Well, this wasn’t easy – but I think I somewhat got it 🙂 I went through a bunch of failed attempts (posted here); relevant code is below.
Basically, the problem in a “next/step until breakpoint” is how to determine whether you’re “on” a breakpoint or not, if the debugger is stopped (at a step). Note also I use GDB 7.2-1ubuntu11 (current for Ubuntu 11.04). So, it went like this:
- I first found about Convenience Variables, and thought – given there are program counters and such available, there must be some GDB convenience variable that gives the “breakpoint” status, and can be used directly in a GDB script. After looking through GDB reference Index for a while, however, I simply cannot find any such variables (my attempts are in nub.gdb)
- In lack of such a “breakpoint status” internal variable – the only thing left to do, is to capture the (‘stdout’) command line output of GDB (in response to commands) as a string, and parse it (looking for “Breakpoint”)
- Then, I found out about Python API to GDB, and the
gdb.execute("CMDSTR", toString=True)
command – which is seemingly exactly what is needed to capture the output: “By default, any output produced by command is sent to gdb’s standard output. If the to_string parameter is True, then output will be collected by gdb.execute and returned as a string[1]“!- So, first I tried to make a script (pygdb-nub.py,gdbwrap) that would utilize
gdb.execute
in the recommended manner; failed here – because of this: - Then, I thought I’d use a python script to
subprocess.Popen
the GDB program, while replacing its stdin and stdout; and then proceed controlling GDB from there (pygdb-sub.py) – that failed too… (apparently, because I didn’t redirect stdin/out right) - Then, I thought I’d use python scripts to be called from GDB (via
source
) which would internally fork into a pty whenevergdb.execute
should be called, so as to capture its output (pygdb-fork.gdb,pygdb-fork.py)… This almost worked – as there are strings returned; however GDB notices something ain’t right: “[tcsetpgrp failed in terminal_inferior: Operation not permitted]“, and the subsequent return strings don’t seem to change.
- So, first I tried to make a script (pygdb-nub.py,gdbwrap) that would utilize
And finally, the approach that worked is: temporarily redirecting the GDB output from a gdb.execute
to a logfile in RAM (Linux: /dev/shm
); and then reading it back, parsing it and printing it from python – python also handles a simple while loop that steps until a breakpoint is reached.
The irony is – most of these bugs, that caused this solution via redirecting the logfile, are actually recently fixed in SVN; meaning those will propagate to the distros in the near future, and one will be able to use gdb.execute("CMDSTR", toString=True)
directly :/ Yet, as I cannot risk building GDB from source right now (and possibly bumping into possible new incompatibilites), this is good enough for me also 🙂
Here are the relevant files (partially also in pygdb-fork.gdb,pygdb-fork.py):
pygdb-logg.gdb
is:
# gdb script: pygdb-logg.gdb
# easier interface for pygdb-logg.py stuff
# from within gdb: (gdb) source -v pygdb-logg.gdb
# from cdmline: gdb -x pygdb-logg.gdb -se test.exe
# first, "include" the python file:
source -v pygdb-logg.py
# define shorthand for nextUntilBreakpoint():
define nub
python nextUntilBreakpoint()
end
# set up breakpoints for test.exe:
b main
b doFunction
# go to main breakpoint
run
pygdb-logg.py
is:
# gdb will 'recognize' this as python
# upon 'source pygdb-logg.py'
# however, from gdb functions still have
# to be called like:
# (gdb) python print logExecCapture("bt")
import sys
import gdb
import os
def logExecCapture(instr):
# /dev/shm - save file in RAM
ltxname="/dev/shm/c.log"
gdb.execute("set logging file "+ltxname) # lpfname
gdb.execute("set logging redirect on")
gdb.execute("set logging overwrite on")
gdb.execute("set logging on")
gdb.execute(instr)
gdb.execute("set logging off")
replyContents = open(ltxname, 'r').read() # read entire file
return replyContents
# next until breakpoint
def nextUntilBreakpoint():
isInBreakpoint = -1;
# as long as we don't find "Breakpoint" in report:
while isInBreakpoint == -1:
REP=logExecCapture("n")
isInBreakpoint = REP.find("Breakpoint")
print "LOOP:: ", isInBreakpoint, "\n", REP
Basically, pygdb-logg.gdb
loads the pygdb-logg.py
python script, sets up the alias nub
for nextUntilBreakpoint
, and initializes the session – everything else is handled by the python script. And here is a sample session – in respect to the test source in OP:
$ gdb -x pygdb-logg.gdb -se test.exe
...
Reading symbols from /path/to/test.exe...done.
Breakpoint 1 at 0x80483ec: file test.c, line 14.
Breakpoint 2 at 0x80483c7: file test.c, line 7.
Breakpoint 1, main () at test.c:14
14 count = 1;
(gdb) nub
LOOP:: -1
15 count += 2;
LOOP:: -1
16 count = 0;
LOOP:: -1
19 doFunction();
LOOP:: 1
Breakpoint 2, doFunction () at test.c:7
7 count += 2;
(gdb) nub
LOOP:: -1
9 count--;
LOOP:: -1
10 }
LOOP:: -1
main () at test.c:20
20 printf("%d\n", count);
1
LOOP:: -1
21 }
LOOP:: -1
19 doFunction();
LOOP:: 1
Breakpoint 2, doFunction () at test.c:7
7 count += 2;
(gdb)
… just as I wanted it 😛 Just don’t know how reliable it is (and whether it will be possible to use in avr-gdb
, which is what I need this for 🙂 EDIT: version of avr-gdb in Ubuntu 11.04 is currently 6.4, which doesn’t recognize the python command 🙁)
Well, hope this helps someone,
Cheers!
Here some references:
- GDB: error detected on stdin
- GDB has problems with getting commands piped to STDIN
- Re: [Gdb] How do i use GDB other input?
- gdb doesn’t accept input on stdin
- Using gdb in an IDE – comp.os.linux.development.apps | Google Groups
- rmathew: Terminal Sickness
- [TUTORIAL] Calling an external program in C (Linux) – GIDForums
- shell – how to use multiple arguments with a shebang (i.e. #!)? – Stack Overflow
- Redirecting/storing output of shell into GDB variable? – Stack Overflow
- Corey Goldberg: Python – Redirect or Turn Off STDOUT and STDERR
- The Cliffs of Inanity › 9. Scripting gdb
- gdb python scripting: where has
parse_and_eval
gone? – Stack Overflow - shell – Invoke gdb to automatically pass arguments to the program being debugged – Stack Overflow
- Storing Files/Directories In Memory With tmpfs | HowtoForge – Linux Howtos and Tutorials
- simple way to touch a file if it does not exist | Python | Python
- os.fork() different in cgi-script? – Python
- java – Writing tests that use GDB – how to capture output? – Stack Overflow
- Debugging with GDB: How to create GDB Commands in Python – Wiki
- GDB reference card