When does ExternalProject_Add build? - windows

Question

I want to build openssl and link my project against it. In my project, I have a library called net which is the part that uses openssl. So in my net\CMakeList, I added
include_directories(
../
+ ../../../ext/openssl/inc32/
)
add_library(net STATIC ${sources})
+ ADD_DEPENDENCIES(net openssl)
In my ext folder that is used for organizing all external library, I have a fresh unzipped openssl source code in a folder named openssl. Then I edited ext\CmakeList
message(STATUS "Configuring OpenSSL")
set(openssl_dir openssl)
if (CMAKE_CL_64)
include(ExternalProject)
set(OPENSSL_CONFIGURE perl\ Configure\ VC-WIN64A)
set(OPENSSL_MAKE ms\\do_win64a\ &&\ nmake\ -f\ ms\\ntdll.mak)
ExternalProject_Add(openssl
PREFIX openssl
#-- Download Step ----------
SOURCE_DIR ${CMAKE_SOURCE_DIR}/ext/openssl
#--Configure step ----------
CONFIGURE_COMMAND ${OPENSSL_CONFIGURE}
#--Build Step ----------
BUILD_COMMAND ${OPENSSL_MAKE}
BUILD_IN_SOURCE 1
#--install Step ----------
INSTALL_COMMAND ""}
)
endif()
When I built, the compiler complained the it can't find include files, and the openssl source code was not built at all, since there is no out32dll and inc32.
My question is: When does ExternalProject_Add actually build the project? If I make my net library depending on openssl, does it mean when I build net it would need to check and build openssl first?

Answer 1

ExternalProject use add_custom_target internally to create project and according to its document
By default nothing depends on the custom target. Use ADD_DEPENDENCIES
to add dependencies to or from other targets.
So if no project depend on it, it will not build by default.
When this ExternalProject_Add actually builds the project?
At build time(not at cmake time).
If I make my net library depends on openssl, does it mean when I build net it would need to check and build openssl first?
No. Since you make the dependency, cmake will handle it and openssl will be build before your library.
PS: There's still a lot of work to do. Because cmake doesn't know where the openssl result is, your project may run into link error. Here is a good example from the answer of "What is the best way to build boost with cmake".It use add_library with IMPORTED GLOBAL and set_target_properties with IMPORTED_LOCATION_* to solve the problem

Answer 2

Install missed
First of all I think you missed an install step. See documentation:
nmake -f "ms\ntdll.mak" install
So simplified version of ExternalProject_Add is:
ExternalProject_Add(
OpenSSL
URL https://github.com/openssl/openssl/archive/OpenSSL_1_0_1h.tar.gz
CONFIGURE_COMMAND perl Configure VC-WIN64A "--prefix=${CMAKE_INSTALL_PREFIX}"
BUILD_COMMAND "ms\\do_win64a.bat"
COMMAND nmake -f "ms\\ntdll.mak"
BUILD_IN_SOURCE 1
INSTALL_COMMAND nmake -f "ms\\ntdll.mak" install
)
Find OpenSSL
Second, if you want to find openssl libraries and headers, you need to use find_package(OpenSSL) command:
find_package(OpenSSL REQUIRED)
message("Libs: ${OPENSSL_LIBRARIES}")
message("Includes: ${OPENSSL_INCLUDE_DIR}")
Builds
Now, if you put all this commands together in one project configure step will fail :) Because it's not how ExternalProject designed. Superbuild that made by few or one ExternalProject_Add commands must be configured first, then build step will install the libraries from external archives. This will conflict with your project, because openssl libraries can't be found - they are not installed by superbuild yet.
Fix
Split cmake code. Run superbuild first (i.e. download and install openssl):
> cmake -Hsuperbuild -B_builds\superbuild "-GVisual Studio 12 2013 Win64" -DCMAKE_INSTALL_PREFIX=ext\install
# note, no ext\install directory yet
> cmake --build _builds\superbuild
# install done, libraries can be found in ext\install
Then build your project, note that the libraries found on configure step:
> cmake -Huse -B_builds\use "-GVisual Studio 12 2013 Win64" -DCMAKE_INSTALL_PREFIX=ext\install
-- Found OpenSSL: .../ext/install/lib/ssleay32.lib;.../ext/install/lib/libeay32.lib (found version "1.0.1h")
Libs: .../ext/install/lib/ssleay32.lib;.../ext/install/lib/libeay32.lib
Includes: .../ext/install/include
> cmake --build _builds\use
# OK
Experimental
I have an experimental project that wrap all of this noise in one command (Windows tested on Visual Studio 2013):
hunter_add_package(OpenSSL)
find_package(OpenSSL REQUIRED)
https://github.com/ruslo/hunter