How to print current compilation flags that are set with target_compile_options()?

Question

I am trying to print compilation flags that are set for target. The best scenario is to print a line with current flags on configure and compilation times, but if it's impossible, then on configure time only (or compilation only) (acceptable solution).

This is my testing .c file:

#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}

And CMakeLists.txt:

cmake_minimum_required(VERSION 3.10)
project(cmake_gcc_options_try_c C)

set(CMAKE_C_STANDARD 11)
set(CMAKE_C_STANDARD_REQUIRED ON)

add_executable(cmake_gcc_options_try_c main.c)

target_compile_options(cmake_gcc_options_try_c 
                       PUBLIC -W -Wall -Wextra -pedantic -pedantic-errors)

# This fails
message("-- Current compiler flags CMAKE_C_FLAGS are: ${CMAKE_C_FLAGS}")
message("-- Current compiler flags C_FLAGS are: ${C_FLAGS}")

and

cmake . && make

Gives this output:

-- The C compiler identification is GNU 7.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Current compiler flags CMAKE_C_FLAGS are: 
-- Current compiler flags C_FLAGS are: 
-- Configuring done
-- Generating done
-- Build files have been written to: /home/user/Projects/cmake-gcc-options-try-c
Scanning dependencies of target cmake_gcc_options_try_c
[ 50%] Building C object CMakeFiles/cmake_gcc_options_try_c.dir/main.c.o
[100%] Linking C executable cmake_gcc_options_try_c
[100%] Built target cmake_gcc_options_try_c

Why CMAKE_C_FLAGS and C_FLAGS are printed as they were undefined?

How to achieve this print on make command:

[ 50%] Building C object CMakeFiles/cmake_gcc_options_try_c.dir/main.c.o
[ 50%] Current compiler flags are: -W -Wall -Wextra -pedantic -pedantic-errors -std=gnu11
[100%] Linking C executable cmake_gcc_options_try_c
[100%] Built target cmake_gcc_options_try_c

?

Update: Viktor Sergienko came with a working solution, but one problem with this it's not so pretty print: Any thoughts how to make it to be in format of other prints? E. g. :

[ 50%] Building C object CMakeFiles/cmake_gcc_options_try_c.dir/main.c.o
[100%] Linking C executable cmake_gcc_options_try_c
[100%] Current compiler flags are: -W -Wall -Wextra -pedantic -pedantic-errors -std=gnu11
[100%] Built target cmake_gcc_options_try_c

Second problem is that -std=gnu11 is not printed (but it is enabled with set(CMAKE_C_STANDARD 11) and set(CMAKE_C_STANDARD_REQUIRED ON))

Answer 1

Use:

• COMPILE_OPTIONS target property to get target-specific compilation flags;
• add_custom_command with a POST_BUILD option to print any variable;

This gave me a ;-list of the options both at configure and compile times:

cmake_minimum_required(VERSION 3.10)
project(cmake_gcc_options_try_c C)

set(CMAKE_C_STANDARD 11)
set(CMAKE_C_STANDARD_REQUIRED ON)

add_executable(cmake_gcc_options_try_c main.c)

target_compile_options(cmake_gcc_options_try_c 
                       PUBLIC -W -Wall -Wextra -pedantic -pedantic-errors)

get_target_property(MAIN_CFLAGS cmake_gcc_options_try_c COMPILE_OPTIONS)
# also see: COMPILE_DEFINITIONS INCLUDE_DIRECTORIES
message("-- Target compiler flags are: ${MAIN_CFLAGS}")

add_custom_command(TARGET cmake_gcc_options_try_c POST_BUILD
COMMAND echo built with the flags: ${MAIN_CFLAGS})

Update after question was updated: to get C/CXX standard, look up C_STANDARD. If you wonder why CMake sets -gnu variant of the flag, it's because of CXX_EXTENSIONS is ON by default.

Update2: To get the full compiler/linker commands for every source file as a JSON file, set CMAKE_EXPORT_COMPILE_COMMANDS to ON (only works in CMake 3.5+, make and ninja generators). Credit for this piece goes to Florian's comment in this question.