GDB: how to change a convenience variable from a built-in python

Question

How do I assign something to a gdb convenience variable from its built-in python? I thought it should be simple, because it naturally arises while trying to implement a user-defined function. However I couldn't find anything.

UPD: Added a simplified example. My real function is more complex though. Anyway, a way to transfer a variable from python up to gdb would help greatly.

An example of usage (achtung! — it raises an error):

#accept a string, return it's lenght
define RetLenOfArg
  py arg0 = gdb.parse_and_eval("$arg0")
  set $Retrn = py len(arg0)
end

Please explain your problem in greater detail and with relevant examples/code if possible — sshashank124
– sshashank124, Commented Apr 18, 2014 at 8:14
I found a quick startup guide about GDB convenience functions and arguments in here: sourceware.org/gdb/onlinedocs/gdb/Functions-In-Python.html — Ivaylo
– Ivaylo, Commented Apr 18, 2014 at 8:31
@Ivaylo yes, you're right, you may post it as an answer. To sum up a steps: instead of implementing a function in .gdbinit we need to create a whole class in a separate python file, load it in a gdb through a command source <python_file>, and then use. In my real case I will need anyway create one more function that will use a python one. Cumbersome, but that's how it is done. — Hi-Angel
– Hi-Angel, Commented Apr 18, 2014 at 9:18

gipi · Accepted Answer · 2021-07-17 09:12:45Z

5

There exists gdb's python API to get/set convenience variable: from the documentation

Function: gdb.convenience_variable (name) Return the value of the convenience variable (see Convenience Vars) named name. name must be a string. The name should not include the ‘$’ that is used to mark a convenience variable in an expression. If the convenience variable does not exist, then None is returned.

Function: gdb.set_convenience_variable (name, value) Set the value of the convenience variable (see Convenience Vars) named name. name must be a string. The name should not include the ‘$’ that is used to mark a convenience variable in an expression. If value is None, then the convenience variable is removed. Otherwise, if value is not a gdb.Value (see Values From Inferior), it is is converted using the gdb.Value constructor.

edited Jul 17, 2021 at 9:12

answered Dec 13, 2019 at 18:31

gipi

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1 Comment

Simon Sobisch Over a year ago

Note: that's a GDB 8.2 feature, in case someone needs it with an older version use gdb.execute as mentioned in the accepted answer.

score 4 · Accepted Answer · 2014-04-18 09:27:52Z

4

accept a string, return it's lenght

If you indeed need strlen you can use $_strlen - a built in gdb convenience function that compute string length:

(gdb) set $Retrn=$_strlen("test")
(gdb) p $Retrn
$4 = 4

how to assign something to a GDB convenience variable from a built-in python?

You can use gdb.execute:

>more my_own_len.py
class my_own_len (gdb.Function):
   def __init__ (self):
     super (my_own_len, self).__init__ ("my_own_len")

   def invoke (self, arg):
      res=len(arg.string())
      gdb.execute( "set $Retrn=" + str(res))
      return res

my_own_len()

This is a test:

>gdb -q -x my_own_len.py
(gdb) p $_strlen("test")
$1 = 4
(gdb) p $my_own_len("test")
$2 = 4
(gdb) p $Retrn
$3 = 4
(gdb)

edited Apr 18, 2014 at 9:27

answered Apr 18, 2014 at 9:21

user184968

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