If the ‘Ctrl-C’ flag is set in the gdb
reply packet (see The F Reply Packet),
the target should behave as if it had
gotten a break message. The meaning for the target is “system call
interrupted by SIGINT”. Consequentially, the target should actually stop
(as with a break message) and return to gdb with a T02
packet.
It's important for the target to know in which state the system call was interrupted. There are two possible cases:
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 — successfully or not — and should behave
as if the break message arrived right after the system call.
gdb must behave reliably. 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 finished by
gdb. This requires sending M or X packets as necessary.
The F packet may only be sent when either nothing has happened
or the full action has been completed.