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由 Glenn Randers-Pehrson 提交于
where it is not appropriate. If mainprog->jmpbuf is used by setjmp, then png_longjmp cannot be used. Reversed patch to remove error handler when the jmp_buf is stored in the main program structure, not the png_struct. The error handler is needed because the default handler in libpng will always use the jmp_buf in the library control structure; this is never set. The gregbook code is a useful example because, even though it uses setjmp/longjmp, it shows how error handling can be implemented using control mechanisms not directly supported by libpng. The technique will work correctly with mechanisms such as Microsoft Structure Exceptions or C++ exceptions (compiler willing - note that gcc does not by default support interworking of C and C++ error handling.)
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