Cmake: How to check if a library is linked to an executable - makefile

Question

I am linking a 3rd party library with my executable. There are buch of other libraries which I have successfully linked against. I'm not able understand what I am doing wrong.
This is the piece which is causing the issue
# Build libssh 0.7.3
ExternalProject_Add(libssh2
URL ${PROJECT_SOURCE_DIR}/packages/libssh.tar.gz
PREFIX ${CMAKE_BINARY_DIR}/external/libssh2
CONFIGURE_COMMAND ""
UPDATE_COMMAND ""
BUILD_COMMAND mkdir -p <SOURCE_DIR>/build && cd <SOURCE_DIR>/build &&
cmake -DWITH_STATIC_LIB=1 -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR> -DCMAKE_BUILD_TYPE=Debug .. && make
INSTALL_COMMAND cd <SOURCE_DIR>/build && make install PREFIX=<INSTALL_DIR>
)
ExternalProject_Get_Property(libssh2 install_dir)
add_library(libssh STATIC IMPORTED)
set_property(TARGET libssh PROPERTY IMPORTED_LOCATION
${install_dir}/lib/libssh.a)
add_dependencies(libssh libssh2)
include_directories(${install_dir}/include)
This is another 3rd party which is linking correctly
ExternalProject_Add(libevent2
URL ${PROJECT_SOURCE_DIR}/packages/libevent-2.0.21-stable.tar.gz
PREFIX ${CMAKE_BINARY_DIR}/external/libevent2
CONFIGURE_COMMAND <SOURCE_DIR>/configure --prefix=<INSTALL_DIR>
UPDATE_COMMAND ""
)
ExternalProject_Get_Property(libevent2 install_dir)
add_library(libevent STATIC IMPORTED)
set_property(TARGET libevent PROPERTY IMPORTED_LOCATION
${install_dir}/lib/libevent.a)
add_dependencies(libevent libevent2)
include_directories(${install_dir}/include)
Error which I am seeing
Linking C executable ../../bin/nixia
CMakeFiles/nixia.dir/main_config.c.o: In function `main_config_dump':
/home/nikhil/nixia/src/main/main_config.c:88: undefined reference to `ssh_new'
/home/nikhil/nixia/src/main/main_config.c:91: undefined reference to `ssh_options_set'
collect2: ld returned 1 exit status
The library is created correctly
[nikhil#nikhil lib]$ ls
cmake libssh.a libssh.so libssh.so.4 libssh.so.4.4.1 libssh_threads.a libssh_threads.so libssh_threads.so.4 libssh_threads.so.4.4.1 pkgconfig
[nikhil#nikhil lib]$ pwd
/home/nikhil/nixia/build/external/libssh2/lib
The library contains the function definition
[nikhil#nikhil lib]$ ar x libssh.a | nm * | grep ssh_new
nm: Warning: 'cmake' is not an ordinary file
U ssh_new
0000000000000000 T ssh_new
00000000000351b0 T ssh_new
00000000000351b0 T ssh_new
00000000000351b0 T ssh_new

Answer 1

I suggest you to compile using CMake verbose mode in order to check the full command used to compile it.
In order to do so you have to compile using the following command:
make VERBOSE=1
In this manner you can check which is the library that was linked identifying the problem.
Possible reasons of the undefined reference error:
The library has not been found (check if -L and -l flags in the g++ command are correct and, in case of errors, modify the CMake to obtain the desired result);
On the same system there are more versions of the same library (in your case libssh): the compiler could use the header files of a version and the library of another which could have a different signature producing linking errors.