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a _REDIR_TIME64 macro is introduced, which the arch's alltypes.h is
expected to define, to control redirection of symbol names for
interfaces that involve time_t and derived types. this ensures that
object files will only be linked to libc interfaces matching the ABI
whose headers they were compiled against.
along with time32 compat shims, which will be introduced separately,
the redirection also makes it possible for a single libc (static or
shared) to be used with object files produced with either the old
(32-bit time_t) headers or the new ones after 64-bit time_t switchover
takes place. mixing of such object files (or shared libraries) in the
same program will also be possible, but must be done with care; ABI
between libc and a consumer of the libc interfaces is guaranteed to
match by the the symbol name redirection, but pairwise ABI between
consumers of libc that define interfaces between each other in terms
of time_t is not guaranteed to match.
this change adds a dependency on an additional "GNU C" feature to the
public headers for existing 32-bit archs, which is generally
undesirable; however, the feature is one which glibc has depended on
for a long time, and thus which any viable alternative compiler is
going to need to provide. 64-bit archs are not affected, nor will
future 32-bit archs be, regardless of whether they are "new" on the
kernel side (e.g. riscv32) or just newly-added (e.g. a new sparc or
xtensa port). the same applies to newly-added ABIs for existing
machine-level archs.
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the memory model we use internally for atomics permits plain loads of
values which may be subject to concurrent modification without
requiring that a special load function be used. since a compiler is
free to make transformations that alter the number of loads or the way
in which loads are performed, the compiler is theoretically free to
break this usage. the most obvious concern is with atomic cas
constructs: something of the form tmp=*p;a_cas(p,tmp,f(tmp)); could be
transformed to a_cas(p,*p,f(*p)); where the latter is intended to show
multiple loads of *p whose resulting values might fail to be equal;
this would break the atomicity of the whole operation. but even more
fundamental breakage is possible.
with the changes being made now, objects that may be modified by
atomics are modeled as volatile, and the atomic operations performed
on them by other threads are modeled as asynchronous stores by
hardware which happens to be acting on the request of another thread.
such modeling of course does not itself address memory synchronization
between cores/cpus, but that aspect was already handled. this all
seems less than ideal, but it's the best we can do without mandating a
C11 compiler and using the C11 model for atomics.
in the case of pthread_once_t, the ABI type of the underlying object
is not volatile-qualified. so we are assuming that accessing the
object through a volatile-qualified lvalue via casts yields volatile
access semantics. the language of the C standard is somewhat unclear
on this matter, but this is an assumption the linux kernel also makes,
and seems to be the correct interpretation of the standard.
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the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.
installation docs have also been updated to reflect this change.
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to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.
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this is ugly and stupid, but now that the *64 symbol names exist, a
lot of broken GNU software detects them in configure, then either
breaks during build due to missing off64_t definition, or attempts to
compile without function declarations/prototypes. "fixing" it here is
easier than telling everyone to add yet another feature test macro to
their builds.
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musl does not support legacy 32-bit-off_t whatsoever. off_t is always
64 bit, and correct programs that use off_t and the standard functions
will just work out of the box. (on glibc, they would require
-D_FILE_OFFSET_BITS=64 to work.) however, some programs instead define
_LARGEFILE64_SOURCE and use alternate versions of all the standard
types and functions with "64" appended to their names.
we do not want code to actually get linked against these functions
(it's ugly and inconsistent), so macros are used instead of prototypes
with weak aliases in the library itself. eventually the weak aliases
may be added at the library level for the sake of using code that was
originally built against glibc, but the macros will still be the
desired solution in the headers.
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previous fix was backwards and propagated the wrong type rather than
the right one...
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some features are not yet supported, and only minimal testing has been
performed. should be considered experimental at this point.
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