Re: How to access userspace c++ strings in systemtap script

[William Cohen <wcohen at redhat dot com>]

On 10/3/18 10:12 AM, William Cohen wrote:
> Hi,
> 
> I would like to get a clearer picture about which sysetmtap scripts
> take a long time to process.  The goal is to provide a little stacked
> bar of the phases for each scripts that is run.  I would like to use
> the the systemtap stap.pass* probe points to give that information.
> Each of these stap.pass** probe points provide the session argument which should
> provides a pointer to the internal information including the script_file.
> 
> The question is how to access c++ string script_file in struct systemtap_session.
> The following snippet of code doesn't work:
> 
>   script_file = @cast(session, "struct systemtap_session")->script_file
>   script = user_string2(script_file, "<unavailable>")
> 
> Ends up with giving the following error:
> 
> semantic error: while processing probe process("/usr/bin/stap").statement(0x7814e) from: process("/usr/bin/stap").mark("pass4__end") from: stap.pass4.end from: stap.pass4.end
> 
> semantic error: 'class basic_string<char, std::char_traits<char>, std::allocator<char> >' (/usr/include/c++/8/bits/basic_string.h:77) is being accessed instead of a member such as '->_M_allocated_capacity': operator '->' at ./stap_time.stp:24:59
>         source:   script_file = @cast(session, "struct systemtap_session")->script_file
>                                                                           ^
> 
> Pass 2: analysis failed.  [man error::pass2]
> 
> 
> Attached is the WIP script.
> 
> -Will
> 

After some digging around I found that the following provided access to the string text:

  script_file = @cast(session, "struct systemtap_session")->script_file->_M_local_buf

-Will