Re: Linking with external lib

[Josh Stone <jistone at redhat dot com>]

On 09/06/2016 12:39 PM, Mahmoud Hanafi wrote:
> 
> 
> On 09/06/2016 12:34 PM, David Smith wrote:
>> On 09/06/2016 02:08 PM, Mahmoud Hanafi wrote:
>>>
>>> On 09/06/2016 11:55 AM, David Smith wrote:
>>>> On 09/06/2016 01:37 PM, Mahmoud Hanafi wrote:
>>>>> In my stap script I have some embedded C function that needs to make
>>>>> some calls to external libraries. Is there I can link my script with the
>>>>> external libs? Or do i need to add all the code I need into my script.
>>>> Basically you can't. A systemtap script gets compiled into a kernel
>>>> module (assuming you are using the default 'linux' runtime). A kernel
>>>> module can't access *any* user library.
>>>>
>>>> Perhaps you should back up a bit and let us know what your external
>>>> library is trying to do.
>>>>
>>> The library is part of lustre. I need the functions that does address
>>> translation to string values.
>> Can you point me to the library/function on lustre.org that you are
>> interested in?
>>
>> You can't really copy userspace C code into systemtap. Systemtap tries
>> very hard to handle memory accesses safely (since otherwise you can
>> crash the kernel). We might be able to "translate" the userspace C code
>> into embedded C.
> the function is defined here
> http://osxr.org:8080/linux/source/drivers/staging/lustre/lustre/libcfs/nidstrings.c#0382
> 
> "libcfs_nid2str"
> 
> It would be nice to compile in something like this, because if there are 
> changes later in the function it wouldn't require stap script changes.

What are your probe locations?  Would it be possible for you to use SDT
probes, where you prepare these strings as arguments ahead of time?

That's what the python function probes do, for instance.  See
systemtap_function_entry() in this patch:
http://pkgs.fedoraproject.org/cgit/rpms/python3.git/tree/00055-systemtap.patch