CMake: Why doesn't POST_BUILD in add_custom_command(...) work? - makefile

Question

I have some trouble when I use PRE_BUILD | PRE_LINK | POST_BUILD in command add_custom_command(...). When I use POST_BUILD, I found the command will execute before the target has been built, like this:
[root#VM_33_35_centos build]# make
Scanning dependencies of target main
[100%] Building C object CMakeFiles/main.dir/main.c.o
Linking C executable main
This is pre build
This is post build
[100%] Built target main
The contents of my CMakeLists.txt are:
cmake_minimum_required(VERSION 2.8)
add_executable(main main.c)
add_custom_command(TARGET main
PRE_BUILD
COMMAND echo "This is pre build "
)
add_custom_command(TARGET main
POST_BUILD
COMMAND echo "This is post build"
)
Why did the command echo "This is post build" on line 8 of CMakeLists.txt not execute after the [100%] Built target main message on line 7(Linux command)?

Answer 1

Command added with add_custom_command(TARGET) signature becomes part of the target, that is target can be assumed to be built (Built target main) only after given command is executed.
Description of POST_BUILD keyword
run after the target has been built
means that command is executed after target file (in your case executable main) is created. This file is created as part of linking process, started after line Linking C executable main.