In all-stop mode, when a program on the remote target is running,
gdb may attempt to interrupt it by sending a ‘Ctrl-C’,
BREAK or a BREAK followed by g, control of which
is specified via gdb's ‘interrupt-sequence’.
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 except for TCP, in which case gdb sends the
telnet BREAK sequence.
‘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 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.
BREAK followed by g is also known as Magic SysRq g.
When Linux kernel receives this sequence from serial port,
it stops execution and connects to gdb.
In non-stop mode, because packet resumptions are asynchronous
(see vCont packet), gdb is always free to send a remote
command to the remote stub, even when the target is running. For that
reason, gdb instead sends a regular packet (see vCtrlC packet) with the usual packet framing instead of the single byte
0x03.
Stubs are not required to recognize these interrupt mechanisms and the precise meaning associated with receipt of the interrupt is implementation defined. If the target supports debugging of multiple threads and/or processes, it should attempt to interrupt all currently-executing threads and processes. If the stub is successful at interrupting the running program, it should send one of the stop reply packets (see Stop Reply Packets) to gdb as a result of successfully stopping the program in all-stop mode, and a stop reply for each stopped thread in non-stop mode. Interrupts received while the program is stopped are queued and the program will be interrupted when it is resumed next time.