CODEWITHSUNDEEP
linuxloopsgdbcommand
Troskyvs
Troskyvs

Reputation: 8087

How gdb command "until" is able to jump over a loop?

I’m quite curious of how gdb implements "until" command.

I mean, I can understand how "finish" works, we know at the beginning and the end of a function, there’s code routine to operate ebp/esp register, so gdb can use this to execute to the end of a function.

But my question is, how does gdb know the end of a loop? If no debug symbol is provided, we know the loop might embed another inner loop, so depending on ecx register is not a solution. Then how does it know the end of a loop? The assembly code of "while" "do/while" and "for" could be different, not sure if there’s a pattern that "finish" command can look for.

Hope to see your explanations.

Upvotes: 1

Views: 1194

Answers (1)

Employed Russian
Employed Russian

Reputation: 213859

how does gdb know the end of a loop?

It doesn't. From documentation:

until
  Continue running until a source line past the current line, in the current stack
  frame, is reached. 

  This command is used to avoid single stepping through a loop more than once.
  It is like the next command, except that when until encounters a jump, it
  automatically continues execution until the program counter is greater
  than the address of the jump.

It's implemented pretty much exactly as described in the last statement above.

Upvotes: 2

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