[CMake] Best way to handle application data path for local run vs. installation

[Alexander Neundorf]

On Thursday, December 03, 2015 09:27:29 Ruslan Baratov via CMake 
wrote:

> On 03-Dec-15 04:34, Alexander Neundorf wrote:
> > On Wednesday, December 02, 2015 12:27:42 Ruslan Baratov wrote:

> > If RPATH was _designed_ to be patchable, tools could just do it,
> > instead of having to implement workarounds for longer strings. E.g.
> > the linker would support a command line argument how much 
space it
> > should reserve for the RPATH entry, or something like that.
> 
> I think it's not possible. As far as I know there is no limitation for
> path size on Linux for example. Hence it's not possible to predict what
> size for path to library user want to use by setting RPATH.

More nitpicking about the meaning of "designed" ;-) :

CMake knows the length of the build RPATH and the length of the install 
RPATH. Let's say the build RPATH has a length of 100, and the install 
RPATH has a length of 150, it could, during linking, give the build RPATH 
to the linker, and also something like --rpath-size=150, and the linker 
would make the field for the RPATH 150 bytes big, so it could be 
patched easly later on.
This is what I would consider the minimum support if it was "designed" 
to be patched.
Instead cmake has to add 50 ":" at the end of the build RPATH. :-/

...
> > The idea above was only for solving the question "am I installed ?"
> > yes/no, not "where am I installed ?".
> 
> Then I don't understand how it's planned to be used. I thought that in
> build directory you changing "$$$$_I_M_NOT_INSTALLED_$$$$" to
> "/path/to/build/directory" so resources can be found there, and on
> install you change it to "/path/to/install/directory". So my guess is
> not correct?


Nothing is planned, just some ideas.
The idea was, during install, to set the first byte of that special string to 
'\0', and in the code you test whether the string is empty ( -> it has been 
patched, so it is installed) or not (still the original 
"$$$$_I_M_NOT_INSTALLED_$$$$").

With that information, the developer can then do whatever he wants. e.g. 
when not installed, search just in $CMAKE_BINARY_DIR (which could 
be configured into some other string), and if installed, search in some 
other way.

Alex

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