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Debugging multiple programs at once

Consider this Makefile:

all: runit

runit: crasher
	./crasher

crasher: crasher.c
	gcc -g -o crasher crasher.c

And, here is the program it is building:

int *x = 0;

int main ()
{
  *x = 52;
}

Now, if you run “make“, eventually you will see a crash.  But how to debug the crash?

Well, obviously, this is a trivial example so you’d just debug the program.  But what if you had a complex script involving extensive and obscure initialization?  Say, in your test suite?  The traditional answer is logging plus cut and paste into gdb; or perhaps hacking an invocation of gdb --args into your script.  Nowadays you can do better, though.

Let’s start by debugging make:

$ gdb -quiet make
Reading symbols from /usr/bin/make...(no debugging symbols found)...done.
Missing separate debuginfos, use: debuginfo-install make-3.82-8.fc16.x86_64

Now set things up for multi-inferior debugging:

(gdb) set detach-on-fork off
(gdb) set target-async on
(gdb) set non-stop on
(gdb) set pagination off

(Yes, it is silly how many settings you have to tweak; and yes, we’re going to fix this.)

Now do it:

(gdb) run
Starting program: /usr/bin/make
gcc -g -o crasher crasher.c
[New inferior 9694]
[New process 9694]
process 9694 is executing new program: /usr/bin/gcc
[New inferior 9695]
[New process 9695]
process 9695 is executing new program: /usr/libexec/gcc/x86_64-redhat-linux/4.6.2/cc1
Missing separate debuginfos, use: debuginfo-install gcc-4.6.2-1.fc16.x86_64
[Inferior 3 (process 9695) exited normally]
[Inferior 9695 exited]
Missing separate debuginfos, use: debuginfo-install cpp-4.6.2-1.fc16.x86_64
(gdb) [New inferior 9696]
[New process 9696]
process 9696 is executing new program: /usr/bin/as
[Inferior 4 (process 9696) exited normally]
[Inferior 9696 exited]
[New inferior 9697]
[New process 9697]
process 9697 is executing new program: /usr/libexec/gcc/x86_64-redhat-linux/4.6.2/collect2
Missing separate debuginfos, use: debuginfo-install binutils-2.21.53.0.1-6.fc16.x86_64
[New inferior 9698]
[New process 9698]
process 9698 is executing new program: /usr/bin/ld.bfd
Missing separate debuginfos, use: debuginfo-install gcc-4.6.2-1.fc16.x86_64
[Inferior 6 (process 9698) exited normally]
[Inferior 9698 exited]
[Inferior 5 (process 9697) exited normally]
[Inferior 9697 exited]
[Inferior 2 (process 9694) exited normally]
[Inferior 9694 exited]
./crasher
[New inferior 9699]
[New process 9699]
process 9699 is executing new program: /tmp/crasher
Missing separate debuginfos, use: debuginfo-install binutils-2.21.53.0.1-6.fc16.x86_64

Program received signal SIGSEGV, Segmentation fault.
0x000000000040047f in main () at crasher.c:5
5      *x = 52;

Cool stuff.  Now you can inspect the crashed program:

(gdb) info inferior
Num  Description       Executable
  7    process 9699      /tmp/crasher
* 1    process 9691      /usr/bin/make
(gdb) inferior 7
[Switching to inferior 7 [process 9699] (/tmp/crasher)]
[Switching to thread 7 (process 9699)]
#0  0x000000000040047f in main () at crasher.c:5
5      *x = 52;

There is still a lot of work to do here — it is still a bit too slow, setting breakpoints is still a pain, etc. These are all things we’re going to be cleaning up in the coming year.

One Comment

  1. That is cool. I didn’t know gdb could attach to multiple programs.

    I’ve been using the following hack for a while for this exact scenario:

    http://people.gnome.org/~walters/gdbifmatch.c

    Thursday, February 16, 2012 at 3:44 pm | Permalink

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