GDB has several different types of target: executable files, core dumps, executing processes etc. At any time, GDB may have several sets of target operations in use. For example target operations for use with an executing process (which can run code) might be different from the operations used when inspecting a core dump.

All the targets GDB knows about are held in a stack. GDB walks down the stack to find the set of target operations suitable for use. The stack is organized as a series of strata of decreasing importance: target operations for threads, then target operations suitable for processes, target operations to download remote targets, target operations for core dumps, target operations for executable files and at the bottom target operations for dummy targets. So GDB when debugging a running process will always select target operations from the process_stratum if available, over target operations from the file stratum, even if the target operations from the file stratum were pushed onto the stack more recently.

At any particular time, there is a current target, held in the global variable current_target. This can never be NULL—if there is no other target available, it will point to the dummy target.

target.h defines a set of convenience macros to access functions and values in the current_target. Thus current_target->to_xyz can be accessed as target_xyz.