cmake external projects command seems to ignore INSTALL_DIR - cmake

Question

First off, I'm relatively new to cmake. I'm trying to use cmake to build a project with a single external dependency. I specify the INSTALL_DIR for the external project to be CMAKE_INSTALL_PREFIX, so it installs to the same place as the parent project. But when I run make, it ignores it and tries to install to /usr/local/lib.
Here's my CMakeList.txt:
cmake_minimum_required( VERSION 2.8 )
include( ExternalProject )
project( capture )
add_library( capture SHARED capture.cc )
set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11" )
ExternalProject_Add( proj_exceptions
GIT_REPOSITORY /home/user/workspace/exceptions
INSTALL_DIR ${CMAKE_INSTALL_PREFIX}
)
add_library( exceptions SHARED IMPORTED )
set_property( TARGET exceptions
PROPERTY IMPORTED_LOCATION ${CMAKE_INSTALL_PREFIX}/lib/libexceptions.so
)
add_dependencies( exceptions proj_exceptions )
include_directories( ${CMAKE_INSTALL_PREFIX}/include )
target_link_libraries( capture exceptions )
install( TARGETS capture DESTINATION lib )
install( FILES capture.h DESTINATION include )
CMakeLists.txt for the external project looks like this:
cmake_minimum_required( VERSION 2.8 )
project( exceptions )
add_library( exceptions SHARED exceptions.cc )
install( TARGETS exceptions DESTINATION lib )
install( FILES exceptions.hh DESTINATION include )
It clones and builds the external project just fine, but it chokes on the install step:
Install the project...
-- Install configuration: ""
-- Installing: /usr/local/lib/libexceptions.so
CMake Error at cmake_install.cmake:42 (file):
file INSTALL cannot copy file
"/home/user/workspace/capture/build/proj_exceptions-prefix/src/proj_exceptions-build/libexceptions.so"
to "/usr/local/lib/libexceptions.so".
Makefile:66: recipe for target 'install' failed
As you can see, the install configuration is empty. Looking through the generated config for the external project, I found this in cmake_install.cmake:
if(NOT DEFINED CMAKE_INSTALL_PREFIX)
set(CMAKE_INSTALL_PREFIX "/usr/local")
endif()
So, it seems that passing INSTALL_DIR to ExternalProject_Add doesn't set the install prefix. The install step succeeds, if I instead use:
ExternalProject_Add( proj_exceptions
GIT_REPOSITORY /home/djones/workspace/exceptions
CMAKE_ARGS "-DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}"
)
So what's the purpose of INSTALL_DIR then?

Answer 1

You're right for the purpose of INSTALL_DIR, but you may have missed some steps.
According the cmake 2.8 doc about external project:
Install Step
The INSTALL_DIR is underneath the calling project’s
binary directory. Use INSTALL_DIR to specify a different location.
Note that in addition to setting INSTALL_DIR, you also have to pass
-DCMAKE_INSTALL_PREFIX or --prefix to the CMake or configure command. It is not used automatically in the configure step since not all
projects follow this convention.
# [INSTALL_DIR dir]
You can refer to the install directory in your configure command, for
example:
CONFIGURE_COMMAND SOURCE_DIR/configure --prefix=INSTALL_DIR
# [INSTALL_COMMAND cmd...]
CMake-based projects use ‘cmake--build’ to build the install target.
Other projects use ‘make install’. Use INSTALL_COMMAND to customize
the install step. Use INSTALL_COMMAND “” to omit the install step. The
install command executes with the working directory set to
.
So try to update your cmake command, or use the custom INSTALL_COMMAND feature.