[CMake] CMAKE uses wrong symlink to so

[Michael Hertling]

On 12/30/2010 12:28 AM, j s wrote:
> I specified the full name to an so in CMAKE 2.8.1.  Unfortunately it links
> against the versioned so name,
> libpython2.6.so.1.0
> 
> instead of the exact name I specified.
> /usr/lib/libpython2.6.so
> 
> SET (PYTHON_ARCHIVE /usr/lib/libpython2.6.so)
> TARGET_LINK_LIBRARIES (myapp  parser engine ${PYTHON_ARCHIVE}
> ${OPENSSL_ARCHIVE})
> 
> Is there any way to tell cmake to do the right thing in Linux?  For some
> strange reason using -l on the link line, and doesn't even use the
> corresponding -L to the path I specify.
>   -lpython2.6

See [1] for an explanation of the latter behavior. Nevertheless, I
can confirm that - on my system - /usr/lib/libpython2.5.so turns to
-lpython2.5 whereas /usr/lib/libpython2.5.so.1{,.0} do not. Here, I
assume that CMake just uses the well-known extensions .so and .a when
deciding whether a file specification denotes a library in an implicit
system location, so /usr/lib/libpython2.5.so is identified as such one,
but /usr/lib/libpython2.5.so.1{,.0} are not. This results in full paths
in the linker's command line for the latters and a -l option without an
appropriate -L one for the former. Use the trick with imported targets
to force a full path for the /usr/lib/libpythonX.X.so library, too.

However, the fact that your target links against libpython2.6.so.1.0
instead of libpython2.6.so is no CMake issue, but relates to that
library's soname; try

readelf -d /usr/lib/libpython2.6.so | grep SONAME

and see ld's manpage, in particular:

"-soname=name
   When creating an ELF shared object, set the internal DT_SONAME field
   to the specified name.  When an executable is linked with a shared
   object which has a DT_SONAME field, then when the executable is run
   the dynamic linker will attempt to load the shared object specified
   by the DT_SONAME field rather than the using the file name given to
   the linker."

'hope that helps.

Regards,

Michael

> [1] http://www.cmake.org/Wiki/CMake_2.6_Notes#Linking_to_System_Libraries