to_open. This function is invoked by the GDB target command. Any additional arguments (beyond the name of the target being invoked) are passed to this function. to_open should establish the communications with the target. It should establish the state of the target (is it already running for example), and initialize data structures appropriately.

This function should not start the target running if it is not currently running—that is the job of the functions (to_create_inferior and to_resume) invoked by the GDB run command.

to_xclose and to_close. Both these functions should close the remote connection. to_close is the legacy function. New implementations should use to_xclose which should also free any memory allocated for this target.

to_attach. For targets which can run without a debugger connected, this function attaches the debugger to a running target (which should first have been opened).

to_detach. Function to detach from a target, leaving it running.

to_disconnect. This is similar to to_detach, but makes no effort to inform the target that the debugger is detaching. It should just drop the connection to the target.

to_terminal_inferior. This function connects the target's terminal I/O to the local terminal. This functionality is not always available with remote targets.

to_rcmd. If the target is capable of running commands, then this function requests that command to be run on the target. This is of most relevance to remote targets.