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The implementation of memalign(3) uses _BSD_SOURCE, so we will too.

renameat(2) is supposed to ignore if old or new is a hardlink to the other.
However, the way this is written, it also ignores if old or new are symlinks
to one another.  This is incorrect.

- Fail if old or new is longer than PATH_MAX with ENAMETOOLONG.

- Fail if old or new is 0 bytes long.

- Fail if old or new has dot or dot-dot as the last component of their
  path name.

The Linux kernel provides none of these behaviours.

These interfaces are all scheduled for addition in Issue 8.

They cannot be available if _XOPEN_SOURCE is <= 700, or
_POSIX_C_SOURCE is <= 200809L.

Quentin Rameau authored Mar 11, 2018 and A. Wilcox committed Jan 08, 2022

As Rick stated, this isn't a clean solution because argv can be
arbirtary long and overflow the stack.

I post it here in case you'd find it useful anyway.

The timespec_get function, and TIME_* macros, are only in C11.

Since musl is compiled with -std=c99, TIME_UTC is unavailable in the
timespec_get implementation, so we use the raw value 1.

aligned_alloc, at_quick_exit, and quick_exit are new in C11 and C++11.
Only make these symbols visible in those versions, to avoid polluting
the namespace of C99 and POSIX 2008 sources.

This is not a POSIX function, it should not be visible there.

Right now, this is a worst-case assumption; some kernels may actually
have a value of 100000 here (100 Hz timers).  This is considered the
easiest implementation.

This is required to be present in <unistd.h>.

This is used to determine what CFLAGS/LDFLAGS are needed to enable
compilation with threads on musl.  We don't have any special ones, so
just return an empty string.

This is required to be present in <unistd.h>.

We definitely don't /support/ UUCP, so return -1 for it.
But this value is required to be present in <unistd.h>.

This should be safer for casting and more compatible with existing code
bases that wrongly assume it must be defined as a pointer.

commit 7be59733 introduced the
hwcap-based branches to support the SPE FPU, but wrongly coded them as
bitwise tests on the computed address of __hwcap, not a value loaded
from that address. replace the add with indexed load to fix it.

the snd_pcm_mmap_control struct used with SNDRV_PCM_IOCTL_SYNC_PTR was
mistakenly defined in the kernel uapi with "before u32" padding both
before and after the first u32 member. our conversion between the
modern struct and the legacy time32 struct was written without
awareness of that mistake, and assumed the time64 version of the
struct was the intended form with padding to match the layout on
64-bit archs. as a result, the struct was not converted correctly when
running on old kernels, with audio glitches as the likely result.

this was discovered thanks to a related bug in the kernel, whereby
32-bit userspace running on a 64-bit kernel also suffered from the
types mismatching. the mistaken layout is now the ABI and can't be
changed -- or at least making a new ioctl to change it would just
result in a worse situation.

our conversion here is changed to treat the snd_pcm_mmap_control
substruct as two separate substructs at locations dependent on
endianness (since the displacement depends on endianness), using the
existing conversion framework.

we make qsort a wrapper by providing a wrapper_cmp function that uses
the extra argument as a function pointer. should be optimized to a tail
call on most architectures, as long as it's built with
-fomit-frame-pointer, so the performance impact should be minimal.

to keep the git history clean, for now qsort_r is implemented in qsort.c
and qsort is implemented in qsort_nr.c.  qsort.c also received a few
trivial cleanups, including replacing (*cmp)() calls with cmp().
qsort_nr.c contains only wrapper_cmp and qsort as a qsort_r wrapper
itself.

When the soft-float ABI for PowerPC was added in commit
5a92dd95, with Freescale cpus using
the alternative SPE FPU as the main use case, it was noted that we
could probably support hard float on them, but that it would involve
determining some difficult ABI constraints. This commit is the
completion of that work.

The Power-Arch-32 ABI supplement defines the ABI profiles, and indeed
ATR-SPE is built on ATR-SOFT-FLOAT. But setjmp/longjmp compatibility
are problematic for the same reason they're problematic on ARM, where
optional float-related parts of the register file are "call-saved if
present". This requires testing __hwcap, which is now done.

