cmake find openssl libraries in non-standard paths - c++

Question

I am having trouble telling cmake to find openssl on my mac. Let me say, I am an amateur at using cmake, as my acquaintance with it, is very recent and occasional. That said, I can do simple things in cmake such as create small projects build them and run them. Some commands (find_library, find_package, ...) always perplex me.
I downloaded and installed openssl (./configure, make and make install) and it has put the files in /opt/openssl
I was googling around to see help for a simple inclusion openssl in my program.
One search result points to a command FindOpenSSL here (https://cmake.org/cmake/help/v3.6/module/FindOpenSSL.html). When i used FindOpenSSL, cmake emitted error saying FindOpenSSL is not a command.
I also tried find_library and find_package commands.
But if I check the variable OPENSSL_FOUND, it is still undefined.
It looks like I have to hardcode (or pass through -D options to cmake) the variables for openssl. What is the best practice here? The reference manual of cmake seems to be like piece of puzzle, as I don't understand the true difference between find_library and find_package, or when to use in preference of the other.
Am I the only one facing this, or more souls around here that are confused and struggling?
Thanks to anyone pointing me the right direction and helping me out of this confusion.
My current CMakeLists.txt looks like below:
ravindranaths-MacBook-Pro:crypt_handlers ravindranath$ cat CMakeLists.txt
cmake_minimum_required (VERSION 3.5.1)
# search for CPP_HOME. If found use this as the install_root dir.
# else, use /usr/local
message (STATUS "Searching for environment var CPP_HOME ...")
if (DEFINED ENV{CPP_HOME})
message (STATUS "Found CPP_HOME: " $ENV{CPP_HOME})
set (CPP_HOME $ENV{CPP_HOME})
else()
message (STATUS "Could not find. Treating /usr/local as CPP_HOME...")
set (CPP_HOME /usr/local)
endif()
set(Boost_USE_STATIC_LIBS ON)
###########################################################
set(CMAKE_INCLUDE_PATH /usr/local/ssl/include/openssl)
set(OPENSSL_USE_STATIC_LIBS ON)
############################################################
find_package(Boost 1.45.0 COMPONENTS system )
find_package (Threads)
set (CMAKE_CXX_STANDARD 11)
include_directories(../../include)
include_directories(../../include/ext)
include_directories(../../include/ext/spdlog)
include_directories(${CPP_HOME}/externals/plog/include)
add_executable(crypt app.cpp)
link_directories(../../build/src)
target_link_libraries (crypt ${CMAKE_THREAD_LIBS_INIT})
find_library(OpenSSL_LIB libcrypto.a libssl.a)
#find_package(OpenSSL)
if (OPENSSL_FOUND)
message (STATUS "OPENSSL found")
message (STATUS "OpenSSL INclude directories:" OPENSSL_INCLUDE_DIR)
else()
message (FATAL_ERROR "OpenSSL Not found.")
endif()
include_directories(${OPENSSL_INCLUDE_DIR})
target_link_libraries(crypt ${CMAKE_DL_LIBS})
TARGET_LINK_LIBRARIES(crypt ${OPENSSL_LIBRARIES} ${LIBYAML_LIBRARIES} pthread -ldl)
#target_link_libraries(crypt /usr/local/ssl/lib/libcrypto.a /usr/local/lib/libssl.a)
target_link_libraries(crypt /usr/local/lib/libcrypto.a /usr/local/lib/libssl.a)
if(Boost_FOUND)
include_directories(${include_directories} ${Boost_INCLUDE_DIRS})
target_link_libraries(crypt nettu ${Boost_LIBRARIES})
endif()
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/dist/bin)
UPDATE
now, I just use find_package(OpenSSL), instead of find_libraries(...). And, I am running cmake as below:
cmake -DCMAKE_BUILD_TYPE=Debug -DOPENSSL_ROOT_DIR=/opt/openssl/ -DOPENSSL_LIBRARIES=/opt/openssl/lib/ ..
cmake succeeds by writing the makefiles. However, I get a linker error when running make. The error points to missing libssl.a like below:
make[2]: *** No rule to make target `/usr/local/lib/libssl.a', needed by `examples/crypt_handlers/crypt'. Stop.
What I don't understand is that the make is still looking for libssl.a in "/usr/local/lib/.." instead of /opt/openssl/lib.
How to fix this?

Answer 1

You can change as follows
#find_library(OpenSSL_LIB libcrypto.a libssl.a)
find_package(OpenSSL)
And run
cmake -DOPENSSL_ROOT_DIR=/opt/openssl
Where OPENSSL_ROOT_DIR is hint for FindOpenSSL.cmake standard module where to find root for OpenSSL installation.