this idea came up when I thought we might need to zero the UNGET
portion of buf as well, but it seems like a useful improvement even
when that turned out not to be necessary.

shgetc sets up to be able to perform an "unget" operation without the
caller having to remember and pass back the character value, and for
this purpose used a conditional store idiom:

    if (f->rpos[-1] != c) f->rpos[-1] = c

to make it safe to use with non-writable buffers (setup by the
sh_fromstring macro or __string_read with sscanf).

however, validity of this depends on the buffer space at rpos[-1]
being initialized, which is not the case under some conditions
(including at least unbuffered files and fmemopen ones).

whenever data was read "through the buffer", the desired character
value is already in place and does not need to be written. thus,
rather than testing for the absence of the value, we can test for
rpos<=buf, indicating that the last character read could not have come
from the buffer, and thereby that we have a "real" buffer (possibly of
zero length) with writable pushback (UNGET bytes) below it.

as reported/analyzed by Pascal Cuoq, the shlim and shcnt
macros/functions are called by the scanf core (vfscanf) with f->rpos
potentially null (if the FILE is not yet activated for reading at the
time of the call). in this case, they compute differences between a
null pointer (f->rpos) and a non-null one (f->buf), resulting in
undefined behavior.

it's unlikely that any observably wrong behavior occurred in practice,
at least without LTO, due to limits on what's visible to the compiler
from translation unit boundaries, but this has not been checked.

fix is simply ensuring that the FILE is activated for read mode before
entering the main scanf loop, and erroring out early if it can't be.

TZ containg a timezone name with >TZNAME_MAX characters currently
breaks musl's timezone parsing. getname() stops after TZNAME_MAX
characters. getoff() will consume no characters (because the next
character is not a digit) and incorrectly return 0. Then, because
there are remaining alphabetic characters, __daylight == 1, and
dst_off == -3600.

getname() must consume the entire timezone name, even if it will not
fit in d/__tzname, so when it returns, s points to the offset digits.

Parsing the timezone name must stop when reaching the null terminator.
In that case, there is no '>' to skip.

Commit d9bdfd16 ("fix memccpy to not access buffer past given size")
correctly added a check for 'n' nonzero, but made the pre-existing test
'*s==c' redundant: n!=0 implies *s==c. Remove the unnecessary check.

Reported by Alexey Izbyshev.

as a result of commit b6a6cd70,
the REL_NONE case is now redundant.

Linux defines MAP_SYNC on powerpc and powerpc64 as of commit
22fcea6f85f2 ("mm: move MAP_SYNC to asm-generic/mman-common.h"),
so we can stop undefining it on those architectures.

change the current O(n) lookup to O(1) based on the machinery
described in "How To Write Shared Libraries" (Appendix B).

kernel commit 4693916846269d633a3664586650dbfac2c5562f (first included
in release v4.14) silently fixed a bug whereby the reserved space
(which was later used for high bits of time) in IPC_STAT structures
was left untouched rather than zeroed. this means that a caller that
wants to read the high bits needs to pre-zero the memory.

since it's not clear that these operations are permitted to modify the
destination buffer on failure, use a temp buffer and copy back to the
caller's buffer on success.

on all mips variants, Linux did (and maybe still does) have some
syscall return paths that wrongly return both the error flag in r7 and
a negated error code in r2. in particular this happened for at least
some causes of ENOSYS.

add an extra check to only negate the error code if it's positive to
begin with.

bug report and concept for patch by Andreas Dröscher.

commit 4221f154 added the r7
constraint apparently out of a misunderstanding of the breakage it was
addressing, and did so because the asm was in a shared macro used by
all the __syscallN inline functions. now "+r" is used in the output
section for the forms 4-argument and up, so having it in input is
redundant, and the forms with 0-3 arguments don't need it as an input
at all.

the r2 constraint is kept because without it most gcc versions (seems
to be all prior to 9.x) fail to honor the output register binding for
r2. this seems to be a variant of gcc bug #87733.

both the r7 and r2 input constraints look useless, but the r2 one was
a quiet workaround for gcc bug 87733, which affects all modern
versions prior to 9.x, so it's kept and documented.

exactly revert commit 604f8d3d which
was wrong; it caused a major regression on Linux versions prior to
2.6.36. old kernels did not properly preserve r2 across syscall
restart, and instead restarted with the instruction right before
syscall, imposing a contract that the previous instruction must load
r2 from an immediate or a register (or memory) not clobbered by the
syscall.