In keeping with the existing powerpc-sf subarch definition, which did
not have fenv, the fenv macros are not defined for SPE and the SPEFSCR
control register is left (and assumed to start in) the default mode.

both passing a null pointer to memcpy with length 0, and adding 0 to a
null pointer, are undefined. in some sense this is 'benign' UB, but
having it precludes use of tooling that strictly traps on UB. there
may be better ways to fix it, but conditioning the operations which
are intended to be no-ops in the k==0 case on k being nonzero is a
simple and safe solution.

commit 6d99ad91 introduced this
regression as part of a larger change, based on an incorrect
assumption that rdhwr being part of the mips r2 ISA level meant that
the TLS register, known in the mips documentation as UserLocal, was
unconditionally present on chips providing this ISA level and would
not need trap-and-emulate. this turns out to be false.

based on research by Stanislav Kljuhhin and Abilio Marques, who
reported the problem as a performance regression on certain routers
using OpenWRT vs older uclibc-based versions, it turns out the mips
manuals document the UserLocal register as a feature that might or
might not be implemented or enabled, reflected by a cpu capability bit
in the CONFIG3 register, and that Linux checks for this and has to
explicitly enable it on models that have it.

thus, it's indeed possible that r2+ chips can lack the feature,
bringing us back to the situation where Linux only has a fast
trap-and-emulate path for the case where the destination register is
$3. so, always read the thread pointer through $3. this may incur a
gratuitous move to the desired final register on chips where it's not
needed, but it really doesn't matter.

len is unsigned and can never be smaller than 0. though unlikely, an
error in read() would have lead to an out of bounds write to name.
Reported-by: Michael Forney <mforney@mforney.org>

due to historical reasons, the mips signal set has 128 bits rather
than 64 like on every other arch. this was special-cased correctly, at
least for 32-bit mips, at one time, but was inadvertently broken in
commit 7c440977, and seems never to
have been right on mips64/n32.

as consequenct of this bug, applications making use of high realtime
signal numbers on mips may have been able to execute application code
in contexts where doing so was unsafe.

the kernel structure has padding of the shm_segsz member up to 64
bits, as well as 2 unused longs at the end. somehow that was
overlooked when the powerpc port was added, and it has been broken
ever since; applications compiled with the wrong definition do not
correctly see the shm_segsz, shm_cpid, and shm_lpid members.

fixing the definition just by adding the missing padding would break
the ABI size of the structure as well as the position of the time64
shm_atime and shm_dtime members we added at the end. instead, just
move one of the unused padding members from the original end (before
time64) of the structure to the position of the missing padding. this
preserves size and preserves correct behavior of any compiled code
that was already working. programs affected by the wrong definition
need to be recompiled with the correct one.

previously, the contents of the TZ variable were considered a
candidate for a file/path name only if they began with a colon or
contained a slash before any comma. the latter was very sloppy logic
to avoid treating any valid POSIX TZ string as a file name, but it
also triggered on values that are not valid POSIX TZ strings,
including 3-letter timezone names without any offset.

instead, only treat the TZ variable as POSIX form if it begins with a
nonzero standard time name followed by +, -, or a digit.

also, special case GMT and UTC to always be treated as POSIX form
(with implicit zero offset) so that a stray file by the same name
cannot break software that depends on setting TZ=GMT or TZ=UTC.

on riscv64 this syscall is called __NR_newfstatat
this helps the name match kernel UAPI for external
programs

the function already returns (void *)

based on the pthread_setname_np implementation

POSIX places an obscure requirement on popen which is like a limited
version of close-on-exec:

    "The popen() function shall ensure that any streams from previous
    popen() calls that remain open in the parent process are closed in
    the new child process."

if the POSIX-future 'e' mode flag is passed, producing a pipe FILE
with FD_CLOEXEC on the underlying pipe, this requirement is
automatically satisfied. however, for applications which use multiple
concurrent popen pipes but don't request close-on-exec, fd leaks from
earlier popen calls to later ones could produce deadlock situations
where processes are waiting for a pipe EOF that will never happen.

to fix this, iterate through all open FILEs and add close actions for
those obtained from popen. this requires holding a lock on the open
file list across the posix_spawn call so that additional popen FILEs
are not created after the list is traversed. note that it's still
possible for another popen call to start and create its pipe while the
lock is held, but such pipes are created with O_CLOEXEC and only drop
close-on-exec status (when 'e' flag is omitted) under control of the
lock.

the newly allocated FILE * has not yet leaked to the application and
is only visible to stdio internals until popen returns. since we do
not change any fields of the structure observed by libc internals,
only the pipe_pid member, locking is not necessary.