Communication protocol

The stub files provided with GDB implement the target side of the communication protocol, and the GDB side is implemented in the GDB source file `remote.c'. Normally, you can simply allow these subroutines to communicate, and ignore the details. (If you're implementing your own stub file, you can still ignore the details: start with one of the existing stub files. `sparc-stub.c' is the best organized, and therefore the easiest to read.)

However, 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.

All GDB commands and responses (other than acknowledgements, which are single characters) are sent as a packet which includes a checksum. A packet is introduced with the character `$', and ends with the character `#' followed by a two-digit checksum:

$packet info#checksum

checksum is computed as the modulo 256 sum of the packet info characters.

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

The host (GDB) sends commands, and the target (the debugging stub incorporated in your program) sends data in response. The target also sends data when your program stops.

Command packets are distinguished by their first character, which identifies the kind of command.

These are some of the commands currently supported (for a complete list of commands, look in `gdb/remote.c.'):

g

Requests the values of CPU registers.

G

Sets the values of CPU registers.

maddr,count

Read count bytes at location addr.

Maddr,count:...

Write count bytes at location addr.

c

caddr

Resume execution at the current address (or at addr if supplied).

s

saddr

Step the target program for one instruction, from either the current program counter or from addr if supplied.

k

Kill the target program.

?

Report the most recent signal. To allow you to take advantage of the GDB signal handling commands, one of the functions of the debugging stub is to report CPU traps as the corresponding POSIX signal values.

T

Allows the remote stub to send only the registers that GDB needs to make a quick decision about single-stepping or conditional breakpoints. This eliminates the need to fetch the entire register set for each instruction being stepped through. The GDB remote serial protocol now implements a write-through cache for registers. GDB only re-reads the registers if the target has run.

If you have trouble with the serial connection, you can use the command set remotedebug. This makes GDB report on all packets sent back and forth across the serial line to the remote machine. The packet-debugging information is printed on the GDB standard output stream. set remotedebug off turns it off, and show remotedebug shows you its current state.

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