Tuesday, March 14, 2017

Auto Generate a 'C' Function Prototype Header File

There is a lot of discussion on the proper way to use header files in Embedded C projects. I don't want to get into that discussion, rather I want to present a tool that is useful for my usage. If you don't agree with my use model, that's OK, you don't have to use this tool that way, it is very adaptable to any situation! [1]

I use header files for three things,

  1. One big one that has all the #includes in it so that every module has the proper references to things like: <stdio.h> and <math.h>.
  2. One that has the global program data in it.
  3. One that has the various public function prototypes (signatures) in it.
The first two are relatively easy to make and maintain. They also settle down quickly in a project as the tasks they maintain are defined early on and while there may be refactoring, they rarely change much.

#3 however is constantly changing and even in a small project it may grow to many, many function prototypes and refactoring causes constant updating.

It is tedious to have to change the data in two places when refactoring or when adding functionality through public functions in an embedded C program.

Auto Generation to the Rescue

I knew that someone somewhere must have written a utility that reads a directory, looks through all the '.c' files and auto-generates a header with all the public function prototypes in it.

Sure enough I found some C code written in 1993, by a Mr. Richard Hipp on the InterWebs that does just that [2].

It's a small program, all in one file and only several pages long. I brought it into Pelles C [3] to see if I could compile it under Win 7 and almost unbelievably it compiled with only a few simple changes. For code written in 1993 that compiles on Windows that seems almost unbelievable to me!

What the program is supposed to do is read every '.c' (and or '.h') file in a directory, extract the public functions and write them as function prototypes into individual '.h' files or one big '.h' file that can be added to the project.

Functions are marked as local (i.e. not for exporting into the '.h' file) by either using the keyword: 'static' or by the use of a 'LOCAL' define (see the program documentation [2]).

The program is wonderfully written, easy to follow and worked straight up out of the box, with one exception.

To read all the '.c' files at once to make a single output file the program depends on the UNIX ability to do wildcard expansion on the command line. MSDOS does not have that capability, so I had to wrap the makeheaders.exe in a MSDOS Batch file to make it work the way I wanted to use it.

MakeHeaders is so complete, it has the ability to pipe in a file list of names on the command line and then make a single big '.h' file. Mr. Hipp thought of everything. The batch file that I used is below.

REM Make one big Header File
dir /B *.c > mkhdr_input.txt
makeheaders -h -f mkhdr_input.txt >AutoGeneratedPrototypes.h
del mkhdr_input.txt


Listing 1 – A MSDOS batch file to operate the MakeHeaders program work the way I wanted it to. Upon running it makes a temporary file with all the names of the '.c' files in the current directory. Then it feeds this list into the makeheaders.exe. The program then parses all the files picking out the public function prototypes to write into one big '.h' file.

The batch file of Listing 1 first makes a file called: “mkhdr_input.txt” that contains just the file names. The “dir /B” switch is for the bare format which will list just the file names.

The makeheaders.exe is then fed with the mkhdr_input.txt file as input. The '-h' switch causes makeheaders to make one big '.h' file as output and then it redirects the output to the standard output which I redirect to the file AutoGeneratedPrototypes.h with the “>” redirect command.

The '-f' switch tells makeheaers that it will get it's input from the file specified, in this case: “mkhdr_input.txt”

Finally I just cleanup by deleting the: “mkhdr_input.txt” file.

There are many other options and ways to make the MakeHeaders program operate so be sure to check out the documentation [2]. For example the same list input can be used to make an individual '.h' file that corresponds to every '.c' file in the directory. This format is preferred by many and in very large programs may be preferable.

I just add the AutoGeneratedHeaders.h in my master 'include .h' file and the rest is automatic.

Perfect Function Prototypes Every Time

Now, anytime I refactor or add public functions to any source file I can just double click on the MakeHeaders.bat file and a new AutoGeneratedPrototypes.h file is made all ready for compiling.

Extra Bonus

If you want to use MakeHeaders to create a single, individual '.h' file for every '.c' file just use the batch file below.

REM Make A Seperate Header File for Each *.C file
for %%G IN (*.c) DO MakeHeaders %%G


[1] To find out more about the various ways to use header files, do a Google Search like,

Then pick a strategy that makes sense for you.

[2] Make Headers program. As of March 2017, the source code and documentation can be found at,

[3] Pelles C – A very good freeware C Compiler for Windows,

Article By: Steve Hageman www.AnalogHome.com     

We design custom: Analog, RF and Embedded systems for a wide variety of industrial and commercial clients. Please feel free to contact us if we can help on your next project.  

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