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With respect to cinfo.err,

With respect to cinfo.err, the current code does work.

However, if you look at the definition of cinfo.err in libjpeg, it is assumed to be a pointer to a structure that contains function pointers for the error handler, exit handler, etc. The cerr pointer you pass in has that structure as its first part, but then also has a jmp_buf structure following this structure as the second part.

Somehow to my mind it is cleaner to have cinfo.err point to a "pure" function pointers block, and use cinfo.client_data to point to the jmp_buf structure. But to each his own.