[CMake] How to use a generated linker map with a shared library?

[Craig Scott]

While not directly answering your question, it seems you may be trying to
deal with symbol visibility. Are you aware of CMake's symbol visibility
features? A good place to start would be the GenerateExportHeader module,
the documentation for which
<https://cmake.org/cmake/help/latest/module/GenerateExportHeader.html> does
a reasonable job of showing how to use the visibility features CMake
provides.

Dealing with your question more directly, you may want to look at the
TARGET form of add_custom_command() and in particular, the PRE_LINK
keyword. See the Build Events section near the bottom of the page for
the add_custom_command()
docs
<https://cmake.org/cmake/help/latest/command/add_custom_command.html#build-events>
.




On Sun, Jan 15, 2017 at 10:46 AM, Paul Smith <paul at mad-scientist.net> wrote:

> I'm really stuck: maybe someone can provide a hint.
>
> I'm trying to create a shared library from a bunch of object files, and
> use a linker script to mark almost all the symbols as local.  The tricky
> thing is I'm auto-generating the linker script via a shell script that
> examines the object files for my library (using readelf) to find global
> symbols.
>
> I basically need a process like this:
>
>    1. Start with source files and a linker map generator shell script
>    2. Compile all the code into object files
>    3. If any file was recompiled (or the generator script) is modified, re-
>       run the generator script to recreate the linker script
>    4. If any file was recompiled or the linker script was recreated,
>       rebuild the shared library, using the linker script.
>
> In "make-ese" it would be trivial: something like:
>
>   libmy.so: $(OBJECTS) link.script ; <link .so> ...
>
>   link.script: $(OBJECTS) genlinkscript ; <generate script> ...
>
> I simply can't figure out how to create something like this in CMake.
>
> I have a custom command to create the linker script:
>
>   add_custom_command(OUTPUT out.script
>       COMMAND genlinkscript ${CMAKE_CURRENT_BINARY_DIR}
>       DEPENDS <<SOMETHING GOES HERE>>
>       VERBATIM)
>
> And I have an add_library command to create the shared library:
>
>   add_library(mylib SHARED foo.cpp foo.h out.script)
>
>   set_property(TARGET mylib APPEND_STRING PROPERTY
>         LINK_FLAGS " -Wl,--version-script=out.script")
>
> But what can my custom_command depend on, so that it's run AFTER all the
> object files are created, but BEFORE the shared library is created?
>
> I tried to do this using an OBJECT library, but see my previous post
> that adding OBJECT libraries as dependencies of custom commands doesn't
> work.
>
> What can I do?  I'm happy with a solution that works only on Linux (I
> can use if()/endif() to turn it off on my other platforms, which don't
> use these linker scripts anyway).
> --
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-- 
Craig Scott
Melbourne, Australia
https://crascit.com
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