[CMake] What is the purpose of INSTALL_DIR in ExternalProject_Add command?

[Cedric Doucet]

Hello Theodore,

thank you for your help.

Well, indeed it greatly depends on the system and the user.
Neverteless, you can alaways provide your own value of CONFIGURATION_COMMAND to install your libraries in the same directory or in different ones.
It does not depend on INSTALL_DIR at all.
You don't need INSTALL_DIR to do that.
It is more convenient to do it at the command line 'cmake -D MY_INSTALL_PREFIX' and make CONFIGURE_COMMAND depend on the value MY_INSTALL_PREFIX (or CMAKE_INSTALL_PREFIX if you want).

But do you understand what INSTALL_DIR do?

For example, what are the differences between this call

ExternalProject_Add(eigen
                    PREFIX third_party
                    INSTALL_DIR third_party/eigen/install
                    DOWNLOAD_COMMAND wget http://bitbucket.org/eigen/eigen/get/3.2.4.tar.gz && tar xvzf 3.2.4.tar.gz -C ../src --strip-components=1
                   )

and this one

ExternalProject_Add(eigen
                    PREFIX third_party
                    DOWNLOAD_COMMAND wget http://bitbucket.org/eigen/eigen/get/3.2.4.tar.gz && tar xvzf 3.2.4.tar.gz -C ../src --strip-components=1
                   )
?

What is the default value of INSTALL_DIR?
What does setting INSTALL_DIR to a value do?

I am sorry if I miss something, but I really try to understand your answers.
I realize that I don't understand what mechanism is behind setting INSTALL_DIR to a value.

In the documentation (http://www.cmake.org/cmake/help/v3.2/module/ExternalProject.html?highlight=external#command:externalproject_add), it is written "Installation prefix".
But what does it mean? 
What is actually done with the value of INSTALL_DIR?

Cédric


----- Mail original -----
> De: "Theodore Papadopoulo" <Theodore.Papadopoulo at inria.fr>
> À: cmake at cmake.org
> Envoyé: Jeudi 9 Avril 2015 15:08:13
> Objet: Re: [CMake] What is the purpose of INSTALL_DIR in ExternalProject_Add command?
> 
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 04/09/2015 01:26 PM, Ryan Pavlik wrote:
> > It's for if your use case requires the project to be installed in
> > a specific location that's not the automatically generated one.
> > For instance, if you're building multiple external projects you
> > might have them all install to the same subdirectory of the binary
> > directory for convenience.
> > 
> > Ryan
> 
> I personally believe that you should never use the files constructed
> in the build directory. This can work for binaries or static libraries
> but is a point of failure with dynamic libraries (often the relative
> paths of those are different in the final install).
> 
> One case where installing is absolutely necessary (at least in my
> somewhat limited experience); windows, which requires that all the
> dynamic libraries used by a program to be in the smae install directory.
> 
> 	Hope it help a little.
> 
> 	Theo.
> 
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