if EINVAL or ENOMEM happened before the first getc_unlocked, it was
possible that the stream orientation had not yet been set.

on some archs, linux support for futex operations (including
robust_list processing) that depend on kernelspace CAS is conditional
on a runtime check. as of linux 4.18, this check fails unconditionally
on nommu archs that perform it, and spurious failure on powerpc64 was
observed but not explained. it's also possible that futex support is
omitted entirely, or that the kernel is older than 2.6.17. for most
futex ops, ENOSYS does not yield hard breakage; userspace will just
spin at 100% cpu load. but for robust mutexes, correct behavior
depends on the kernel functionality.

use the get_robust_list syscall to probe for support at the first call
to pthread_mutexattr_setrobust, and block creation of robust mutexes
with a reportable error if they can't be supported.

do_tzset() did't always reset the DST transition rules r0 and r1. That
means the rules from older TZ settings could leak into newer ones.

in order to produce FILE objects to pass to the intscan/floatscan
backends without any (prohibitively costly) extra buffering layer, the
strto* functions set the FILE's rend (read end) buffer pointer to an
invalid value at the end of the address space, or SIZE_MAX/2 past the
beginning of the string. this led to undefined behavior comparing and
subtracting the end pointer with the buffer position pointer (rpos).

the comparison issue is easily eliminated by using != instead of <.
however the subtractions require nontrivial changes:

previously, f->shcnt stored the count that would have been read if
consuming the whole buffer, which required an end pointer for the
buffer. the purpose for this was that it allowed reading it and adding
rpos-rend at any time to get the actual count so far, and required no
adjustment at the time of __shgetc (actual function call) since the
call would only happen when reaching the end of the buffer.

to get rid of the dependency on rend, instead offset shcnt by buf-rpos
(start of buffer) at the time of last __shlim/__shgetc call. this
makes for slightly more work in __shgetc the function, but for the
inline macro it's still just as easy to compute the current count.

since the scan helper interfaces used here are a big hack, comments
are added to document their contracts and what's going on with their
implementations.

POSIX allows ttyname(_r) and isatty to return EBADF if passed file
descriptor is invalid.

maintainer's note: these are optional ("may fail") errors, but it's
non-conforming for ttyname_r to return ENOTTY when it failed for a
different reason.

this significantly improves codegen in functions that need to access
errno but otherwise have no need for a GOT pointer.

we could probably improve it much more by including an inline version
of the &errno accessor function, but that depends on having the
definitions of struct __pthread and __pthread_self(), which at present
would expose a lot more than is appropriate. moving them to a small
tls.h later might make this more reasonable.

commit 5ce37379 removed the inclusion
of libc.h from this file as spurious, but it's needed to get PAGE_SIZE
on archs where PAGE_SIZE is not a constant defined by limits.h.

there is no good reason to wait to find and process the plural rules
for a translated message file until a gettext form requesting plural
rule processing is used. it just imposes additional synchronization,
here in the form of clunky use of atomics.

it looks like there may also have been a race condition where nplurals
could be seen without plural_rule being seen, possibly leading to null
pointer dereference. if so, this commit fixes it.

in our memory model, all atomics are supposed to be full barriers;
stores are not release-only. this is important because store is used
as an unlock operation in places where it needs to acquire the waiter
count to determine if a futex wake is needed. at least in the
malloc-internal locks, but possibly elsewhere, soft deadlocks from
missing futex wake (breakable by poking the threads to restart the
syscall, e.g. by attaching a tracer) were reported to occur.

once the malloc lock is replaced with Jens Gustedt's new lock
implementation (see commit 47d0bcd4),
malloc will not be affected by the issue, but it's not clear that
other uses won't be. reducing the strength of the ordering properties
required from a_store would require a thorough analysis of how it's
used.

to fix the problem, I'm removing the powerpc[64]-specific a_store
definition; now, the top-level atomic.h will implement a_store using
a_barrier on both sides of the store.

it's not clear to me yet whether there might be issues with the other
atomics. it's possible that a_post_llsc needs to be replaced with a
full barrier to guarantee the formal semanics we want, but either way
I think the difference is unlikely to impact the way we use them.

