Compiling and injecting code in GDB: C++ in addition to C?

Compiling and injecting code in GDB: C++ in addition to C?

[Robin Whittle]

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A more detailed description of my experiences is at: 
https://stackoverflow.com/questions/75387541/compiling-and-injecting-code-in-gdb-c-rather-than-c 
.  Briefly:

    I am running gdb 10.1.90.20210103-git and gcc/g++ 10.2.1 20210110 on
    x86-64 Debian 11.

    I can compile C code and have it run in the context of the paused
    inferior, but as soon as I introduce one or more lines of C++ code,
    no compilation or at least no run of the code occurs.  I get no
    error messages.

    https://www.sourceware.org/gdb/news/ mentions that GDB 8.3.1 was
    released on 2019-09-20, with: "Experimental support for compilation
    and injection of C++ source code into the inferior (requires GCC 7.1
    or higher, built with libcp1.so)."

    I have /usr/lib/x86_64-linux-gnu/libcc1.so.o but no libcp1.so
    there.  Little seems to have changed in libcp1.so in recent years:
    https://github.com/gcc-mirror/gcc/commits/master/libcc1/libcp1.cc .

Is it possible to inject C++ code, with any version of gdb and gcc/g++?

Is there any other documentation I should be looking at?  The main gdb 
documentation of code compilation and injection 
https://sourceware.org/gdb/onlinedocs/gdb/Compiling-and-Injecting-Code.html#set-debug-compile 
only gives C examples.

I want this capability so I can easily dump the contents of large, 
complex, vectors of class objects to a text file, likely multiple times 
the middle of a debug session.  Also I would like to be able to modify 
such data programmatically.

Thanks very much for gdb and gcc!


   Best regards

     Robin Whittle

Re: Compiling and injecting code in GDB: C++ in addition to C?

[Tom Tromey]

>>>>> "Robin" == Robin Whittle <rw@firstpr.com.au> writes:

Robin> Is it possible to inject C++ code, with any version of gdb and gcc/g++?

My experience is that the C++ plugin isn't very functional.  It seems to
have some bugs and crash pretty frequently as well.  Also, it seems like
nobody is actively working on it.

Robin> I want this capability so I can easily dump the contents of large,
Robin> complex, vectors of class objects to a text file, likely multiple
Robin> times the middle of a debug session.  Also I would like to be able to
Robin> modify such data programmatically.

I wonder if you could do some trick like have a library, then use
'compile' in C mode to dlopen it.  Not great of course.

Tom

Re: Compiling and injecting code in GDB: C++ in addition to C?

[Robin Whittle]

Hi Tom,

Thanks very much for your reply.  Here is what I learned, as I wrote as 
an answer to my question at: 
https://stackoverflow.com/questions/75387541/compiling-and-injecting-code-in-gdb-c-rather-than-c/75419090#75419090.

   Best regards

     Robin


Summary: GDB's C++ compilation and injection system does not work for 
me, and it is reported to be buggy or non-functional.  I only tested 
this "compile" command with single lines of C code, but it worked fine 
for these.  I was able to run my function, which writes a text dump file 
containing the valeus of a vector of class objects' data members, while 
the inferior was paused after a breakpoint, by including it in a source 
file and calling it with GDB's "print" or "call" commands, giving it the 
required input argument of the name of the vector to dump.


I haven't tried using GDB on its own, but at present I have no reason to 
believe that Codelite is interfering with my ability to communicate with 
GDB via Codelite's "Debugger > Output" console.

I wrote to the GDB Developer list and received what I assume is a well 
informed reply.  I was told that in my respondent's experience "the C++ 
plugin isn't very functional. It seems to have some bugs and crash 
pretty frequently as well. Also, it seems like nobody is actively 
working on it."  This is consistent with my experience to date.

Perhaps later versions of GDB would work better.  However, there has 
been minimal change to the library 
https://github.com/gcc-mirror/gcc/commits/master/libcc1/libcp1.cc since 
2017, and I could find no indication of new work on C++ compilation and 
injection in https://www.sourceware.org/gdb/news/ since the announcement 
of its introduction:

GDB 8.3.1 was released on 2019-09-20, with: "Experimental support for 
compilation and injection of C++ source code into the inferior (requires 
GCC 7.1  or higher, built with libcp1.so)."

It is possible that my problem is related to not having this library.  I 
do have the C equivalent: /usr/lib/x86_64-linux-gnu/libcc1.so.o.  I 
could find no such library in a Debian package.  It is possible I could 
get this going by compiling the latest GCC/G++ and GDB, but that is 
pretty daunting, involving installing later standard libraries than my 
current Debian installation has, while making those and the newer G++ 
and GDB available for Codelite sessions, but not interfering with other 
software on machine which needs the older compilation system.

The respondent suggested that instead of compiling and injecting fresh 
code in the middle of a debugging session, I could write it as a 
library, and run it by using the compile command, on a purely C piece of 
code which would call it.  However, for my purposes, the function needs 
to be passed one or more C++ constructs, and any code which called a C++ 
function would be C++ code and so, presumably, not be compiled by GDB.

For my immediate purpose of doing text dumps of large vectors of class 
objects' data members, I was able to succeed by writing such a function 
in an existing source file, which also contains the functions I am 
debugging.  Then I could call it with GDB's "print" or "call" commands.  
I imagine this would work if the debugging function was in any other 
linked source file, or in a linked library. (Perhaps there is a way of 
writing such a library, compiling and linking it while the inferior is 
halted.)

I was able to run this, from a debugging session in which the inferior 
was halted somewhere, with either the "print" or "call" command followed 
by the name of the function, with the name of the vector in brackets.  
There must be no semicolon.  I found the "print" command better, since 
GDB would report the return value, which was "void".  My function used 
C++ constructs such as "<<" to write to the text file.

A call within my dump function of the C function "system("pwd")" 
resulted in the current directory being output on the inferior's 
terminal window.  Likewise: "print system("pwd")".  "print system("mc")" 
dutifully ran Midnight Commander in the inferior's terminal window.

I altered the function to return a string, which GDB reported in the GDB 
"Debugger > Output" console.  I added a line:  std::cout << "xyz"; - but 
nothing appeared on either the inferior's terminal window or in the GDB 
console.

My attempts to use the compile command to call my dump function in 
exactly the same way produced no results, with or without a trailing 
semicolon.

I found that"print" and "call" work with C and C++ code while "compile" 
only works with C code.

I tried print vvv[6].mmm for data members which were integers, floats, 
and strings and GDB returned their values correctly.  Adding a ++ at the 
end caused it to return the original integer value, but the value itself 
was incremented.

When I tried "print" with a compound line of code, with a semicolon 
between the two logical lines, with and without one at the end, there 
was no response.

The GDB Developers list is not the place for support requests, but I did 
not know a better way to find out about the status of this facility.



On 2023-02-11 4:01 am, Tom Tromey wrote:
> Robin> Is it possible to inject C++ code, with any version of gdb and gcc/g++?
>
> My experience is that the C++ plugin isn't very functional.  It seems to
> have some bugs and crash pretty frequently as well.  Also, it seems like
> nobody is actively working on it.
>
> Robin> I want this capability so I can easily dump the contents of large,
> Robin> complex, vectors of class objects to a text file, likely multiple
> Robin> times the middle of a debug session.  Also I would like to be able to
> Robin> modify such data programmatically.
>
> I wonder if you could do some trick like have a library, then use
> 'compile' in C mode to dlopen it.  Not great of course.
>
> Tom