Using Dynamically Generated Header Files with CMake

Sometimes it can be useful to dynamically generate header files and include them from a C source file. However, it can be a little difficult getting CMake to recognize when to generate the files and when to recompile the source files that include those headers. What follows is a method to do this.

Telling CMake to Generate the Header File

To generate the header file we need to create a custom_command which will run the command to generate the header file, when the file it is based on has changed. The following shows how to use the hypothetical generatewordlistheader command to create wordlist.h everytime wordlist.txt changes.

add_custom_command(
    PRE_BUILD
    OUTPUT wordlist.h
    COMMAND generatewordlistheader wordlist.txt wordlist.h
    DEPENDS wordlist.txt
)

Telling CMake When to Recompile

Next we need to tell CMake to recompile any source files which include the generated header file, when the header file has changed. To do this we create a statically linked library, here called poet, that relies on the source files that include the header and the header itself.

add_library(poet STATIC poet.c ${MyApp_SOURCE_DIR}/src/wordlist.h)

Finally we need to ensure that the statically generated library above is linked into the application.

target_link_libraries(myapp poet)

Full example

Using a fictional project called MyApp we can see how this is put together.

cmake_minimum_required(VERSION 2.6)
project(MyApp)
add_executable(myapp myapp.c)

add_custom_command(
    PRE_BUILD
    OUTPUT wordlist.h
    COMMAND generatewordlistheader wordlist.txt wordlist.h
    DEPENDS wordlist.txt
)

add_library(poet STATIC poet.c ${MyApp_SOURCE_DIR}/src/wordlist.h)

target_link_libraries(myapp poet)

Conclusion

This isn't perfect but it does work. Unfortunately CMake is an ugly, but very useful tool. I'm sure there is a better way to do this and if anyone can think of a way, please leave a comment or email me. Until then, this achieves what I and a number of other people have been trying to do.

Creative Commons License
Using Dynamically Generated Header Files with CMake by Lawrence Woodman is licensed under a Creative Commons Attribution 4.0 International License.

Share This Post

Feedback/Discuss

Sign up to get new articles straight to your inbox.

Delivered by FeedBurner

Related Articles

Double-Width C64 Style Font on the Commodore VIC-20

The C64 used a fatter double-width font because the video quality of early VIC-IIs wasn't very good. Therefore they had to make the C64's font fatter than the VIC-20's. However, this fatter font look...   Read More

The Pilot Programming Language on CP/M

Pilot was created by John A. Starkweather in the early 1960s as a programming language for Computer Assisted Instruction. It has often been compared to Logo because of its use with children. However,...   Read More

SUBLEQ on the Commodore VIC-20

I have created a SUBLEQ Virtual Machine for the Commodore VIC-20. SUBLEQ is a computer architecture that has only one instruction: SUBLEQ. The instruction stands for SUbtract and Branch if Less than ...   Read More

SUBLEQ - A One Instruction Set Computer (OISC)

SUBLEQ has to be one of the easiest architectures to implement in either software or hardware and this is the main reason for its design as a teaching aid. It has only one instruction, hence why it is...   Read More

The Mouse Programming Language on CP/M

Mouse is an interpreted stack orientated language designed by Peter Grogono around 1975. It was designed to be a small but powerful language for microcomputers, similar to Forth, but much simpler. On...   Read More