effectivly revert commit ddc7c4f9
which was wrong; it caused a major regression on Linux versions prior
to 2.6.36. old kernels did not properly preserve r2 across syscall
restart, and instead restarted with the instruction right before
syscall, imposing a contract that the previous instruction must load
r2 from an immediate or a register (or memory) not clobbered by the
syscall.

since other changes were made since, including removal of the struct
stat conversion that was replaced by separate struct kstat, this is
not a direct revert, only a functional one.

the "0"(r2) input constraint added back seems useless/erroneous, but
without it most gcc versions (seems to be all prior to 9.x) fail to
honor the output register binding for r2. this seems to be a variant
of gcc bug #87733. further changes should be made later if a better
workaround is found, but this one has been working since 2012. it
seems this issue was encountered but misidentified then, when it
inspired commit 4221f154.

these were leftover from early beginnings when arpa/inet.h was not
including netinet/in.h.

this is added for POSIX-future as the outcome of Austin Group issue
599. since it's in the reserved namespace for pthread.h, there are no
namespace considerations for adding it early.

commit 59324c8b added __socketcall
analogous to __syscall, returning the negated error rather than
setting errno. use it to simplify the fallback path of socket(),
avoiding extern calls and access to errno.

Author: Rich Felker <dalias@aerifal.cx>
Date:   Tue Jul 30 17:51:16 2019 -0400

    make __socketcall analogous to __syscall, error-returning

this reverts commit 4ee039f3, which
added the helper as a hack to make vdprintf usable before relocation,
contingent on strong assumptions about the arch and tooling, back when
the dynamic linker did not have a real staged model for
self-relocation. since commit f3ddd173
this has been unnecessary and the function was just wasting size and
execution time.

The final rounding operation should be done with the correct sign
otherwise huge results may incorrectly get rounded to or away from
infinity in upward or downward rounding modes.

This affected sinh and sinhf which set the sign on the result after
a potentially overflowing mul. There may be other non-nearest rounding
issues, but this was a known long standing issue with large ulp error
(depending on how ulp is defined near infinity).

The fix should have no effect on sinh and sinhf performance but may
have a tiny effect on cosh and coshf.

Handle when after reduction |y| > pi/4+tiny. This happens in directed
rounding modes because the fast round to int code does not give the
nearest integer. In such cases the reduction may not be symmetric
between x and -x so e.g. cos(x)==cos(-x) may not hold (but polynomial
evaluation is not symmetric either with directed rounding so fixing
that would require more changes with bigger performance impact).

The fix only adds two predictable branches in nearest rounding mode,
simple ubenchmark does not show relevant performance regression in
nearest rounding mode.

The code could be improved: e.g reducing the medium size threshold
such that two step reduction is enough instead of three, and the
single precision case can avoid the issue by doing the round to int
differently, but this fix was kept minimal.

because struct stat is no longer assumed to correspond to the
structure used by the stat-family syscalls, it's not valid to make any
of these syscalls directly using a buffer of type struct stat.

commit 94938920 moved all logic around
this change for stat-family functions into fstatat.c, making the
others wrappers for it. but a few other direct uses of the syscall
were overlooked. the ones in tmpnam/tempnam are harmless since the
syscalls are just used to test for file existence. however, the uses
in fchmodat and __map_file depend on getting accurate file properties,
and these functions may actually have been broken one or more mips
variants due to removal of conversion hacks from syscall_arch.h.

as a low-risk fix, simply use struct kstat in place of struct stat in
the affected places.

these did not truncate excess precision in the return value. fixing
them looks like considerable work, and the current C code seems to
outperform them significantly anyway.

long double functions are left in place because they are not subject
to excess precision issues and probably better than the C code.

this commit is for the sake of reviewable history.

analogous to commit 1c9afd69 for
atan[2][f].

for functions implemented in C, this is a requirement of C11 (F.6);
strictly speaking that text does not apply to standard library
functions, but it seems to be intended to apply to them, and C2x is
expected to make it a requirement.

failure to drop excess precision is particularly bad for inverse trig
functions, where a value with excess precision can be outside the
range of the function (entire range, or range for a particular
subdomain), breaking reasonable invariants a caller may expect.

this extends commit 5a105f19, removing
timer[fd]_settime and timer[fd]_gettime. the timerfd ones are likely
to have been used in software that started using them before it could
rely on libc exposing functions.

this extends commit 5a105f19, removing
clock_settime, clock_getres, clock_nanosleep, and settimeofday.