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C11 removed the requirement that FILE be a complete type, which was
deemed erroneous, as part of the changes introduced by N1439 regarding
completeness of types (see footnote 6 for specific mention of FILE).
however the current version of POSIX is still based on C99 and
incorporates the old requirement that FILE be a complete type.
expose an arbitrary, useless complete type definition because the
actual object used to represent FILE streams cannot be public/ABI.
thanks to commit 13d1afa46f8098df290008c681816c9eb89ffbdb, we now have
a framework for suppressing the public complete-type definition of FILE
when stdio.h is included internally, so that a different internal
definition can be provided. this is perfectly well-defined, since the
same struct tag can refer to different types in different translation
units. it would be a problem if the implementation were accessing the
application's FILE objects or vice versa, but either would be
undefined behavior.
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by ABI, the public stdin/out/err macros use extern pointer objects,
and this is necessary to avoid copy relocations that would be
expensive and make the size of the FILE structure part of the ABI.
however, internally it makes sense to access the underlying FILE
objects directly. this avoids both an indirection through the GOT to
find the address of the stdin/out/err pointer objects (which can't be
computed PC-relative because they may have been moved to the main
program by copy relocations) and an indirection through the resulting
pointer object.
in most places this is just a minor optimization, but in the case of
getchar and putchar (and the unlocked versions thereof), ipa constant
propagation makes all accesses to members of stdin/out PC-relative or
GOT-relative, possibly reducing register pressure as well.
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