ucrt_xp — Universal CRT for Windows XP

One ABI-stable C runtime for Windows XP (SP2/SP3, x86): any module, any MSVC version, same ucrt_xp.dll.

Why

Classic CRT state (heap, locale, errno, FILE*, sync) differs across VC versions and cannot be mixed safely in one process. Vista+ APIs (CONDITION_VARIABLE, InitOnceExecuteOnce, …) do not exist on XP. ucrt_xp centralizes those pieces behind a version-checked, XP-safe ABI.

Quick start

#define UCRT_XP_USE_STD_NAMES
#define UCRT_XP_IMPLEMENT_AUTOSTART // For one source only
#include "ucrt_xp_compat.h"

int main(void) {
    printf("hello %d\n", 42);
    return 0;
}

add_subdirectory(ucrt_xp)          # or link the prebuilt DLL

# app.exe
add_executable(app ...)
target_link_libraries(app ucrt_xp) # DLL: headers use dllimport automatically
# or: target_link_libraries(app ucrt_xp_static)  # UCRT_XP_STATIC propagates via CMake

Deploy ucrt_xp.dll next to the EXE (app-local). Static link is also supported via ucrt_xp_static.

UCRT_XP_API (in ucrt_xp.h) selects the attribute: UCRT_XP_BUILD_DLL (DLL build only) → dllexport; UCRT_XP_STATIC → none; otherwise → dllimport. When linking the static library without CMake, define UCRT_XP_STATIC yourself. Do not define UCRT_XP_BUILD_DLL in applications.

Coverage (high level)

Area	Notes
Heap	malloc/free, LFH + per-thread small-object cache
Locale	Thread-local, refcounted, immutable; strcoll/strxfrm/_stricoll
Sync	Vista-style CONDITION_VARIABLE / InitOnce* (native on Vista+, fallback on XP)
stdio	fd table + buffered FILE*, text CRLF, stdin/stdout/stderr
printf / scanf	Integer, float, string, wide variants
string / mem	Full str*/mem* + MSVC extras (_strlwr, _memicmp, …)
wide	wcs*, _wopen/_wfopen, wide printf/scanf, wide fs/process
ctype / wctype	Standard + iscsym, isw*, tow*
convert	strtol/strtoll/strtod/strtof, _itoa, rand/rand_s
process	getenv/_putenv, _pipe/_popen, spawn/exec, _beginthread*
time	64-bit time_t, reentrant localtime/gmtime, strftime, asctime
fs	_access/_stat/_findfirst, _fullpath/_splitpath, wide path helpers
fd	_dup/_dup2/_setmode/_get_osfhandle/_eof/_lseeki64
locale names	setlocale / localeconv
multibyte	mblen/mbtowc/mbstowcs/wcstombs
exception	SEH trampoline + ucrt_xp_guarded_call
assert	assert macro → ucrt_xp_assert
secure CRT	*_s string/mem/printf/scanf/file/time/path/itoa

Not covered (by design): <math.h> transcendentals (use the compiler’s), setjmp/longjmp (compiler intrinsics). Secure CRT *_s is included (string/mem/printf/file/time/mb).

Design rules that matter

1. ABI negotiation — call ucrt_xp_init(UCRT_XP_ABI_VERSION) once. Mismatch → diagnostic and hard exit (no silent partial compat).
2. Append-only ABI — never change existing signatures or struct layouts; only append; bump UCRT_XP_ABI_MINOR (or MAJOR + new DLL name for breaks). Ordinals in ucrt_xp.def are never reused.
3. No host CRT stdio/heap — implementations talk to Win32 directly so behavior does not depend on which msvcrXX.dll a module linked.
4. Thread-local where it counts — errno, locale, rand state; _beginthread* runs CRT attach/detach (errno, locale, heap cache).
5. XP-only APIs — _WIN32_WINNT=0x0501; no Vista+ imports at link time. Vista-shaped sync is resolved at runtime when present.

Layout

include/ucrt_xp.h         Public C ABI
include/ucrt_xp_compat.h  Optional `#define UCRT_XP_USE_STD_NAMES` macros
include/ucrt_xp.hpp       Optional C++ RAII helpers
src/                      Implementation + ucrt_xp.def
tests/                    test_basic, test_sync
examples/                 hello.c / hello.cpp

Build

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
ctest --test-dir build

Targets XP subsystem (5.01) when built with MSVC.

ABI checklist (before changing ucrt_xp.h)

1. Do not change existing function signatures or struct layouts.
2. Only append; bump UCRT_XP_ABI_MINOR.
3. Breaking change → bump UCRT_XP_ABI_MAJOR and ship a new DLL name.
4. Every new export gets the next ordinal in ucrt_xp.def.

Current ABI: 1.13 (UCRT_XP_ABI_MAJOR.MINOR).