GDB: Returning from user defined function in script

Question

I am trying to inspect the contents of a data structure in a core dump. There are many nodes in this. And I need to compare two nodes. I am trying to implement the same compare function that the program used to handle the data structure as a user defined gdb function. A node has multiple parameters and the compare function checks each of them and returns -1, 0 or 1.

I did the following,

define single_compare
    if $arg0 > $arg1
        set $arg2 = 1
    end
    if $arg0 < $arg1
        set $arg2 = -1
    end
    if $arg0 == $arg1
        set $arg2 = 0
    end
end

define compare_nodes
    set $obj_inst_a = (nodetype*)$arg0
    set $obj_inst_b = (nodetype*)$arg1
    set $compr = 0

    single_compare $obj_inst_a->a $obj_inst_b->a $compr
    if $compr != 0
        set $arg2 = $compr
    end

    single_compare $obj_inst_a->b $obj_inst_b->b $compr
    if $compr != 0
        set $arg2 = $compr
    end
...

The problem here is, if i find the a values in each node is different, I don't need to compare b, just return from there, and use $arg2 to take a decision. Is there a way to do that in gdb script? finish/return will cause the current program functions to return/finish. Not the script.

Tom Tromey · Accepted Answer · 2018-07-12 14:04:36Z

1

There is not a direct way to do this. gdb's CLI language is, as you've discovered, somewhat limited.

One way to sort of get what you want is to wrap your function's body in a loop that will only execute once:

set var $w = 1
while $w
  if something
    # Here's how to exit early.
    loop_break
  end
  # Make sure this only loops once.
  set var $w = 0
end

Another approach is to write more complicated scripts using Python.

answered Jul 12, 2018 at 14:04

Tom Tromey

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1 Comment

Subin Over a year ago

I did something like if else if else end end end And thank you very much, I need the loop part for something else :)

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