

Insert (Z4) or remove (z4) an access watchpoint.

Reply:

`OK'

success

`'

not supported

`ENN'

for an error

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 poorly defined. In general one of the UNIX signal numbering conventions is used.

`SAA'

AA is the signal number

`TAAn...:r...;n...:r...;n...:r...;'

AA = two hex digit signal number; n... = register number (hex), r... = target byte ordered register contents, size defined by REGISTER_RAW_SIZE; n... = `thread', r... = thread process ID, this is a hex integer; n... = (`watch' | `rwatch' | `awatch', r... = data address, this is a hex integer; n... = other string not starting with valid hex digit. GDB should ignore this n..., r... pair and go on to the next. This way we can extend the protocol.

`WAA'

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

`XAA'

The process terminated with signal AA.

`NAA;t...;d...;b... (obsolete)'

AA = signal number; t... = address of symbol _start; d... = base of data section; b... = base of bss section. Note: only used by Cisco Systems targets. The difference between this reply and the `qOffsets' query is that the `N' packet may arrive spontaneously whereas the `qOffsets' is a query initiated by the host debugger.

`OXX...'

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

`Fcall-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.7 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.7 File-I/O remote protocol extension, for more details.

D.4 General Query Packets

The following set and query packets have already been defined.

qC -- current thread

Return the current thread id.

Reply:

`QCpid'

Where pid is a HEX encoded 16 bit process id.

`*'

Any other reply implies the old pid.

qfThreadInfo -- all thread ids

qsThreadInfo

Obtain a list of active thread ids from the target (OS). Since there may be too many active threads to fit into one reply packet, this query works iteratively: it may require more than one query/reply sequence to obtain the entire list of threads. The first query of the sequence will be the qfThreadInfo query; subsequent queries in the sequence will be the qsThreadInfo query.

NOTE: replaces the qL query (see below).

Reply:

`mid'

A single thread id

`mid,id...'

a comma-separated list of thread ids

`l'

(lower case 'el') denotes end of list.

In response to each query, the target will reply with a list of one or more thread ids, in big-endian hex, separated by commas. GDB will respond to each reply with a request for more thread ids (using the qs form of the query), until the target responds with l (lower-case el, for 'last').

qThreadExtraInfo,id -- extra thread info

Where id is a thread-id in big-endian hex. Obtain a printable string description of a thread's attributes from the target OS. This string may contain anything that the target OS thinks is interesting for GDB to tell the user about the thread. The string is displayed in GDB's `info threads' display. Some examples of possible thread extra info strings are "Runnable", or "Blocked on Mutex".

Reply:

`XX...'

Where XX... is a hex encoding of ASCII data, comprising the printable string containing the extra information about the thread's attributes.

qLstartflagthreadcountnextthread -- query LIST or threadLIST (deprecated)

Obtain thread information from RTOS. Where: startflag (one hex digit) is one to indicate the first query and zero to indicate a subsequent query; threadcount (two hex digits) is the maximum number of threads the response packet can contain; and nextthread (eight hex digits), for subsequent queries (startflag is zero), is returned in the response as argthread.

NOTE: this query is replaced by the qfThreadInfo query (see above).

Reply:

`qMcountdoneargthreadthread...'

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().

qCRC:addr,length -- compute CRC of memory block

Reply:

`ENN'

An error (such as memory fault)

`CCRC32'

A 32 bit cyclic redundancy check of the specified memory region.

qOffsets -- query sect offs

Get section offsets that the target used when re-locating the downloaded image. Note: while a Bss offset is included in the response, GDB ignores this and instead applies the Data offset to the Bss section.

Reply:

`Text=xxx;Data=yyy;Bss=zzz'

qPmodethreadid -- thread info request

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

Reply:

`*'

See remote.c:remote_unpack_thread_info_response().

qRcmd,command -- remote 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.

`ENN'

Indicate a badly formed request.

``''

When `q'`Rcmd' is not recognized.

qSymbol:: -- symbol lookup

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 -- symbol value

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.

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

MIPS32

All registers are transfered as thirty-two bit quantities in the order: 32 general-purpose; sr; lo; hi; bad; cause; pc; 32 floating-point registers; fsr; fir; fp.

MIPS64

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

D.6 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... -> +

D.7 File-I/O remote protocol extension

D.7.1 File-I/O Overview
D.7.2 Protocol basics
D.7.3 The F request packet
D.7.4 The F reply packet
D.7.5 Memory transfer
D.7.6 The Ctrl-C message
D.7.7 Console I/O
D.7.8 The isatty(3) call
D.7.9 The system(3) call
D.7.10 List of supported calls
D.7.11 Protocol specific representation of datatypes
D.7.12 Constants
D.7.13 File-I/O Examples

D.7.1 File-I/O Overview

The File I/O remote protocol extension (short: File-I/O) allows the target to use the hosts file system and console I/O when calling 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 with an adequate response packet to the target system. This simulates file system operations even on targets that lack file systems.

