Remote gdb and shared libraries

Remote gdb and shared libraries

[Doug Abbott]

Hi,

This should be a fairly simple problem of inadequate documentation.  
I've searched the archives and found quite a bit of stuff on shared 
libraries, but nothing that exactly matches the problem I'm seeing.

I'm running gdb 6.3 under Red Hat 9 using gdbserver on an ARM9 target 
board to debug a simple Hello World program.  I've worked through the 
stuff about setting solib-absolute-prefix and both the host and target 
copies of the libraries seem to be identical.

The first suspicious thing that happens is when gdb connects to the 
target, it returns:

    0x40001470 in ?? ()

implying to me that it can't find the name of the function at that 
particular address.  After the program starts and halts at a breakpoint, 
the info share command returns:

    From               To                  Syms Read       Shared Object
    Library
    0x40035cd0     0x401162cc  No                 
    /usr/local/arm/arm-linux/lib/libc.so.6
    0x40001470     0x40011598  No                
    /usr/local/arm/arm-linux/lib/ld-linux.so.2

Hmmm.  The most suspicious aspect of this is the Syms Read column that 
says "No".  Why didn't it read the symbols?

With each next command, gdb says "Cannot access memory at address 0x0".  
When I try to step over a call to printf() (with next), it says  
"0x000082cc in ?? ()".  Executing next again returns "Cannot find bounds 
of current function".

At that point things are pretty much hung up.  Surely this is a simple 
problem.  I've used gdbserver before in x86 and Motorola Dragonball 
environments without a problem.  But in those situations, I didn't have 
to build the shared libraries (if there were any?).

And BTW, the hello program runs fine on the target without gdb.

Any help would be greatly appreciated.

Doug

Re: Remote gdb and shared libraries

[Daniel Jacobowitz]

On Mon, May 01, 2006 at 10:15:39PM -0600, Doug Abbott wrote:
> The first suspicious thing that happens is when gdb connects to the 
> target, it returns:
> 
>    0x40001470 in ?? ()
> 
> implying to me that it can't find the name of the function at that 
> particular address.

You're in the dynamic loader but GDB doesn't yet know where the dynamic
loader has been loaded.  This is typical.

>    From               To                  Syms Read       Shared Object
>    Library
>    0x40035cd0     0x401162cc  No                 
>    /usr/local/arm/arm-linux/lib/libc.so.6
>    0x40001470     0x40011598  No                
>    /usr/local/arm/arm-linux/lib/ld-linux.so.2
> 
> Hmmm.  The most suspicious aspect of this is the Syms Read column that 
> says "No".  Why didn't it read the symbols?

Because it couldn't find them.  You have to tell it where they are.
Have you used set solib-absolute-prefix?  If not, try that.  Do your
libraries precisely match the target's?

> With each next command, gdb says "Cannot access memory at address 0x0".  
> When I try to step over a call to printf() (with next), it says  
> "0x000082cc in ?? ()".  Executing next again returns "Cannot find bounds 
> of current function".

You have to use nexti / stepi when you've gotten into somewhere that
GDB doesn't know about.

The fact that it doesn't know about 0x82cc is suspicious.  You did load
your application into GDB, right?  Using "file" or on the command line?


-- 
Daniel Jacobowitz
CodeSourcery