libstdc++: Use of non-standard memalign instead of C11 aligned_alloc

[stephanosio]

libstdc++ seems to internally make use of the GNU memalign function instead of the ISO C aligned_alloc function.

Looking at the libstdc++ internals, it should be using the aligned_alloc instead of memalign when aligned_alloc is detected as available:

namespace __gnu_cxx {
#if _GLIBCXX_HAVE_ALIGNED_ALLOC
using ::aligned_alloc;
[...]
#elif _GLIBCXX_HAVE_MEMALIGN
static inline void*
aligned_alloc (std::size_t al, std::size_t sz)
{
  // Solaris requires al >= sizeof a word and QNX requires >= sizeof(void*)
  // but they both provide posix_memalign, so will use the definition above.
  return memalign (al, sz);
}
[...]
  while ((p = __gnu_cxx::aligned_alloc (align, sz)) == nullptr)

Find out why _GLIBCXX_HAVE_ALIGNED_ALLOC is not being defined and make sure it is defined and the non-standard memalign function is not used by libstdc++.

[keith-packard]

Good debugging -- it looks like gcc is just missing an AC_DEFINE(HAVE_ALIGNED_ALLOC) in libstdc++-v3/configure.ac in the newlib section. It has no mechanism to autodetect the available libc API, so it hard-codes the list of available functions and aligned_alloc() is just missing here. Super easy to fix; I'll get a patch made for the SDK.

However, I don't think that eliminates the need for the memalign() alias in the common C library -- every gcc-based toolchain which doesn't have the necessary patch will fail unless that alias is provided. If we can detect when a fixed toolchain is in use, we can skip the definition there.

[keith-packard]

The proposed change is here: zephyrproject-rtos/gcc#22
I haven't tested it.