CMake undefined reference to `mg_printf' - c++

Question

I've been trying to work this out for a while and I've decided to ask for help.
I'm trying to include mongoose in my project, so in my api_and_json.cpp file I have:
extern "C"{
#include "mongoose/mongoose.h"
}
My cmake file currently looks like this:
cmake_minimum_required(VERSION 2.8)
include(ProcessorCount)
ProcessorCount(N)
if(NOT N EQUAL 0)
set(CTEST_BUILD_FLAGS -j${N})
set(ctest_test_args ${ctest_test_args} PARALLEL_LEVEL ${N})
endif()
project(api_and_json)
set(CMAKE_BINARY_DIR ${CMAKE_SOURCE_DIR}/bin)
set(EXECUTABLE_OUTPUT_PATH ${CMAKE_BINARY_DIR})
set(LIBRARY_OUTPUT_PATH ${CMAKE_BINARY_DIR})
include_directories("${PROJECT_SOURCE_DIR}")
add_executable(api_and_json ${PROJECT_SOURCE_DIR}/api_and_json.cpp)
I've tried a few permutations, so I've stripped it back to how it was before.
CMake works fine, but when I try make I get:
...
[ 50%] Linking CXX executable ../bin/api_and_json
CMakeFiles/api_and_json.dir/api_and_json.cpp.o: In function `ev_handler(mg_connection*, int, void*)':
api_and_json.cpp:(.text+0xa1): undefined reference to `mg_printf'
api_and_json.cpp:(.text+0xb7): undefined reference to `mg_printf_http_chunk'
api_and_json.cpp:(.text+0xcd): undefined reference to `mg_send_http_chunk'
CMakeFiles/api_and_json.dir/api_and_json.cpp.o: In function `main':
api_and_json.cpp:(.text+0x10c): undefined reference to `mg_mgr_init'
api_and_json.cpp:(.text+0x184): undefined reference to `mg_bind_opt'
api_and_json.cpp:(.text+0x1dc): undefined reference to `mg_set_protocol_http_websocket'
api_and_json.cpp:(.text+0x32b): undefined reference to `mg_mgr_poll'
collect2: error: ld returned 1 exit status
CMakeFiles/api_and_json.dir/build.make:94: recipe for target '../bin/api_and_json' failed
make[2]: *** [../bin/api_and_json] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/api_and_json.dir/all' failed
make[1]: *** [CMakeFiles/api_and_json.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
I'm sure I'm missing something simple, any help will be appreciated.
I've looked elsewhere including:
What is an undefined reference/unresolved external symbol error and how do I fix it?
This reference did not help me, but the suggestion by Rama did.

Answer 1

You forgot to link against mongoose.
Add this line at the end of your CmakeList.txt:
target_link_libraries (api_and_json mongoose)