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gawk: Debugging Terms

 14.1.2 Debugging Concepts
 Before diving in to the details, we need to introduce several important
 concepts that apply to just about all debuggers.  The following list
 defines terms used throughout the rest of this major node:
 "Stack frame"
      Programs generally call functions during the course of their
      execution.  One function can call another, or a function can call
      itself (recursion).  You can view the chain of called functions
      (main program calls A, which calls B, which calls C), as a stack of
      executing functions: the currently running function is the topmost
      one on the stack, and when it finishes (returns), the next one down
      then becomes the active function.  Such a stack is termed a "call
      For each function on the call stack, the system maintains a data
      area that contains the function's parameters, local variables, and
      return value, as well as any other "bookkeeping" information needed
      to manage the call stack.  This data area is termed a "stack
      'gawk' also follows this model, and gives you access to the call
      stack and to each stack frame.  You can see the call stack, as well
      as from where each function on the stack was invoked.  Commands
      that print the call stack print information about each stack frame
      (as detailed later on).
      During debugging, you often wish to let the program run until it
      reaches a certain point, and then continue execution from there one
      statement (or instruction) at a time.  The way to do this is to set
      a "breakpoint" within the program.  A breakpoint is where the
      execution of the program should break off (stop), so that you can
      take over control of the program's execution.  You can add and
      remove as many breakpoints as you like.
      A watchpoint is similar to a breakpoint.  The difference is that
      breakpoints are oriented around the code: stop when a certain point
      in the code is reached.  A watchpoint, however, specifies that
      program execution should stop when a _data value_ is changed.  This
      is useful, as sometimes it happens that a variable receives an
      erroneous value, and it's hard to track down where this happens
      just by looking at the code.  By using a watchpoint, you can stop
      whenever a variable is assigned to, and usually find the errant
      code quite quickly.
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