Re: cross-compiling & debugging embedded-linux apps

[Jim Lewis <jlewis@mvista.com>]

Brendan J Simon wrote:

> I have a powerpc embedded system (MPC860, 4MB Flash, 16MB RAM, ethernet,
> rs232).  I have compiled the kernel and can boot it using a root
> filesystem via initrd or nfs.  The root filesystem is a minimal one that
> was on the linuxppc-embedded ftp site.  It basically has /bin/sh,
> /bin/ls and a few libraries in /lib.
>
> I NEED to be able to compile apps from the sources.  I have managed to
> cross-compile ncurses and bash.  I can't get bash to run at all (even a
> statically compiled version).  I get segmentaion faults.  I'm currently
> using SASH which I have cross-compiled as a static binary.  I compiled a
> test app (bjs1.c) which outputs a string every second.  It is compiled
> as a static binary (bjs1-static) and a shared binary (bjs1-shared).  The
> static binary works but the shared one does not.  I assume it is some
> library problem but I can't figure out what.

I think you are right. What libraries are you linking against? I think the the
minroot FS from the embedded FTP site uses libc 1.99. I have had nothing but
trouble when trying to mix libraries.

If you did build against different libraries, one way you could do is to
create a new directory for your shared application and the shared objects you
linked against. Make sure to include the ld.so.1 executable. Then, at the
target SASH prompt, issue the following:
> /test/ld.so.1 --library-path /test /test/bjs1-shared

> The output of the sash
> session is below.
>
> Stand-alone shell (version 1.0)
> > ./bjs1-static
> BJS1: Brendan was here
> BJS1: Brendan was here
> BJS1: Brendan was here
> pid 7: killed (signal 2)
> >
> > ./bjs1-shared
> pid 8: killed (signal 11)
> >
>
> I have all the libraries on the root filesystem.  The rpc.nfsd daemon
> seems to read the entire file but sash says the process is killed with
> signal 11 (segmentation fault).  I have no idea how to debug this.  I
> don't think there is a simulator for the mpc860 as part of gdb.  Is
> there a way of debugging this on the target with powerpc-gdb and an
> ethernet or serial connection ?

Yes, there is. Dan posted some gdbserver sources a while back. That allows you
to do cross debug. But you are likely to have the same problems building
gdbserver that you are having with your simple test application.

>
>
> How does the kernel know where to look for libraries ?  I assume there
> are some default locations like /lib.  I haven't got an ld.so.conf setup
> nor do I have ldconfig.

The 'default' location seems to be built in to the executable. Usually,
/lib/ld.so.1 is invoked and it tries to open ld.so.cache to determine where
libraries are. If it cannot find /etc/ld.so.cache, then it just uses default
locations that were "compiled in". The ld.so.cache is created by ldconfig,
which reads ld.so.conf to determine how to build ld.so.cache.

>
>
> It can't be that hard to get a simple 10 line program to execute as a
> shared binary.  It must be something really simple that I am missing.
>
> Thanks for any help,
> Brendan Simon.

--
Jim Lewis
Sr. Field Applications Engineer
MontaVista Software, Inc.
(817)261-9088 http://www.mvista.com


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