GDB scope does not work quite right for Pascal - Gpc

12 Oct 2001


      At 14:44 12/10/01 , vous avez écrit:
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Number:         222
Category:       gdb
Synopsis:       GDB scope does not work quite right for Pascal
Confidential:   no
Severity:       non-critical
Priority:       medium
Responsible:    unassigned
State:          open
Class:          sw-bug
Submitter-Id:   net
Arrival-Date:   Fri Oct 12 05:48:01 PDT 2001
Closed-Date:
Last-Modified:
Originator:     Adam Oldham
Release:        gdb 20010813
Organization:
Environment:
Mandrake 8.1, Kernel 2.4.8, glibc 2.2, Intel Platform
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Description:
Source is compiled with FPC (Free Pascal Compiler).  This can happen with 
any source of pascal.  When you declare nested functions, gdb's scope does 
not allow the viewing of the variables that are from the parent function 
that should be within the scope.
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How-To-Repeat:
The simple program attached will produce the problem.  When compiled and 
run, the correct output is displayed, but if you run GDB on it, and break 
on function test2 and print out int1 which is a global variable for the 
function test1, you receive:
"No symbol "INT1" in current context."
There are several remarks to that bug report:
   1) I don't know at all how nested functions work in C
Are they allowed? Do they also use a paremt fram pushing method ?
If yes, can locals from the calling function be found in C?
Maybe someone from the gdb mailing list can answer that question.
If this is already implemented for C its probably easier to just take
the same method.
2) There is a solution, I implemented it in the IDE for Free Pascal
but it is rather complicated.
   Let me try to explain this solution to everyone.
   Nested functions in pascal get a hidden parameter that contains the
frame pointer (ebp value for i386 cpu) of the calling function. This allows the
program to access correctly local variable of the function that called the 
nested
function. This value is searched in the backtrace at the location where
the frame pointer are usually saved ( i.e. at offset 0 of the local ebp 
value for i386)
If we find the corresponding frame (which is not always the first frame 
above the current one)
then we can try if we find the unfound local symbol in that frame).
To ease this search, I inserted a pseudo-parameter to the function called
parent_ebp (in lower case, while normal Free Pascal symbols are uppercased)
that indicate the location of this parnt frame pointer.
3)  As maintainer of the pascal language specific part of GDB,
I am of course willing to introduce this into the gdb sources, but I would
really like to get first some feedback from the gpc list.
    Does gpc do anything about this calling frame hidden parameter ?
Could we agree to use the same method for both compiler
in order to get better support for Pascal ?
Pierre Muller
Institut Charles Sadron
6,rue Boussingault
F 67083 STRASBOURG CEDEX (France)
mailto:muller@ics.u-strasbg.fr
Phone : (33)-3-88-41-40-07  Fax : (33)-3-88-41-40-99