There are multiple classes of targets such as: processes, executable files or
recording sessions. Core files belong to the process class, making core file
and process mutually exclusive. Otherwise, gdb can work concurrently
on multiple active targets, one in each class. This allows you to (for
example) start a process and inspect its activity, while still having access to
the executable file after the process finishes. Or if you start process
recording (see Reverse Execution) and reverse-step there, you are
presented a virtual layer of the recording target, while the process target
remains stopped at the chronologically last point of the process execution.
Use the core-file and exec-file commands to select a new core
file or executable target (see Commands to Specify Files). To
specify as a target a process that is already running, use the attach
command (see Debugging an Already-running Process).