Threading and synchronization utilities

libuv provides cross-platform implementations for multiple threading and synchronization primitives. The API largely follows the pthreads API.

Data types

type uv_thread_t

Thread data type.

typedef void (*uv_thread_cb)(void *arg)

Callback that is invoked to initialize thread execution. arg is the same value that was passed to uv_thread_create().

type uv_key_t

Thread-local key data type.

type uv_once_t

Once-only initializer data type.

type uv_mutex_t

Mutex data type.

type uv_rwlock_t

Read-write lock data type.

type uv_sem_t

Semaphore data type.

type uv_cond_t

Condition data type.

type uv_barrier_t

Barrier data type.



type uv_thread_options_t

Options for spawning a new thread (passed to uv_thread_create_ex()).

typedef struct uv_thread_options_s {
  enum {
    UV_THREAD_NO_FLAGS = 0x00,
  } flags;
  size_t stack_size;
} uv_thread_options_t;

More fields may be added to this struct at any time, so its exact layout and size should not be relied upon.

New in version 1.26.0.

int uv_thread_create(uv_thread_t *tid, uv_thread_cb entry, void *arg)

Changed in version 1.4.1: returns a UV_E* error code on failure

int uv_thread_create_ex(uv_thread_t *tid, const uv_thread_options_t *params, uv_thread_cb entry, void *arg)

Like uv_thread_create(), but additionally specifies options for creating a new thread.

If UV_THREAD_HAS_STACK_SIZE is set, stack_size specifies a stack size for the new thread. 0 indicates that the default value should be used, i.e. behaves as if the flag was not set. Other values will be rounded up to the nearest page boundary.

New in version 1.26.0.

uv_thread_t uv_thread_self(void)
int uv_thread_join(uv_thread_t *tid)
int uv_thread_equal(const uv_thread_t *t1, const uv_thread_t *t2)

Thread-local storage


The total thread-local storage size may be limited. That is, it may not be possible to create many TLS keys.

int uv_key_create(uv_key_t *key)
void uv_key_delete(uv_key_t *key)
void *uv_key_get(uv_key_t *key)
void uv_key_set(uv_key_t *key, void *value)

Once-only initialization

Runs a function once and only once. Concurrent calls to uv_once() with the same guard will block all callers except one (it’s unspecified which one). The guard should be initialized statically with the UV_ONCE_INIT macro.

void uv_once(uv_once_t *guard, void (*callback)(void))

Mutex locks

Functions return 0 on success or an error code < 0 (unless the return type is void, of course).

int uv_mutex_init(uv_mutex_t *handle)
int uv_mutex_init_recursive(uv_mutex_t *handle)
void uv_mutex_destroy(uv_mutex_t *handle)
void uv_mutex_lock(uv_mutex_t *handle)
int uv_mutex_trylock(uv_mutex_t *handle)
void uv_mutex_unlock(uv_mutex_t *handle)

Read-write locks

Functions return 0 on success or an error code < 0 (unless the return type is void, of course).

int uv_rwlock_init(uv_rwlock_t *rwlock)
void uv_rwlock_destroy(uv_rwlock_t *rwlock)
void uv_rwlock_rdlock(uv_rwlock_t *rwlock)
int uv_rwlock_tryrdlock(uv_rwlock_t *rwlock)
void uv_rwlock_rdunlock(uv_rwlock_t *rwlock)
void uv_rwlock_wrlock(uv_rwlock_t *rwlock)
int uv_rwlock_trywrlock(uv_rwlock_t *rwlock)
void uv_rwlock_wrunlock(uv_rwlock_t *rwlock)


Functions return 0 on success or an error code < 0 (unless the return type is void, of course).

int uv_sem_init(uv_sem_t *sem, unsigned int value)
void uv_sem_destroy(uv_sem_t *sem)
void uv_sem_post(uv_sem_t *sem)
void uv_sem_wait(uv_sem_t *sem)
int uv_sem_trywait(uv_sem_t *sem)


Functions return 0 on success or an error code < 0 (unless the return type is void, of course).


  1. Callers should be prepared to deal with spurious wakeups on uv_cond_wait() and uv_cond_timedwait().

  2. The timeout parameter for uv_cond_timedwait() is relative to the time at which function is called.

  3. On z/OS, the timeout parameter for uv_cond_timedwait() is converted to an absolute system time at which the wait expires. If the current system clock time passes the absolute time calculated before the condition is signaled, an ETIMEDOUT error results. After the wait begins, the wait time is not affected by changes to the system clock.

int uv_cond_init(uv_cond_t *cond)
void uv_cond_destroy(uv_cond_t *cond)
void uv_cond_signal(uv_cond_t *cond)
void uv_cond_broadcast(uv_cond_t *cond)
void uv_cond_wait(uv_cond_t *cond, uv_mutex_t *mutex)
int uv_cond_timedwait(uv_cond_t *cond, uv_mutex_t *mutex, uint64_t timeout)


Functions return 0 on success or an error code < 0 (unless the return type is void, of course).


uv_barrier_wait() returns a value > 0 to an arbitrarily chosen “serializer” thread to facilitate cleanup, i.e.

if (uv_barrier_wait(&barrier) > 0)
int uv_barrier_init(uv_barrier_t *barrier, unsigned int count)
void uv_barrier_destroy(uv_barrier_t *barrier)
int uv_barrier_wait(uv_barrier_t *barrier)