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Fun with gengetopt

Posted on | January 8, 2008 | 6 Comments

The GNU operating system is full of easter eggs hidden in plain view. Depending on what repositories you elect to tell your packager about, you could spend an entire day discovering useful things that you never knew existed. Many of these trinkets are part of (or work with) the GNU build system.

I’ve known about a program named gengetopt for a few years, its a neat tool to generate GNU-kosher C code that parses all of your command line arguments and displays help. It even generates a function to parse options from a configuration file. I’ve been meaning to use it for a while because it plays nicely with autotools, but kept forgetting about it. Its been around since (at least) 2003, but many people don’t know that its there, so here’s a little blurb about what it does and how to use it. Read on, it will save you hours.

I’m not sure how you do it, but I like to plan the scope of my program well in advance of getting started. I’ll typically write my usage() function first while thinking about just how everything should work. Once I’m satisfied, its easy to start writing the rest and I’m less likely to wander off into more functionality than I really need once I begin. With gengetopt, Its simple to do this in a plain text file.

The program itself is very simple. You define your options, the type each argument should be (string, flag, double, long, etc), if its required, its initial value and its short/long switch options. My dummy file is below, more or less the same as the example given in the man page which I named test.ggo: package "dummy" version "0.0.1-1" purpose "To play with gengetopt"

option "larry" s "Larry Stooge" string no option "moe" m "Moe Stooge" int no option "curly" i "Curly Stooge" int yes option "shemp" - "Shemp? Sometimes" flag off option "stooge" F "The Stooges" no option "nyuk" - "Perhaps nyuks should float?" long no option "funny" - "Stooges are funny!" string default="Yes" no

Since I was playing with it, I wanted to generate code that I could just compile and run on its own, so my command to produce everything would be a little different than common use: gengetopt --file-name=cmds --conf-parser --unamed-opts \ --func-name=main < test.ggo

This generated cmds.c and cmds.h, specifying that I wanted the function that parsed the arguments to be named main() (so it could be run without writing a dummy program to include it). I also passed –unamed-opts so that the generated code could handle a list of files and –conf-parser to make a function to deal with a configuration file.

The end result was perfect. All of the code (including printed help) was created, easily compiled and ready for me to finish and adjust. 500 lines of code done in less than a second, you can’t beat that!

The output is well done, all of your options are stored in a structure and the proper autotools hooks are present. The manual page is complete enough to get going quickly, more options (such as option grouping) are explained in the included HTML documentation.

The only thing it does not do is allow an auto-insertion of an option to parse the configuration file. A –parse-cfgfile-option would be neat, or syntax for it in the input file. A switch to specify tabs or spaces for indenting would be neat, but I’m not going to nitpick.

The output of gengetopt is not copyrighted or licensed, its released to the public domain. gengetopt itself is GPL, but there are no restrictions on the output. As such, you aren’t required to license whatever uses the generated code under the GPL.

I’m glad that I finally remembered that this existed, I have to re-write a bunch of daemons done in perl and PHP to C. This will save quite a few hours :) In the past couple of years, many companies have opted to hire system integrators who have at least a (basic) knowledge of C so that they can write stuff on the fly to just make something work. Tools like this save a ton of time for anyone in such a position.

Even though its been around for a while, if you’ve never really used it .. its new to you :) Hopefully this blurb saves others some time, too.


6 Responses to “Fun with gengetopt”

  1. Lorenzo Bettini
    January 8th, 2008 @ 4:06 pm

    Hey Tim! I’m Lorenzo, the author and maintainer of GNU gengetopt.

    I’m so glad you enjoyed gengetopt and you find it easy to use! Many compliments for this article: it gives a very nice introduction on gengetopt and how to use it! You know, I never thought about the trick of calling the function main :-)

    I saw that you subscribed to the mailing list, so please, feel free to provide suggestions/enhancements/patches.

    Hope to hear from you soon! cheers Lorenzo

  2. tinkertim
    January 9th, 2008 @ 2:48 pm


    The program is really, really useful. I really like it because it saves a ton of time and helps when planning out just how everything is going to work.

    I’ll send some suggestions for small feature enhancements (hopefully) along with patches in the not too distant future, when I have a little more time.

    This is a really cool companion to autotools, I’m going to have fun messing around with having configure generate the input file. This would mean switches that weren’t appropriate for the system would simply not appear. I haven’t tried it yet, but I think it would work. Surely, others have done this :)

    I might start going through my aptitude cache to find more really useful things that many might not know about and write some posts similar to this one. Lots of admins are getting to know a bit of programming, but aren’t aware of some really useful tools.

    Anyway, thanks for something cool and useful :)

  3. Lorenzo Bettini
    January 9th, 2008 @ 4:18 pm

    It’s really my pleasure :-)

    by the way, the new release of gengetopt, 2.22, should be out soon (I’m finishing some testing) and the code generation was heavily modified in order to be cleaner and (hopefully) easier to understand/modify; I’ll let you know when it’ll be ready (w.t.r. I can also subscribe you to the info mailing list where new release announcements will be posted – I also post announcements on my blog).

    By the way, I hope you’re using the .tar.gz version, since the version you find in many distributions is not very up-to-date.

    Please, let me take the chance to advertise my other two GNU programs (oriented to code development):

    cheers Lorenzo

  4. tinkertim
    January 9th, 2008 @ 6:37 pm


    I was actually just playing with gengen :) I always grab soureballs from the GNU servers, packagers delight in doing odd, mindless things and cherry picking patches.

    I’ll chirp in on the lists soon, hopefully with a few patches to suggest new features :)

  5. Lorenzo Bettini
    January 9th, 2008 @ 7:04 pm

    I look forward to! :-)

  6. Interesting internationalization : Echoreply
    September 18th, 2008 @ 3:12 pm

    [...] its a tool that simplifies generating the code required for programs to accept options. I talked about gengetopt a while back on this [...]

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