Commits on Oct 15, 2024

1. dev: Make the use of stdio FILE robust. …

   ABI changes:
       The three API functions png_init_io, png_begin_read_from_stdio and
       png_image_write_to_stdio now require the host fread, fwrite and
       fflush functions where appropriate in addition to the host FILE.
   
       Internally libpng uses the provided functions for all operations on
       the FILE; it does not use the implementations available when libpng
       was built.
   
   API changes:
       The original APIs of the above three functions are now implemented
       as function-like macros which automatically pass the correct ISO-C
       functions.  This ensures that there are no net API changes and that
       the correct arguments are passed.
   
   Build changes:
       The read stdio functionality is now dependent on SEQUENTIAL_READ
       rather than READ.  This is done for clarity; the progressive read
       mechanism does not use FILE.
   
   Internal changes:
       png_struct::io_ptr has been supplemented with png_struct::stdio_ptr
       used when stdio is used directly by libpng for IO as opposed to
       caller-provided callbacks.
   
       Error checking has been added to detect mismatched use of the stdio
       (png_init_io etc) API with the callback (png_set_read_fn,
       png_set_write_fn APIs.)  Changing from one API to the other
       mid-stream should work but has not been tested and is not currently
       a documented option.
   
   The changes address an issue which is frequently encountered on
   Microsoft Windows systems because Windows NT DLLs each have their own
   ISO-C hosted environment.  This means that a FILE from one DLL cannot be
   used safely from a different DLL unless all access to the object is done
   using functionality from the creating DLL.  The problem is more general;
   passing objects across DLL or other boundaries is frequently supported
   but direct access to those objects' internal structure in the receiving
   environment is not safe.  Other such uses were address early on in the
   development of libpng, this addresses the main, almost certainly, sole
   remaining issue.
   
   The idea of adding additional function pointers png_init_io was
   suggested by github.com user pp383 in pull request pnggroup#208.
   
   Signed-off-by: John Bowler <[email protected]>

   

   jbowler committed Oct 15, 2024
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