this change eliminates the internal __memalign function and makes the memalign and posix_memalign functions completely independent of the malloc implementation, written portably in terms of aligned_alloc.
libc.h was intended to be a header for access to global libc state and related interfaces, but ended up included all over the place because it was the way to get the weak_alias macro. most of the inclusions removed here are places where weak_alias was needed. a few were recently introduced for hidden. some go all the way back to when libc.h defined CANCELPT_BEGIN and _END, and all (wrongly implemented) cancellation points had to include it. remaining spurious users are mostly callers of the LOCK/UNLOCK macros and files that use the LFS64 macro to define the awful *64 aliases. in a few places, new inclusion of libc.h is added because several internal headers no longer implicitly include libc.h. declarations for __lockfile and __unlockfile are moved from libc.h to stdio_impl.h so that the latter does not need libc.h. putting them in libc.h made no sense at all, since the macros in stdio_impl.h are needed to use them correctly anyway.
commit 618b18c78e33acfe54a4434e91aa57b8e171df89 removed the previous detection and hardening since it was incorrect. commit 72141795d4edd17f88da192447395a48444afa10 already handled all that remained for hardening the static-linked case. in the dynamic-linked case, have the dynamic linker check whether malloc was replaced and make that information available. with these changes, the properties documented in commit c9f415d7ea2dace5bf77f6518b6afc36bb7a5732 are restored: if calloc is not provided, it will behave as malloc+memset, and any of the memalign-family functions not provided will fail with ENOMEM.
this change serves multiple purposes: 1. it ensures that static linking of memalign-family functions will pull in the system malloc implementation, thereby causing link errors if an attempt is made to link the system memalign functions with a replacement malloc (incomplete allocator replacement). 2. it eliminates calls to free that are unpaired with allocations, which are confusing when setting breakpoints or tracing execution. as a bonus, making __bin_chunk external may discourage aggressive and unnecessary inlining of it.
the generated code should be mostly unchanged, except for explicit use of C_INUSE in place of copying the low bits from existing chunk headers/footers. these changes also remove mild UB due to dubious arithmetic on pointers into imaginary size_t[] arrays.
commit c9f415d7ea2dace5bf77f6518b6afc36bb7a5732 included checks to make calloc fallback to memset if used with a replaced malloc that didn't also replace calloc, and the memalign family fail if free has been replaced. however, the checks gave false positives for replacement whenever malloc or free resolved to a PLT entry in the main program. for now, disable the checks so as not to leave libc in a broken state. this means that the properties documented in the above commit are no longer satisfied; failure to replace calloc and the memalign family along with malloc is unsafe if they are ever called. the calloc checks were correct but useless for static linking. in both cases (simple or full malloc), calloc and malloc are in a source file together, so replacement of one but not the other would give linking errors. the memalign-family check was useful for static linking, but broken for dynamic as described above, and can be replaced with a better link-time check.
replacement is subject to conditions on the replacement functions. they may only call functions which are async-signal-safe, as specified either by POSIX or as an implementation-defined extension. if any allocator functions are replaced, at least malloc, realloc, and free must be provided. if calloc is not provided, it will behave as malloc+memset. any of the memalign-family functions not provided will fail with ENOMEM. in order to implement the above properties, calloc and __memalign check that they are using their own malloc or free, respectively. choice to check malloc or free is based on considerations of supporting __simple_malloc. in order to make this work, calloc is split into separate versions for __simple_malloc and full malloc; commit ba819787ee93ceae94efd274f7849e317c1bff58 already did most of the split anyway, and completing it saves an extra call frame. previously, use of -Bsymbolic-functions made dynamic interposition impossible. now, we are using an explicit dynamic-list, so add allocator functions to the list. most are not referenced anyway, but all are added for completeness.
the case where mem was already aligned is handled earlier in the function now.