Python API questions and use cases

Tom Tromey
Wed Sep 15 18:01:00 GMT 2010

>>>>> "Joel" == Joel Borggrén-Franck <> writes:

Joel> 1) Getting the value of a global
Joel> foo myGlobalFoo;
Joel> in some C file, how do I access the value of myGlobalFoo from
Joel> python?

Joel> The only working solution I have at the moment is to escape to
Joel> gdb-script with:
Joel> gdb.parse_and_eval("myGlobalFoo")
Joel> is this intended?

This is simplest.

Joel> I know I can iterate over symbols in the symbol table, but I
Joel> haven't found a way to go from symbol to value.

Hmm, we don't seem to expose a way to do that.  Sorry about that.
Could you file a bug report for this?

Joel> 2) Casts

Joel> given that I know that at address 0xdeadbeef there will be a
Joel> struct of type foo how do I get that struct (or a pointer to that
Joel> struct) as a gdb.Value object?

Joel> Currently i have written a cast function:

Joel> def cast(type_string, address):
Joel>     """Returns a gdb.Value object with type 'type_string' from memory
Joel>     starting at 'address'."""
Joel>     return gdb.parse_and_eval("(%s)((void *)%s)" % (type_string, address))

Joel> that I think works, but is there some other way?

I think the tricky part is getting a Value holding the appropriate
constant.  For that you might not have anything better, at present, than
parse_and_eval.  (This is a non-issue if the address is already a

You can get a (simple) type using gdb.lookup_type.  So you should be
able to do something like:

   address.cast (gdb.lookup_type ('struct foo').pointer())

Joel> Another usecase for casts would be if I wanted to mask an address
Joel> in python, given:
Joel> foo *fooP;
Joel> how do I do the equivalent of this in pyhton:
Joel> set $check = ((size_t)foo) & 1

Simplest is to do it using Python math:

  foo = ...
  check = long (foo) & 1

Otherwise you can look up size_t:

  check = foo.cast (gdb.lookup_type ('size_t')) & 1

If you really want to set a convenience variable, then for the time
being you will have to use parse_and_eval.  We don't expose those any
other way.


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