Re: CYGWIN JPEG question

["Charles S. Wilson" <cwilson at ece dot gatech dot edu>]

Daniel G. Aliaga wrote:

> I recently installed CYGWIN. Its great!
> 
> I'm porting code that uses the standard JPEG library. I specify the library
> on the link line, but I get error messages such as the following (one for
> every jpeg call):
> 
> ....: undefined reference to 'import stub for
> jpeg_read_header(jpeg_decompress_struct *, int)'


Did you "interpret" this error message?  I've never seen gcc/ld spell 
stuff out in that much detail.  Ordinarily, the error would be 
"undefined reference to __imp__jpeg_read_header" in this case.


> and so forth, etc.
> 
> 
> I suppose this has something to do with DLLs vs libraries. It seems
> libjpeg.a and libjpeg.dll.a are already in /usr/lib. I suppose the
> installation properly created them. cjpeg and djpeg work just fine.


Yes, static lib, import lib, and DLL are all included in the dist.


> Is there something special I need to do during link time? I come from a Unix
> background. My compile line is something simple like:
> 
> "g++ -o myprogram myprogram.C -ljpeg -lm"
> 


First, you should probably split compilation and linking into two 
separate steps (not *really* necessary, but often illuminating). Second, 
you need to decide whether you want to link statically or dynamically.

Static:
add -DJPEG_STATIC (or -DALL_STATIC) when compiling
add -static when linking (this will cause the linker to hunt for 
"libjpeg.a" instead of "libjpeg.dll.a" when given "-ljpeg".

Dynamic:
Strange -- what you have should work (minus the -lm, but that shouldn't 
affect anything).  The system header files are set up to define/declare 
things appropriately for a dynamic link.  Please try:

g++ -c -o myprogram.o myprogram.C
g++ -o myprogram myprogram.o -ljpeg

(BTW, I'm not sure that the jpeg library is C++ safe...)

--Chuck

P.S. personal mail makes me LESS likely to respond, not more.  I get 
five or more messages a day of cygwin-related personal mail that belongs 
on the list.  Most of it goes in the trash.




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