Edit: im now using http://code.google.com/p/jpeg-compressor/ so i dont care about making this work anymore.
I downloaded the http://ijg.org/ source code, i tried to build it as a DLL file. This is farthest i could build until i went into dead-end.
I noticed there was some .c files which i had to delete, but i dont know if i deleted the correct ones, there was at least these files jmemmac.c and jmemdos.c which i figured i dont need. I had to delete jmemmac.c because i compile on windows, but it gave more errors so i kept deleting those useless files, but now i dont know what to do anymore.
Here are all the errors i got when building:
1>.\ansi2knr.c(273) : warning C4013: 'exit' undefined; assuming extern returning int
1>.\example.c(116) : warning C4013: 'exit' undefined; assuming extern returning int
1>.\example.c(379) : warning C4013: 'put_scanline_someplace' undefined; assuming extern returning int
1>ckconfig.obj : error LNK2005: _main already defined in wrjpgcom.obj
1>djpeg.obj : error LNK2005: _main already defined in wrjpgcom.obj
1>rdjpgcom.obj : error LNK2005: _main already defined in wrjpgcom.obj
1>jpegtran.obj : error LNK2005: _main already defined in wrjpgcom.obj
1>cjpeg.obj : error LNK2005: _main already defined in wrjpgcom.obj
1>ansi2knr.obj : error LNK2005: _main already defined in wrjpgcom.obj
1>jmemmgr.obj : error LNK2001: unresolved external symbol _jpeg_open_backing_store
1>jmemmgr.obj : error LNK2001: unresolved external symbol _jpeg_get_small
1>jmemmgr.obj : error LNK2001: unresolved external symbol _jpeg_mem_term
1>jmemmgr.obj : error LNK2001: unresolved external symbol _jpeg_free_large
1>jmemmgr.obj : error LNK2001: unresolved external symbol _jpeg_get_large
1>jmemmgr.obj : error LNK2001: unresolved external symbol _jpeg_mem_available
1>jmemmgr.obj : error LNK2001: unresolved external symbol _jpeg_free_small
1>jmemmgr.obj : error LNK2001: unresolved external symbol _jpeg_mem_init
1>example.obj : error LNK2001: unresolved external symbol _image_height
1>example.obj : error LNK2001: unresolved external symbol _image_buffer
1>example.obj : error LNK2001: unresolved external symbol _put_scanline_someplace
1>example.obj : error LNK2001: unresolved external symbol _image_width
Ok, if you're compiling a DLL, you generally don't want any "main" functions. I assume the source files referencing a a function called "main" or "_main" are examples of how to use the library, so it should be save to get rid of those.
In jmemmac.c, there is a function called "jpeg_open_backing_store", which is one of the unresolved external symbols jmemmgr is complaining about. The function appears to have platform-dependant calls, such as the "FindFolder" function for mac.
The only thing I can think of to do is to study jmemmac.c, jmemdos.c and jmemansi.c and rewrite those functions so they will work on a windows platform.
Oh, and as for "exit" being undefined, include "stdlib.h", as it defines the exit function.