Ggtcf

If I define the function in the main, everything works fine. But if I define the function in another c file, this weir float multiplication problem happens: it gives 0 always.

example1/main.c:

int multiply_by_2(float scalar) 
 printf("result: %fn", scalar*2);


int main()

 multiply_by_2(3);

example1/CMakeLists.txt

cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c)

Output:

result: 6.000000

example2/main.c:

 int main()
 
 multiply_by_2(3);
 

example2/a.c:

int multiply_by_2(float scalar) 
 printf("result: %fn", scalar*2);

example2/CMakeLists.txt:

cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c a.c)

Output:

result: 0.000000

int multiply_by_2(float scalar) 
 printf("result: %fn", scalar*2);

you have an int returning function which does not return any integer. This triggers undefined behavior (once you call that multiply_by_2 function elsewhere). Be scared, very bad things could happen (even outside of that multiply_by_2 function or outside of its caller).

Next time, compile with all warnings and debug info, so with gcc -Wall -Wextra -g if using GCC. Improve your code to get no warnings.

Read also How to debug small programs.

If you use floating point numbers, be very cautious. They are difficult to understand and behave counter-intuitively (e.g. addition is not associative). Read http://floating-point-gui.de/ first.

If you have several translation units (e.g. source files such as a.c and main.c) you should in practice at least have some common header file and #include it in every *.c file. At a first approximation, your header file should declare all your  [global] types and functions. And you'll need to configure your build automation tool (e.g. write your Makefile) to take into account such dependencies: your header file should somehow be mentioned.

Read much more about C programming. See also some C reference site. Read the documentation of every function you are using (e.g. of printf)

You'll learn a lot by studying the source code of existing small free software programs (e.g. on github, gitlab, in a Linux distribution, etc).

The problem was that I should add a declaration of the functions in main.c, even though my compiler wouldn't warne me about it. With the declarations, all bugs are gone