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Re: GDB as a program analyzer - some thoughts
- From: Ramana Radhakrishnan <ramana dot radhakrishnan at codito dot com>
- To: Alexandre Courbot <Alexandre dot Courbot at lifl dot fr>
- Cc: gdb at sources dot redhat dot com
- Date: Tue, 18 May 2004 21:14:27 +0530
- Subject: Re: GDB as a program analyzer - some thoughts
- References: <40AA23FD.6040103@lifl.fr>
Hi Alexandre,
This post deals about using GDB for non-debugging purposes, I hope
it's not offtopic. I'm also hoping to find some people interested in
the same matters than I to discuss about how GDB could be improved for
analysis purposes.
My few posts here have been basically about it. GDB is great to
extract datas out of a program and output them to a file. It's very
different to gprof - for instance, it's useful to graphically measure
the efficiency of one memory manager against another, by placing
breakpoints at memory allocation functions and recording the amount of
memory used there. The recorded datas can then proove that, with some
strategy, we saved a few garbage collections and therefore gained speed.
This is of course nothing you can't do with printfs enclosed inside
#ifdef DEBUGs, but the advantage is that GDB allows you to do it in a
non-intrusive and much more flexible way.
Well non-intrusive / rather less intrusive :-) ? There is still the
overhead of a ptrace call in case of native debugging. in such cases ?
Flexible yes .
I'm surprised that I haven't found a solutions dedicated to that - and
so far, gdb is by far the best solution I've found. Writing a gdb
script + the corresponding gnuplot script results in easy to get
graphes of whatever you want in your program.
If people are interested in it, I can post some of the scripts I'm
using with their result. I can also write a tutorial page on that topic.
I have some non-elegant bits in my scripts however. They mainly
concern breakpoints. Since the breakpoints are set into gdb scripts,
it's better if they reference symbols like function names instead of
file:line_number pairs. The line of the breakpoint might move in
future code modifications, and the gdb script won't be updated
accordingly. Unfortunately, breaking at the very beginning of the
function you are interested doesn't give you the data that has been
computed inside.
So, I wonder if some breakpoint settings would be implementable with
gdb (or if they can already be expressed and I missed them):
- Setting a breakpoint at the returning of a function
Could be done by maybe adding a pre-finish command to gdb ? Though
ofcourse in modern toolchains since the size of a function is present a
hack might be to write a script using
using objdump / awk to generate a gdb script per function to put
breakpoints at all possible return instructions ?
There does not seem to be a pre-finish function. finish runs through
the current activation record and returns .What you seem to want is for
gdb to stop just before the function returns
so that the data can be collected.
- Setting a breakpoint at some fixed point of a function (for
instance, a C label)
- Any other breakpoint setting that is not line-number based and would
support source modification
In continuation with my fancy for awk , grep and objdump maybe generate
your script with respect to the functions that you are interested in ?
objdump -x executable; grep for the symbol, get the address and then
generate the script with the correct address. Ofcourse might sound like
overkill for something so simple ?
cheers
Ramana