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

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# A target that is always out of date.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:
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.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

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preinstall/fast:
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.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
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.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:
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.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).