Why does cmake not add a library link command in the makefile? - c++

Question

I Ubuntu, I am learning about cmake and make, and just trying a simple example. I have two directories: src and build. In src, I have two files: main.cpp, with just a simple hello world example, and CMakeLists.txt, with the following text:
add_executable(test main.cpp)
target_link_libraries(test blas)
Here, blas is just some arbitrary library that I've found at /usr/lib/libblas.so. My main.cpp file does not actually use it, but I am just testing things out...
Now, if I enter the build directory, and run cmake ../src, then a makefile is generated in build. The contents of that makefile is pasted below.
What I am puzzled about, is why there is no mention of the blas library at all? Shouldn't cmake be adding a line such as -lblas?
Thanks :)
# CMAKE generated file: DO NOT EDIT!
# Generated by "Unix Makefiles" Generator, CMake Version 2.8
# Default target executed when no arguments are given to make.
default_target: all
.PHONY : default_target
#=============================================================================
# Special targets provided by cmake.
# Disable implicit rules so canonical targets will work.
.SUFFIXES:
# Remove some rules from gmake that .SUFFIXES does not remove.
SUFFIXES =
.SUFFIXES: .hpux_make_needs_suffix_list
# Suppress display of executed commands.
$(VERBOSE).SILENT:
# A target that is always out of date.
cmake_force:
.PHONY : cmake_force
#=============================================================================
# Set environment variables for the build.
# The shell in which to execute make rules.
SHELL = /bin/sh
# The CMake executable.
CMAKE_COMMAND = /usr/bin/cmake
# The command to remove a file.
RM = /usr/bin/cmake -E remove -f
# Escaping for special characters.
EQUALS = =
# The program to use to edit the cache.
CMAKE_EDIT_COMMAND = /usr/bin/cmake-gui
# The top-level source directory on which CMake was run.
CMAKE_SOURCE_DIR = /home/ejohns/Projects/Test2/src
# The top-level build directory on which CMake was run.
CMAKE_BINARY_DIR = /home/ejohns/Projects/Test2/build
#=============================================================================
# Targets provided globally by CMake.
# Special rule for the target edit_cache
edit_cache:
#$(CMAKE_COMMAND) -E cmake_echo_color --switch=$(COLOR) --cyan "Running CMake cache editor..."
/usr/bin/cmake-gui -H$(CMAKE_SOURCE_DIR) -B$(CMAKE_BINARY_DIR)
.PHONY : edit_cache
# Special rule for the target edit_cache
edit_cache/fast: edit_cache
.PHONY : edit_cache/fast
# Special rule for the target rebuild_cache
rebuild_cache:
#$(CMAKE_COMMAND) -E cmake_echo_color --switch=$(COLOR) --cyan "Running CMake to regenerate build system..."
/usr/bin/cmake -H$(CMAKE_SOURCE_DIR) -B$(CMAKE_BINARY_DIR)
.PHONY : rebuild_cache
# Special rule for the target rebuild_cache
rebuild_cache/fast: rebuild_cache
.PHONY : rebuild_cache/fast
# The main all target
all: cmake_check_build_system
$(CMAKE_COMMAND) -E cmake_progress_start /home/ejohns/Projects/Test2/build/CMakeFiles /home/ejohns/Projects/Test2/build/CMakeFiles/progress.marks
$(MAKE) -f CMakeFiles/Makefile2 all
$(CMAKE_COMMAND) -E cmake_progress_start /home/ejohns/Projects/Test2/build/CMakeFiles 0
.PHONY : all
# The main clean target
clean:
$(MAKE) -f CMakeFiles/Makefile2 clean
.PHONY : clean
# The main clean target
clean/fast: clean
.PHONY : clean/fast
# Prepare targets for installation.
preinstall: all
$(MAKE) -f CMakeFiles/Makefile2 preinstall
.PHONY : preinstall
# Prepare targets for installation.
preinstall/fast:
$(MAKE) -f CMakeFiles/Makefile2 preinstall
.PHONY : preinstall/fast
# clear depends
depend:
$(CMAKE_COMMAND) -H$(CMAKE_SOURCE_DIR) -B$(CMAKE_BINARY_DIR) --check-build-system CMakeFiles/Makefile.cmake 1
.PHONY : depend
#=============================================================================
# Target rules for targets named test
# Build rule for target.
test: cmake_check_build_system
$(MAKE) -f CMakeFiles/Makefile2 test
.PHONY : test
# fast build rule for target.
test/fast:
$(MAKE) -f CMakeFiles/test.dir/build.make CMakeFiles/test.dir/build
.PHONY : test/fast
main.o: main.cpp.o
.PHONY : main.o
# target to build an object file
main.cpp.o:
$(MAKE) -f CMakeFiles/test.dir/build.make CMakeFiles/test.dir/main.cpp.o
.PHONY : main.cpp.o
main.i: main.cpp.i
.PHONY : main.i
# target to preprocess a source file
main.cpp.i:
$(MAKE) -f CMakeFiles/test.dir/build.make CMakeFiles/test.dir/main.cpp.i
.PHONY : main.cpp.i
main.s: main.cpp.s
.PHONY : main.s
# target to generate assembly for a file
main.cpp.s:
$(MAKE) -f CMakeFiles/test.dir/build.make CMakeFiles/test.dir/main.cpp.s
.PHONY : main.cpp.s
# Help Target
help:
#echo "The following are some of the valid targets for this Makefile:"
#echo "... all (the default if no target is provided)"
#echo "... clean"
#echo "... depend"
#echo "... edit_cache"
#echo "... rebuild_cache"
#echo "... test"
#echo "... main.o"
#echo "... main.i"
#echo "... main.s"
.PHONY : help
#=============================================================================
# Special targets to cleanup operation of make.
# Special rule to run CMake to check the build system integrity.
# No rule that depends on this can have commands that come from listfiles
# because they might be regenerated.
cmake_check_build_system:
$(CMAKE_COMMAND) -H$(CMAKE_SOURCE_DIR) -B$(CMAKE_BINARY_DIR) --check-build-system CMakeFiles/Makefile.cmake 0
.PHONY : cmake_check_build_system

Answer 1

CMake uses a 3-level hierarchy of makefiles. The line you're looking for will be either in CMakeFiles/Makefile2 or in build.make somewhere under CMakeFiles (and probably a test directoty as well).