Description:kernelspace musl code

Team:OTHERS
Feature or Bugfix:Feature
Binary Source:NA
PrivateCode(Yes/No):No

Change-Id: I99874a7c570b7d22b4a3d34840260eb48ea3ffa1
Reviewed-on: http://mgit-tm.rnd.huawei.com/10276995Reviewed-by: Nshenwei 00579521 <denny.shenwei@huawei.com>
Tested-by: Npublic jenkins <public_jenkins@notesmail.huawei.com>

commit ddc947ed fixed the
corresponding bug for exit which was introduced when commit
0b80a7b0 added support for
caller-provided buffers, making it possible for stderr to be a
buffered stream.

morally, for null pointers a and b, a-b, a<b, and a>b should all be
defined as 0; however, C does not define any of them.

the stdio implementation makes heavy use of such pointer comparison
and subtraction for buffer logic, and also uses null pos/base/end
pointers to indicate that the FILE is not in the corresponding (read
or write) mode ready for accesses through the buffer.

all of the comparisons are fixed trivially by using != in place of the
relational operators, since the opposite relation (e.g. pos>end) is
logically impossible. the subtractions have been reviewed to check
that they are conditional the stream being in the appropriate reading-
or writing-through-buffer mode, with checks added where needed.

in fgets and getdelim, the checks added should improve performance for
unbuffered streams by avoiding a do-nothing call to memchr, and should
be negligible for buffered streams.

commit c002668e inadvertently moved
the check for unflushed write buffer outside of the scope of the
existing lock.

previously, fflush_unlocked was an alias for an internal backend that
was called by fflush, either for its argument or in a loop for each
file if a null pointer was passed. since the logic for the latter was
in the main fflush function, fflush_unlocked crashed when passed a
null pointer, rather than flushing all open files. since
fflush_unlocked is not a standard function and has no specification,
it's not clear whether it should be expected to accept null pointers
like fflush does, but a reasonable argument could be made that it
should.

this patch eliminates the helper function, simplifying fflush, and
makes fflush_unlocked an alias for fflush, which is valid because the
two functions agree in their behavior in all cases where their
behavior is defined (the unlocked version has undefined behavior if
another thread could hold locks).

functions which open in-memory FILE stream variants all shared a tail
with __fdopen, adding the FILE structure to stdio's open file list.
replacing this common tail with a function call reduces code size and
duplication of logic. the list is also partially encapsulated now.

function signatures were chosen to facilitate tail call optimization
and reduce the need for additional accessor functions.

with these changes, static linked programs that do not use stdio no
longer have an open file list at all.

previously we detected this bug in configure and issued advice for a
workaround, but this turned out not to work. since then gcc 4.9.0 has
appeared in several distributions, and now 4.9.1 has been released
without a fix despite this being a wrong code generation bug which is
supposed to be a release-blocker, per gcc policy.

since the scope of the bug seems to affect only data objects (rather
than functions) whose definitions are overridable, and there are only
a very small number of these in musl, I am just changing them from
const to volatile for the time being. simply removing the const would
be sufficient to make gcc 4.9.1 work (the non-const case was
inadvertently fixed as part of another change in gcc), and this would
also be sufficient with 4.9.0 if we forced -O0 on the affected files
or on the whole build. however it's cleaner to just remove all the
broken compiler detection and use volatile, which will ensure that
they are never constant-folded. the quality of a non-broken compiler's
output should not be affected except for the fact that these objects
are no longer const and thus possibly add a few bytes to data/bss.

this change can be reconsidered and possibly reverted at some point in
the future when the broken gcc versions are no longer relevant.

there is no need/use for a flush hook. the write function serves this
purpose already. i originally created the hook for implementing mem
streams based on a mistaken reading of posix, and later realized it
wasn't useful but never removed it until now.

previously, stdio used spinlocks, which would be unacceptable if we
ever add support for thread priorities, and which yielded
pathologically bad performance if an application attempted to use
flockfile on a key file as a major/primary locking mechanism.

i had held off on making this change for fear that it would hurt
performance in the non-threaded case, but actually support for
recursive locking had already inflicted that cost. by having the
internal locking functions store a flag indicating whether they need
to perform unlocking, rather than using the actual recursive lock
counter, i was able to combine the conditionals at unlock time,
eliminating any additional cost, and also avoid a nasty corner case
where a huge number of calls to ftrylockfile could cause deadlock
later at the point of internal locking.

this commit also fixes some issues with usage of pthread_self
conflicting with __attribute__((const)) which resulted in crashes with
some compiler versions/optimizations, mainly in flockfile prior to
pthread_create.

the biggest change in this commit is that stdio now uses readv to fill
the caller's buffer and the FILE buffer with a single syscall, and
likewise writev to flush the FILE buffer and write out the caller's
buffer in a single syscall.

making this change required fundamental architectural changes to
stdio, so i also made a number of other improvements in the process:

- the implementation no longer assumes that further io will fail
  following errors, and no longer blocks io when the error flag is set
  (though the latter could easily be changed back if desired)

- unbuffered mode is no longer implemented as a one-byte buffer. as a
  consequence, scanf unreading has to use ungetc, to the unget buffer
  has been enlarged to hold at least 2 wide characters.

- the FILE structure has been rearranged to maintain the locations of
  the fields that might be used in glibc getc/putc type macros, while
  shrinking the structure to save some space.

- error cases for fflush, fseek, etc. should be more correct.

- library-internal macros are used for getc_unlocked and putc_unlocked
  now, eliminating some ugly code duplication. __uflow and __overflow
  are no longer used anywhere but these macros. switch to read or
  write mode is also separated so the code can be better shared, e.g.
  with ungetc.

- lots of other small things.