The protocol is defined host- and target-system independent. It uses it's own independent representation of datatypes and values. Both, GDB and the target's GDB stub are responsible for translating the system dependent values into the unified protocol values when data is transmitted.

The communication is synchronous. A system call is possible only when GDB is waiting for 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 interuptible by target signals. It is possible to interrupt File-I/O by a user interrupt (Ctrl-C), though.

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

D.7.2 Protocol basics

The File-I/O protocol uses the F packet, as request as well as as reply packet. Since a File-I/O system call can only occur when GDB is waiting for the continuing or stepping target, the File-I/O request is a reply that GDB has to expect as a result of a former `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 values are given in a protocol specific representation.

At that point GDB has to perform the following actions.

• If parameter pointer values are given, which point 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 pointer parameters in the request packet point to buffer space in which a system call is expected to copy data to, 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.

D.7.3 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 real values in case of scalar datatypes, as pointers to target buffer space in case of compound datatypes and unspecified memory areas or as pointer/length pairs in case of string parameters. These are appended to the call-id, each separated from its predecessor by a comma. All values are transmitted in ASCII string representation, pointer/length pairs separated by a slash.

D.7.4 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 send if the user requested a break. In this case, errno must be send 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 interupted before the host call has been performed:

F-1,4,C

assuming 4 is the protocol specific representation of EINTR.

D.7.5 Memory transfer

Structured data which is transferred using a memory read or write as e.g. a struct stat is expected to be in a protocol specific format with all scalar multibyte datatypes being big endian. This should be done by the target before the F packet is sent resp. by GDB before it transfers memory to the target. Transferred pointers to structured data should point to the already coerced data at any time.

D.7.6 The Ctrl-C message

A special case is, if the Ctrl-C flag is set in the GDB reply packet. In this case the target should behave, as if it had gotten a break message. The meaning for the target is "system call interupted by SIGINT". Consequentially, the target should actually stop (as with a break message) and return to GDB with a T02 packet. In this case, it's important for the target to know, in which state the system call was interrupted. Since this action is by design not an atomic operation, we have to differ between two 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 -- successful or not -- and should behave as if the break message arrived right after the system call.

GDB must behave reliable. 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 finshed by GDB. This requires sending M or X packets as they fit. The F packet may only be send when either nothing has happened or the full action has been completed.

D.7.7 Console I/O

By default and if not explicitely 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 presses Ctrl-C. The behaviour is as explained above, the read system call is treated as finished.

• The user presses Enter. This is treated as end of input with a trailing line feed.

• The user presses Ctrl-D. This is treated as end of input. No trailing character, especially no Ctrl-D is appended to the input.

If the user has typed more characters as 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 on users request.

D.7.8 The isatty(3) call

A special case in this protocol is the library call isatty which is implemented as it's own call inside of this protocol. It returns 1 to the target if the file descriptor given as parameter is attached to the GDB console, 0 otherwise. Implementing through system calls would require implementing ioctl and would be more complex than needed.

D.7.9 The system(3) call

The other special case in this protocol is the system call which is implemented as it's own call, too. GDB is taking over the full task of calling the necessary host calls to perform the system call. The return value of system is simplified before it's returned to the target. Basically, the only signal transmitted back is EINTR in case the user pressed Ctrl-C. Otherwise the return value consists entirely of the exit status of the called command.

Due to security concerns, the system call is refused to be called by GDB by default. The user has to allow this call explicitly by entering

`set remote system-call-allowed 1'

Disabling the system call is done by

`set remote system-call-allowed 0'

The current setting is shown by typing

`show remote system-call-allowed'

D.7.10 List of supported calls

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

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 length 0.

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.

Each other bit is 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.

Each other bit is silently ignored.

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

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.

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

buf is an invalid pointer value.

EINTR

The call was interrupted by the user.

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

buf 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.

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.

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.

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.

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.

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.

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.

system

Synopsis: int system(const char *command); Request: Fsystem,commandptr/len Return value: 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 hosts 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.

D.7.11 Protocol specific representation of datatypes

Integral datatypes
Pointer values
struct stat
struct timeval

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.

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. Example:

``hello, world'' at address 0x123456

is transmitted as

123456/d

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 are conforming to the definitions given in the approriate 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: 0 file 1 console st_ino: No valid meaning for the target. Transmitted unchanged. st_mode: Valid mode bits are described in Appendix C. Any other bits have currently no meaning for the target. st_uid: No valid meaning for the target. Transmitted unchanged. st_gid: No valid meaning for the target. Transmitted unchanged. 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 systems don't support exact timing values.

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

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

struct timeval

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

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

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

D.7.12 Constants

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

Open flags
mode_t values
Errno values
Lseek flags
Limits

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

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

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.

Lseek flags

SEEK_SET 0 SEEK_CUR 1 SEEK_END 2

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

D.7.13 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