How to create plugins in plain C?

[AJ ONeal]

How does one go about creating C plugins?

Example Scenario:

I have an application which generates rgb16 files.

I have a number libraries with a particular interface that output in various formats

This is the interface:

int encode(void* buffer, int width, int height, int depth)

There is a settings.conf which is parsed at startup or SIGHUP that looks like this:

jpeg: rgb16tojpeg.so

png: rgb16topng.so

// bmp: rgb16tobmp.so

By default none of the modules are loaded.

In this case the modules jpeg and png are going to be loaded (and used), but bmp will not.

In this fashion I only load the module into memory if I plan to use it. Otherwise I do not.

More importantly, if I come up with a new format I should be able to put the .so into the plugins directory and update the settings file and voila, the output should suddenly also be encoded in the new format.

Any ideas?

AJ ONeal

[Shawn]

Check out dlopen().

[AJ ONeal]

Thanks Shawn,

I was googling the wrong terms previously. What I should have been searching for was "dynamic loading" rather than "plugins".

I found a few good articles with examples using dlopen:

"Dynamic loading and un-loading of shared libraries using libdl" about halfway down this page:

http://www.yolinux.com/TUTORIALS/LibraryArchives-StaticAndDynamic.html

Also

http://pubs.opengroup.org/onlinepubs/009695399/functions/dlopen.html

http://pubs.opengroup.org/onlinepubs/009695399/functions/dlsym.html

I'm reading through them now.

AJ ONeal

[AJ ONeal]

I don't quite understand the difference between RTLD_LOCAL and RTLD_GLOBAL.

More importantly, I don't understand the 'recolation' lingo.

Can someone shed some light on the subject?

Here's an example where I don't understand what the difference would be:

Assume the libs

  rgb16tojpeg.so

  rgb16png.so

And each has a function

  int encode(void* buffer, int width, int height, int depth);

pngh = dlopen("rgb16topng.so", RTLD_LAZY | RTLD_GLOBAL);

jpegh = dlopen("rgb16tojpeg.so", RTLD_LAZY | RTLD_LOCAL);

jpegfn = dlsym(jpegh, "encode");

pngfn = dlsym(pngh, "encode");

*jpegfn(data, 320, 240, 16);

*pngfn(data, 320, 240, 16);

However if I were to do

encfn = dlsym(RTLD_DEFAULT, "encode");

then encfn would be the RTLD_GLOBAL loaded "encode"?

AJ ONeal

On Tue, Feb 1, 2011 at 3:04 PM, Shawn <shawnw...@gmail.com> wrote: