cannot find library under /usr/local/lib - c++

Question

when I use CMakeLists.txt with:
find_library(cryptoppV libcryptopp.a)
target_link_libraries(${PROJenter code hereECT_NAME} ${cryptoppV})
then i can find a library under /usr/local/lib,and make the C++ programe right and got the right result.
but when i replace it with:
-- find_library(cryptoppV libcryptopp.a)
target_link_libraries(${PROJECT_NAME} cryptopp)
then i got the error message：
ld: library not found for -lcryptopp
why cmake do not link /usr/local/lib by default? did i do something wrong?
-- add by aijinsong Oct 7, 2018 6:37 AM
i'm in more confused. when the CMakeLists.txt was:
set(SOURCE_FILES main.cpp)
add_executable(${PROJECT_NAME} ${SOURCE_FILES})
find_package(Boost 1.58 REQUIRED thread)
target_link_libraries(${PROJECT_NAME} Boost::thread)
find_library(cryptoppV libcryptopp.a)
target_link_libraries(${PROJECT_NAME} ${cryptoppV})
the compiler can find cryptopp/sha.h. but when the CMakeLists.txt was:
set(SOURCE_FILES main.cpp)
add_executable(${PROJECT_NAME} ${SOURCE_FILES})
## find_package(Boost 1.58 REQUIRED thread)
## target_link_libraries(${PROJECT_NAME} Boost::thread)
find_library(cryptoppV libcryptopp.a)
target_link_libraries(${PROJECT_NAME} ${cryptoppV})
the error message was:
fatal error: 'cryptopp/sha.h' file not found
#include <cryptopp/sha.h>
when the CMakeLists.txt was:
set(SOURCE_FILES main.cpp)
add_executable(${PROJECT_NAME} ${SOURCE_FILES})
find_package(Boost 1.58 REQUIRED thread)
## target_link_libraries(${PROJECT_NAME} Boost::thread)
find_library(cryptoppV libcryptopp.a)
target_link_libraries(${PROJECT_NAME} ${cryptoppV})
the error message was still:
fatal error: 'cryptopp/sha.h' file not found
#include <cryptopp/sha.h>
why i use cryptopp that the cmake ask me to link with library Boost::thread? i'm in more confused.
-- add by aijinsong Oct 7, 2018 11:56 AM
And if i use g++ main.cpp -o main -lcryptopp, i can get the right result. This shows that the library cryptopp has been installed correcttly, and g++ can find the library. why when i do it by make, it can't find the library?
-- add for KamilCuk start
-- add by aijinsong at Oct 7, 2018 3:27 PM
when i make it by make VERBOSE=1, i got the following message:
cd /Users/aijinsong/Documents/projects/com.aijs.cxx/bolochain/src && /usr/local/Cellar/cmake/3.12.3/bin/cmake -E cmake_link_script CMakeFiles/bolochain.dir/link.txt --verbose=1
and the text in link.txt is:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -g -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/bolochain.dir/main.cpp.o -o bolochain /usr/local/lib/libboost_thread-mt.dylib -lcryptopp /usr/local/lib/libboost_chrono-mt.dylib /usr/local/lib/libboost_system-mt.dylib /usr/local/lib/libboost_date_time-mt.dylib /usr/local/lib/libboost_atomic-mt.dylib
this command cause the failure link but when i edit it like following, then c++ link command process very well:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -g -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/bolochain.dir/main.cpp.o -o bolochain /usr/local/lib/libboost_thread-mt.dylib -lcryptopp /usr/local/lib/libboost_chrono-mt.dylib /usr/local/lib/libboost_system-mt.dylib /usr/local/lib/libboost_date_time-mt.dylib /usr/local/lib/libboost_atomic-mt.dylib
just delete:
-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk
I'm still working on this problem.
-- add for KamilCuk end

Answer 1

Fist, Thanks #Kamil Cuk. The argument -VERBOSE=1 was so useful that I can get more detail messages that show me what happens when I use make.
The point is that when I use cmake under OSX system. It will generate a txt be named 'link.txt' which includes commands and part of it is as the following:
-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk
when I delete this part, the commands will execute right. but when I add this line, the commands execute wrong. so the point is that this line maybe limited the search path of c++. Thanks #tsyvarev . You are right, -isysroot limited the c++ linkers searches /usr/local/lib.
Second, I did't find out how to remove the line -isysroot ... generated by OSX cmake. So， I need to find another solution.
When I search more information about find_package/include_directories/target_link_libraries, I found out that find_package need a FindXXX.cmake file to help it to find out the header file and libraries of the target. So I googled a FindCyptoPP.cmake file. and in this file it find out tow vars, one hold the value of cryptopp's header directory path, and one hold the value of cryptopp's library path. Then I use include_directories/target_link_libraries as following, the problem
war solved.
find_package(CryptoPP)
include_directories(${CRYPTOPP_INCLUDE_DIRS})
target_link_libraries(${PROJECT_NAME} ${CRYPTOPP_LIBRARIES})
and then when I use make -VERBOSE = 1, I fond that the output was a little different when I use the CMakeLists.txt as following:
target_link_libraries(${PROJECT_NAME} cryptopp)
When I use three lines, the output contains a line /usr/local/lib/libcryptopp.dylib. When I use one line, the output contains a line -lcryptopp.
So, with the command line -isysroot, the command line -lcryptopp will search library under the directory defined by -isysroot, and under the directory, there is no library named cryptopp but under /usr/local/lib. But with command line /usr/local/lib/libcryptopp.dylib, it gaves the absolute path of the library, so the linkes just do the linking task and need not search. Thanks #Kamil Cuk again.
Thrid, I knew include_directories/target_link_libraries are two separate steps which one is used for include header file and one is used for link libraries.
Still, there were some problems not soled:
- how to remove -isysroot?
- how to create a FindXXX.cmake file?
- how to make /usr/local/lib as a default search directory and was it a practice way to do so?
I will continue working on them, and come back a few days or weeks later.