[CMake] Fwd: Save stripped debugging information

Michael Hertling mhertling at online.de
Sun Oct 2 10:07:17 EDT 2011


On 10/01/2011 10:07 AM, Yuri Timenkov wrote:
> that's the problem: you don't know neither file name nor it's location,
> especially in multi-configuration generators.

You *do* know a debug file's name and location, either because you must
generate it explicitly (objcopy approach) or via the concerned target's
$<TARGET_FILE_DIR:...> generator expression in custom targets/commands
(Visual approach). Otherwise, a debug file with unknown name and/or
location would be rather useless.

W.r.t. a debug file's installation, one must currently take generator-
specific measures: Either generate the file at all (objcopy approach)
or copy it to a configuration-independent location (Visual approach).
This might be considered as annoying, but it can be easily achieved
with CMake's present means, so I'm not fully aware of the actual
problem we're discussing ATM.

> It's also bad idea to mix build and install steps. [...]

Absolutely.

> [...] Install command doesn't
> understand generators expressions.

Regarding files that

- should be installed,
- are not target binaries,
- are put in configuration-specific locations,

it would be handy to have generator expressions available in
the INSTALL() command; perhaps; that's worth a feature request.

> If it were possible to emulate vs behavior for gcc things would be much
> easier in some cases. [...]

In which cases?

> [...] However extracting debug info from binaries is
> performed by packager (at least rpmbuild does this)

Sometimes - or quite often, as the case may be - packages are installed
without any packager's ado, and with a simple tar archive, e.g., you'll
be out of luck, too, so one shouldn't rely on a packager's capabilities
when it comes to the extraction of debug information or the like, IMO.

Regards,

Michael

> On Oct 1, 2011 8:09 AM, "Michael Hertling" <mhertling at online.de> wrote:
>> On 09/30/2011 08:39 AM, Rolf Eike Beer wrote:
>>>> On 09/29/2011 06:15 AM, Yuri Timenkov wrote:
>>>>> When I was investigating similar problem, I found alternative approach
>>>>> at
>>>>> http://code.google.com/p/dynamorio/source/browse/trunk/CMakeLists.txt.
>>>>>
>>>>> The thing is to change linker rules, to something like this:
>>>>> set(CMAKE_C_CREATE_SHARED_LIBRARY
>>>>> # standard rule
>>>>> "<CMAKE_C_COMPILER> <CMAKE_SHARED_LIBRARY_C_FLAGS>
>>>>> <LANGUAGE_COMPILE_FLAGS> <LINK_FLAGS>
>>>>> <CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS>
>>>>> <CMAKE_SHARED_LIBRARY_SONAME_C_FLAG><TARGET_SONAME> -o <TARGET>
>>>>> <OBJECTS>
>>>>> <LINK_LIBRARIES>"
>>>>> # now create a .debug copy
>>>>> "${CMAKE_OBJCOPY} --only-keep-debug <TARGET> <TARGET>.debug"
>>>>> # link original to point at .debug copy
>>>>> # directory components are removed, so "../lib/" is fine
>>>>> "${CMAKE_OBJCOPY} --add-gnu-debuglink=<TARGET>.debug <TARGET>"
>>>>> # Strip all information from target binary.
>>>>> "${CMAKE_STRIP} --strip-debug --discard-all --preserve-dates
>>>>> <TARGET>"
>>>>> )
>>>>>
>>>>> I don't exactly remember benefits from this approach but it kind of
>>>>> works.
>>>>
>>>> The benefits are that one needs to define these rule variables once as
>>>> they're inherited by the subdirectories. The downside is that the rule
>>>> variables are used by Makefile generators only, whereas the target-
>>>> associated custom commands are a more generic approach.
>>>>
>>>>> And I agree that functionality like installing debug symbols in
>>>>> install()
>>>>> rules out of box would be quite handy.
>>>>
>>>> INSTALL() is essentially about copying files and directories, so it
>>>> doesn't depend on the toolchain; in particular, you can use INSTALL()
>>>> for projects which are configured with PROJECT(... NONE), i.e. without
>>>> any toolchain. By contrast, extracting debug symbols does highly depend
>>>> on the toolchain, e.g. the objcopy(1) utility isn't mentioned in POSIX,
>>>> and even with the GNU tools, you have multiple possibilities to connect
>>>> the stripped binary with the unstripped one, note --add-gnu-debuglink
>>>> vs. build IDs. Windows and MacOSX will further enrich this entire zoo
>>>> of utilities and command line switches, not to mention toolchains for
>>>> specific platforms. So, opening and parameterizing INSTALL() - w.r.t.
>>>> its interface and its implementation - in order to provide reasonable
>>>> support for the extraction of debug symbols during installation is a
>>>> major undertaking, IMO, besides the conceptional issue of toolchain-
>>>> dependence.
>>>
>>> The idea was not to generate those during install, but to be able to let
>>> them being installed. For e.g. MSVC you don't have anything to do, the
>>> linker will already generate the PDB file already. So all you would have
>>> to do would be to copy the generated debug file to the proper place. This
>>> whish comes from the fact that for multi-configuration generators you
>>> don't know which configuration is active so you don't know where to
> search
>>> the PDB file. And INSTALL() and ADD_CUSTOM_TARGET()/_COMMAND() don't
>>> understand generator expressions.
>>
>> INSTALL() doesn't, but ADD_CUSTOM_TARGET/COMMAND() are right the two
>> commands which *do* understand generator expressions. As long as you
>> know the location of the debug files relative to the location of the
>> concerned target's binary, the $<TARGET_FILE_DIR:target> expressions
>> should do the trick. So, the worst case you might suffer is that you
>> must use a custom target/command to copy or move the debug files to
>> a suitable location in order to apply INSTALL(FILES ...) on them in
>> the end. IMO, that's bearable; does it not work for you? However,
>> adding generator expressions to the INSTALL() command might be
>> worth a thought.
>>
>>> So my idea would be to generate the debug file during or after the link
>>> step and save the position to this file somewhere internally, so
>>> INSTALL(... DEBUG_SYMBOLS) would know which to take. Or to do just
> nothing
>>> if we do not support external debug symbols on this platform.
>>
>> What do you mean with "save the position to this file somewhere
>> internally"? Saving by a user's action, i.e. declaring a file as a
>> debug file? If you know the file's name and location, you can simply
>> apply INSTALL(FILES ...) on it, perhaps with an intermediate step as
>> suggested above. Saving by action of CMake, i.e. without specifying
>> the debug file's name and/or location? For this to work, you would
>> need to teach CMake to recognize a debug file by itself, and that's
>> probably a comparably complicated undertaking as teaching CMake how
>> to generate debug files. E.g., with a typical *nix toolchain, CMake
>> has no chance to know in advance *if* a debug file is generated,
>> *where* it is written to and *what* is its name.
>>
>> Regards,
>>
>> Michael


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