MAX_HANDLE_SZ is described in name_to_handle_at() to contain maximum
expected size for a file handle

as originally published, the C99 syntax only allowed static index
parameter declarators when a gratuitous parameter name was included.
gcc 3, which some projects use for bootstrapping, is a supported C99
compiler, but does not have the fix to the standard incorporated, so
edit the affected declaration to conform to the earlier buggy C99
syntax.

other compilers don't need this option, but gcc 3 and perhaps others
accept it despite not understanding it, then print warnings about it
at build time.

omitting it when not needed will also help shorten the command lines.

since commit dc2f368e this has been
disabled by default, but was left available in case users unhappy with
the resulting size or performance regressions wanted to try to make it
work. now that we make widespread use of hidden visibility for
internal interfaces, this no longer makes sense. if any costly calls
remain they can be fixed with hidden aliases.

this further reduces the number of source files which need to include
libc.h and thereby be potentially exposed to libc global state and
internals.

this will also facilitate further improvements like adding an inline
fast-path, if we want to do so later.

pthread_atfork.c does not actually include pthread_impl.h and has no
reason to, so it wasn't getting the declaration. move it to libc.h
which is already included by both fork.c and pthread_atfork.c. this
makes more sense anyway since the function has little to do with
pthreads anyway aside from the name.

the LFS64 macro was not self-documenting and barely saved any
characters. simply use weak_alias directly so that it's clear what's
being done, and doesn't depend on a header to provide a strange macro.

libc.h was intended to be a header for access to global libc state and
related interfaces, but ended up included all over the place because
it was the way to get the weak_alias macro. most of the inclusions
removed here are places where weak_alias was needed. a few were
recently introduced for hidden. some go all the way back to when
libc.h defined CANCELPT_BEGIN and _END, and all (wrongly implemented)
cancellation points had to include it.

remaining spurious users are mostly callers of the LOCK/UNLOCK macros
and files that use the LFS64 macro to define the awful *64 aliases.

in a few places, new inclusion of libc.h is added because several
internal headers no longer implicitly include libc.h.

declarations for __lockfile and __unlockfile are moved from libc.h to
stdio_impl.h so that the latter does not need libc.h. putting them in
libc.h made no sense at all, since the macros in stdio_impl.h are
needed to use them correctly anyway.

not all prefixed symbols can be made hidden. some are part of
ABI-compat (e.g. __nl_langinfo_l) and others are ABI as a consequence
of the way copy relocations for weak aliases work in ELF shared
libraries. most, however, can be made hidden.

with this commit, there should be no remaining unintentionally visible
symbols exported from libc.so.

this was added so that posix_spawn and possibly other functionality
could be implemented in terms of vfork, but that turned out to be
unsafe. any such usage needs __clone with proper handling of stack
lifetime.

this is perhaps not the ideal place, but no better alternatives stand
out.

the direct syscall or various thin and mostly-inline wrappers around
it are used instead internally. at some point a public futex function
should be added, but it's not yet clear what the signature should be,
and in the mean time this file is not useful.

this is a special case that does not need a declaration, because it's
not even a libc-internal interface between translation units. instead
it's a poor hack around compilers' inability to shrink-wrap critical
code paths. after vis.h was disabled, it became more of a
pessimization on many archs due to the extra layer of machinery to
support a call through the PLT, but now it should be efficient again.

the __-prefixed filename does not make sense when the only purpose of
this file is implementing a public function that's not used as a
backend for implementing the standard dirent functions.

these were overlooked for various reasons in earlier stages.

these were overlooked in the declarations overhaul work because they
are not properly declared, and the current framework even allows their
declared types to vary by arch. at some point this should be cleaned
up, but I'm not sure what the right way would be.

this makes significant differences to codegen on archs with an
expensive PLT-calling ABI; on i386 and gcc 7.3 for example, the sin
and sinf functions no longer touch call-saved registers or the stack
except for pushing outgoing arguments. performance is likely improved
too, but no measurements were taken.

commits leading up to this one have moved the vast majority of
libc-internal interface declarations to appropriate internal headers,
allowing them to be type-checked and setting the stage to limit their
visibility. the ones that have not yet been moved are mostly
namespace-protected aliases for standard/public interfaces, which
exist to facilitate implementing plain C functions in terms of POSIX
functionality, or C or POSIX functionality in terms of extensions that
are not standardized. some don't quite fit this description, but are
"internally public" interfacs between subsystems of libc.

rather than create a number of newly-named headers to declare these
functions, and having to add explicit include directives for them to
every source file where they're needed, I have introduced a method of
wrapping the corresponding public headers.

parallel to the public headers in $(srcdir)/include, we now have
wrappers in $(srcdir)/src/include that come earlier in the include
path order. they include the public header they're wrapping, then add
declarations for namespace-protected versions of the same interfaces
and any "internally public" interfaces for the subsystem they
correspond to.

along these lines, the wrapper for features.h is now responsible for
the definition of the hidden, weak, and weak_alias macros. this means
source files will no longer need to include any special headers to
access these features.

over time, it is my expectation that the scope of what is "internally
public" will expand, reducing the number of source files which need to
include *_impl.h and related headers down to those which are actually
implementing the corresponding subsystems, not just using them.