C11 defined the optional secure extensions to be demanded by __STDC_WANT_LIB_EXT1__.

From the following tested libc implementations:

glibc
musl
FreeBSD and DragonFly libc
FreeBSD-derived darwin libc
OpenBSD libc
newlib
dietlibc
uClibc
minilibc
Microsoft Windows w/ secure API
Android Bionic
Embarcadero C++ libc

only the last 3 implement the secure C11 extensions:

Microsoft Windows
Android Bionic
Embarcadero C++ libc

General quirks

Generally most libc's use a 4 byte wchar_t (which resembles UTF-32), but windows/cygwin/solaris/aix use 2-byte (UTF-16), therefore they need to support surrogate pairs.

The wide sprintf variants do not allow a NULL buffer argument to return the size of the resulting buffer. If the initial buffer is too small, you need to realloc and redo.

There's still no locale-independent utf8 support, not even in C11 with its new u8"" type.

Many wchar and mb conversions and searches are locale-dependent, hence unusable for utf8.

Nobody implements a proper wchar case-conversion, foldcase and normalization API. ICU or libunistring are way too heavy for this simple problem. There's only utf8proc for utf8 encoded strings (now with julia).

C11 Annex K/safec caveats

snprintf, vsnprintf, snwprintf, vsnwprintf, tmpnam_s:

They are all considered unsafe. The 4 'n' truncating printf versions don't guarantee null-delimited destination buffers.
tmpnam_s and tmpnam are racy.

Microsoft Windows/MINGW_HAS_SECURE_API

fopen_s, freopen_s deviate in the API: restrict is missing.
strtok_s, wcstok_s,vsnprintf_s miss the dmax argument.
vsnprintf_s adds a maxarg argument.
vswprintf adds a maxarg argument on w32. (with __STRICT_ANSI__ undefined)
no strnlen on mingw32.
no errno_t return type for qsort_s, only void.
reversed argument order for localtime_s and gmtime_s.
older mingw versions have wchar.h with only 2 functions: wcscmp, wcslen

safeclib

safeclib does not check optional NULL parameters to the vararg scanf_s and printf_s functions. This would need tighter integration into the upstream libc.
safeclib fgets_s permits temporary writes of dmax+1 characters into dest.

Other caveats

glibc

SEGV with freopen(NULL, "rb", stdin) with asan on some systems, calling an invalid strlen() on NULL.
quirky declaration of various standards, which conflict with each other.

glibc needs the correct standard to include some extensions when we declare the standard by ourselves. e.g. clang-4.0 -std=c99 misses several reentrant versions. when defining _XOPEN_SOURCE 700 to define strnlen and the reentrant time versions, we need also __STDC_WANT_LIB_EXT1__ 1 for errno_t, and this would break several other older struct members, such as tm_gmtoff

newlib

vswscanf is broken with a format string containing L"%%n"
The following multibyte API's are missing, and can be defined like this:

mbstate_t st;
#define wctomb(dest, wc) wcrtomb((dest), (wc), &st)
#define wcstombs(dest, src, len) wcsrtombs((dest), &(src), (len), &st)
#define mbstowcs(dest, src, len) mbsrtowcs((dest), &(src), (len), &st)

FreeBSD libc

vswscanf is broken with a format string containing L"%%n"
mbstowcs is broken with (NULL, '\0')

musl

wmemcmp returns not -1,0,1 but the full ptr diff.
mbtowc and wctomb accept and convert illegal 4 byte characters in the ASCII locale to surrogate pairs, as it would be unicode. e.g. it converts \xa0 to \xdfa0.

It's now 10 years after the secure libc extensions were designed, C11 adopted them, and still almost nobody implements them. This library was written 2008 under the MIT license, thanks Cisco.

Reini Urban 2017