Custom ICU4C Synchronization


  1. Build Time User Provided Synchronization

:warning: Support for including an alternate implementation of atomic and mutex operations has been withdrawn and removed from ICU4C. See issue ICU-20185.

Build Time User Provided Synchronization

Build time user synchronization provides a mechanism for platforms with special requirements to provide their own mutex and one-time initialization implementations to ICU. This facility was introduced in ICU 53. It may change over time.

The alternative implementations are compiled directly into the ICU libraries. Alternative implementations cannot be plugged in at run time.

The tables below show the items that must be defined by a custom ICU synchronization implementation. The list includes both functions that are used throughout ICU code and additional functions are for internal by other ICU synchronization primitives.

Low Level Atomics, a set of platform or compiler dependent typedefs and inlines. Provided in the internal header file umutex.h.

Type/Function Description
typedef u_atomic_int32_t A 32 bit integer that will work with low level atomic operations. (typedef)
umtx_loadAcquire(u_atomic_int32_t &var)  
umtx_storeRelease(u_atomic_int32_t &var, int32_t val)  
umtx_atomic_inc(u_atomic_int32_t &var)  
umtx_atomic_dec(u_atomic_int32_t &var)  

Mutexes. Type declarations for ICU mutex wrappers. Provided in a header file.

Type Description
struct UMutex An ICU mutex. All instances will be static. Typically just contains an underlying platform mutex.
U_MUTEX_INITIALIZER A C style initializer for a static instance of a UMutex.

Mutex and InitOnce implementations. Out-of-line platform-specific code. Provided in a .cpp file.

Function Description
umtx_lock(UMutex *mutex) Lock a mutex.
umtx_unlock(UMutex* mutex) Unlock a mutex.
umtx_initImplPreInit(UInitOnce &uio) umtx_initOnce() implementation function.
umtx_initImplPostInit(UInitOnce &uio) umtx_initOnce() implementation function.

UInitOnce and umtx_initOnce() are used internally by ICU for thread-safe one-time initialization. Their implementation is split into a platform-independent part (contained in umutex.h), and the pair of platform-dependent implementation functions listed above.

Build Setup

Compiler preprocessor variables are used to name the custom files to be included in the ICU build. If defined, the files are included at the top of the normal platform #ifdef chains in the ICU sources, and effectively define a new platform.

Macro Description
U_USER_ATOMICS_H Set to the name of the low level atomics header file.
U_USER_MUTEX_H Mutexes header file.
U_USER_MUTEX_CPP Mutexes and InitOnce implementation file.

It is possible (and reasonable) to supply only the two mutex files, while retaining the ICU default implementation for the low level atomics.

Example ICU configure with user mutexes specified:

CPPFLAGS='-DU_USER_ATOMICS_H=atomic_c11.h -DU_USER_MUTEX_H=mutex_c11.h -DU_USER_MUTEX_CPP=mutex_c11.cpp' ./runConfigureICU --enable-debug Linux


This interface may change between ICU releases. The required set of functions may be extended, or details of the behavior required may be altered.

The types and functions defined by this interface reach deeply into the ICU implementation, and we need to retain the ability to make changes should the need arise.


The code below shows a complete set of ICU user synchronization files.

This implementation uses C++11 language mutexes and atomics. These make for a convenient reference implementation because the C++11 constructs are well defined and straight forward to use.

Similar implementations for POSIX and Windows can be found in files common/umutex.h and common/umutex.cpp, in the platform #ifdef chains; these are part of the standard ICU distribution.

Mutex Header

// Example of an ICU build time customized mutex header.
// Must define struct UMutex and an initializer that will work with static instances.
// All UMutex instances in ICU code will be static.

#ifndef ICU_MUTEX_C11_H
#define ICU_MUTEX_C11_H
#include <mutex>
#include <condition_variable>
struct UMutex {
    std::mutex fMutex;

Atomics Header

#include <atomic>

typedef std::atomic<int32_t> u_atomic_int32_t;

inline int32_t umtx_loadAcquire(u_atomic_int32_t &var) {
    return var.load(std::memory_order_acquire);

inline void umtx_storeRelease(u_atomic_int32_t &var, int32_t val) {, std::memory_order_release);

inline int32_t umtx_atomic_inc(u_atomic_int32_t &var) {
    return var.fetch_add(1) + 1;

inline int32_t umtx_atomic_dec(u_atomic_int32_t &var) {
    return var.fetch_sub(1) - 1;

Mutex and InitOnce implementations

// Example ICU build time custom mutex cpp file.
// Must implement these functions:
// umtx_lock(UMutex *mutex);
// umtx_unlock(UMutex *mutex);
// umtx_initImplPreInit(UInitOnce &uio);
// umtx_initImplPostInit(UInitOnce &uio);

umtx_lock(UMutex *mutex) {
    if (mutex == NULL) {
        // Note: globalMutex is pre-defined in the platform-independent ICU code.
        mutex = &globalMutex;

umtx_unlock(UMutex* mutex) `{
    if (mutex == NULL) {
        mutex = &globalMutex;

// A mutex and a condition variable are used by the implementation of umtx_initOnce()
// The mutex is held only while the state of the InitOnce object is being changed or
// tested. It is not held while initialization functions are running.
// Threads needing to block, waiting for an initialization to complete, will wait
// on the condition variable.
// All InitOnce objects share a common mutex and condition variable. This means that
// all blocked threads will wake if any (possibly unrelated) initialization completes.
// Which does no harm, it should be statistically rare, and any spuriously woken
// threads will check their state and promptly wait again.

static std::mutex initMutex;
static std::condition_variable initCondition;

// This function is called from umtx_initOnce() when an initial test of a UInitOnce::fState flag
// reveals that initialization has not completed, that we either need to call the
// function on this thread, or wait for some other thread to complete the initialization.
// The actual call to the init function is made inline by template code
// that knows the C++ types involved. This function returns true if
// the inline code needs to invoke the Init function, or false if the initialization
// has completed on another thread.
// UInitOnce::fState values:
// 0: Initialization has not yet begun.
// 1: Initialization is in progress, not yet complete.
// 2: Initialization is complete.
UBool umtx_initImplPreInit(UInitOnce &uio) {
    std::unique_lock<std::mutex> initLock(initMutex);
    int32_t state = uio.fState;
    if (state == 0) {
        umtx_storeRelease(uio.fState, 1);
        return true; // Caller will next call the init function.
    } else {
        while (uio.fState == 1) {
            // Another thread is currently running the initialization.
            // Wait until it completes.
        U_ASSERT(uio.fState == 2);
        return false;

// This function is called from umtx_initOnce() just after an initializationfunction completes.
// Its purpose is to set the state of the UInitOnce object to initialized, and to
// unblock any threads that may be waiting on the initialization.
// Some threads may be waiting on the condition variable, requiring the notify_all().
// Some threads may be racing to test the fState flag outside of the mutex, 
// requiring the use of store-release when changing its value.

void umtx_initImplPostInit(UInitOnce &uio) {
    std::unique_lock<std::mutex> initLock(initMutex);
    umtx_storeRelease(uio.fState, 2);