otherwise offs in ucontext_t will be wrong, and break code that
inspects or modifies the signal makes (including cancellation code).

after the thread unmaps its own stack/thread structure, the kernel,
performing child tid clear and futex wake, could clobber a new mapping
made at the same location as the just-removed thread's tid field.
disable kernel clearing of child tid to prevent this.

the mips abi reserves stack space equal to the size of the in-register
args for the callee to save the args, if desired. this would cause the
beginning of the thread structure to be clobbered...

the old code worked in qemu app-level emulation, but not on real
kernels where the clone syscall does not copy the register values to
the new thread. save arguments on the new thread stack instead.

with this change, threads on mips seem to be working

on other archs, like x86[_64], asm version is required

basically, this version of the code was obtained by starting with
rdp's work from his ellcc source tree, adapting it to musl's build
system and coding style, auditing the bits headers for discrepencies
with kernel definitions or glibc/LSB ABI or large file issues, fixing
up incompatibility with the old binutils from aboriginal linux, and
adding some new special cases to deal with the oddities of sigaction
and pipe syscall interfaces on mips.

at present, minimal test programs work, but some interfaces are broken
or missing. threaded programs probably will not link.

the type doesn't actually matter, just the size, but it's nice to be
consistent...

this file can be overridden by a same-named file in an arch dir.

if libc.a is compiled PIC for use in static PIE code, this should not
cause the dynamic linker (which still does not support static-linked
main program) to be built into libc.a.

most importantly, the name for such libs was being set from an
uninitialized buffer. also, shortname always had an initial '/'
character, making it useless for looking up already-loaded libraries
by name, and thus causing repeated searches through the library path.

major changes now:

- shortname is the base name for library lookups with no explicit
  pathname. it's initially clear for libraries loaded with an explicit
  pathname (and for the main program), but will be set if the same
  library (detected via inodes match) is later found by a search.

- exact name match is never used to identify libraries loaded with an
  explicit pathname. in this case, there's no explicit search, so we
  can just stat the file and check for inode match.

apparently somebody wants this for something... and it doesn't hurt.

no need to pass zero for unused arguments; just omit them.

this hidden endian dependency had left big endian arm badly broken.

previously this was being handled the same as a library-specific,
dependency-order lookup on the next library in the global chain, which
is likely to be utterly meaningless. instead the lookup needs to be in
the global namespace, but omitting the initial portion of the global
library chain up through the calling library.

this is not a standard but it's the traditional behavior and it's more
useful because the caller can reliably detect errors.

this option is expensive and only used on old gcc's that lack
-fexcess-precision=standed, but it's not needed on non-i386 archs
where floating point does not have excess precision anyway.

if musl ever supports m68k, i think it will need to be special-cased
too. i'm not aware of any other archs with excess precision.

on arm, the location of the saved-signal-mask flag and mask were off
by one between sigsetjmp and siglongjmp, causing incorrect behavior
restoring the signal mask. this is because the siglongjmp code assumed
an extra slot was in the non-sig jmp_buf for the flag, but arm did not
have this. now, the extra slot is removed for all archs since it was
useless.

also, arm eabi requires jmp_buf to have 8-byte alignment. we achieve
that using long long as the type rather than with non-portable gcc
attribute tags.

no idea why gcc refuses to compile the C code to use a tail call, but
it's best to use asm anyway so we don't have to rely on the quality of
the compiler's optimizations for correct code.

the new version is largely the work of Solar Designer, with minor
changes for integration with musl. compared to the old code, text size
is reduced by about 7k, stack space usage by about 70k, and
performance is greatly improved by avoiding expensive calculation of
constant tables on each run.

this version also adds support for extended des-based password hashes,
which allow for unlimited key (password) length and configurable
iteration counts.

i've also published the interface for crypt_r in a new crypt.h header.
especially since this is not a standard interface, i did not feel
compelled to match the glibc abi for the crypt_data structure. the
glibc structure is way too big to allocate on the stack; in fact it's
so big that the first usage may cause the main thread to exceed its
pre-committed stack size of 128k and thus could cause the program to
crash even on systems with overcommit disabled. the only legitimate
use of crypt_data for crypt_r is to store the hash string to return,
so i've reserved 256 bytes, which should be more than sufficient
(longest known password hashes are ~60 characters, and beyond that is
possibly even exceeding some implementations' passwd file field size
limit).

lr must be saved because init/fini-section code from the compiler
clobbers it. this was not a problem when i tested without gcc's
crtbegin/crtend files present, but with them, musl on arm fails to
work (infinite loop in _init).

based on a patch submitted by Kristian L. <email@thexception.net>

patch submitted by Kristian L. <email@thexception.net>

on old kernels, there's no way to detect errors; we must assume
negative syscall return values are pgrp ids. but if the F_GETOWN_EX
fcntl works, we can get a reliable answer.

The long double adjustment was wrong:
The usual check is
  mant_bits & 0x7ff == 0x400
before doing a mant_bits++ or mant_bits-- adjustment since
this is the only case when rounding an inexact ld80 into
double can go wrong. (only in nearest rounding mode)

After such a check the ++ and -- is ok (the mantissa will end
in 0x401 or 0x3ff).

fma is a bit different (we need to add 3 numbers with correct
rounding: hi_xy + lo_xy + z so we should survive two roundings
at different places without precision loss)

The adjustment in fma only checks for zero low bits
  mant_bits & 0x3ff == 0
this way the adjusted value is correct when rounded to
double or *less* precision.
(this is an important piece in the fma puzzle)

Unfortunately in this case the -- is not a correct adjustment
because mant_bits might underflow so further checks are needed
and this was the source of the bug.

unicode char data has both "W" and "F" wide types and the old table
only included the "W" ones. this omitted U+3000 (ideographic space)
and all the wide-ascii, etc.

this is silly, but it makes apps that read binary junk and interpret
it as ld80 "safer", and it gets gnulib to stop replacing printf...

it should return the error code rather than 0/-1 and setting errno.