Concurrency support library

C includes built-in support for threads, atomic operations, mutual exclusion, condition variables, and thread-specific storages.

These features are optionally provided:

• if the macro constant __STDC_NO_THREADS__ is defined by the compiler, the header <threads.h> and all of the names provided in it are not provided;
• if the macro constant __STDC_NO_ATOMICS__ is defined by the compiler, the header <stdatomic.h> and all of the names provided in it are not provided.

See also _Atomic type specifier and qualifier .

Threads

Defined in header <threads.h>
thrd_t	implementation-defined complete object type identifying a thread
thrd_create (C11)	creates a thread (function)
thrd_equal (C11)	checks if two identifiers refer to the same thread (function)
thrd_current (C11)	obtains the current thread identifier (function)
thrd_sleep (C11)	suspends execution of the calling thread for the given period of time (function)
thrd_yield (C11)	yields the current time slice (function)
thrd_exit (C11)	terminates the calling thread (function)
thrd_detach (C11)	detaches a thread (function)
thrd_join (C11)	blocks until a thread terminates (function)
thrd_success thrd_timedout thrd_busy thrd_nomem thrd_error (C11)	indicates a thread error status (constant)
thrd_start_t (C11)	a typedef of the function pointer type int ( * ) ( void * ) , used by thrd_create (typedef)

Atomic operations

Defined in header <stdatomic.h>
Operations on atomic types
ATOMIC_BOOL_LOCK_FREE ATOMIC_CHAR_LOCK_FREE ATOMIC_CHAR16_T_LOCK_FREE ATOMIC_CHAR32_T_LOCK_FREE ATOMIC_WCHAR_T_LOCK_FREE ATOMIC_SHORT_LOCK_FREE ATOMIC_INT_LOCK_FREE ATOMIC_LONG_LOCK_FREE ATOMIC_LLONG_LOCK_FREE ATOMIC_POINTER_LOCK_FREE (C11)	indicates that the given atomic type is lock-free (macro constant)
atomic_is_lock_free (C11)	indicates whether the atomic object is lock-free (function)
atomic_store atomic_store_explicit (C11)	stores a value in an atomic object (function)
atomic_load atomic_load_explicit (C11)	reads a value from an atomic object (function)
atomic_exchange atomic_exchange_explicit (C11)	swaps a value with the value of an atomic object (function)
atomic_compare_exchange_strong atomic_compare_exchange_strong_explicit atomic_compare_exchange_weak atomic_compare_exchange_weak_explicit (C11)	swaps a value with an atomic object if the old value is what is expected, otherwise reads the old value (function)
atomic_fetch_add atomic_fetch_add_explicit (C11)	atomic addition (function)
atomic_fetch_sub atomic_fetch_sub_explicit (C11)	atomic subtraction (function)
atomic_fetch_or atomic_fetch_or_explicit (C11)	atomic bitwise OR (function)
atomic_fetch_xor atomic_fetch_xor_explicit (C11)	atomic bitwise exclusive OR (function)
atomic_fetch_and atomic_fetch_and_explicit (C11)	atomic bitwise AND (function)
Flag type and operations
atomic_flag (C11)	lock-free atomic boolean flag (struct)
atomic_flag_test_and_set atomic_flag_test_and_set_explicit (C11)	sets an atomic_flag to true and returns the old value (function)
atomic_flag_clear atomic_flag_clear_explicit (C11)	sets an atomic_flag to false (function)
Initialization
atomic_init (C11)	initializes an existing atomic object (function)
ATOMIC_VAR_INIT (C11) (deprecated in C17) (removed in C23)	initializes a new atomic object (function macro)
ATOMIC_FLAG_INIT (C11)	initializes a new atomic_flag (macro constant)
Memory synchronization ordering
memory_order (C11)	defines memory ordering constraints (enum)
kill_dependency (C11)	breaks a dependency chain for memory_order_consume (function macro)
atomic_thread_fence (C11)	generic memory order-dependent fence synchronization primitive (function)
atomic_signal_fence (C11)	fence between a thread and a signal handler executed in the same thread (function)
Convenience type aliases
Typedef name	Full type name
atomic_bool (C11)	_Atomic _Bool
atomic_char (C11)	_Atomic char
atomic_schar (C11)	_Atomic signed char
atomic_uchar (C11)	_Atomic unsigned char
atomic_short (C11)	_Atomic short
atomic_ushort (C11)	_Atomic unsigned short
atomic_int (C11)	_Atomic int
atomic_uint (C11)	_Atomic unsigned int
atomic_long (C11)	_Atomic long
atomic_ulong (C11)	_Atomic unsigned long
atomic_llong (C11)	_Atomic long long
atomic_ullong (C11)	_Atomic unsigned long long
atomic_char8_t (C23)	_Atomic char8_t
atomic_char16_t (C11)	_Atomic char16_t
atomic_char32_t (C11)	_Atomic char32_t
atomic_wchar_t (C11)	_Atomic wchar_t
atomic_int_least8_t (C11)	_Atomic int_least8_t
atomic_uint_least8_t (C11)	_Atomic uint_least8_t
atomic_int_least16_t (C11)	_Atomic int_least16_t
atomic_uint_least16_t (C11)	_Atomic uint_least16_t
atomic_int_least32_t (C11)	_Atomic int_least32_t
atomic_uint_least32_t (C11)	_Atomic uint_least32_t
atomic_int_least64_t (C11)	_Atomic int_least64_t
atomic_uint_least64_t (C11)	_Atomic uint_least64_t
atomic_int_fast8_t (C11)	_Atomic int_fast8_t
atomic_uint_fast8_t (C11)	_Atomic uint_fast8_t
atomic_int_fast16_t (C11)	_Atomic int_fast16_t
atomic_uint_fast16_t (C11)	_Atomic uint_fast16_t
atomic_int_fast32_t (C11)	_Atomic int_fast32_t
atomic_uint_fast32_t (C11)	_Atomic uint_fast32_t
atomic_int_fast64_t (C11)	_Atomic int_fast64_t
atomic_uint_fast64_t (C11)	_Atomic uint_fast64_t
atomic_intptr_t (C11)	_Atomic intptr_t
atomic_uintptr_t (C11)	_Atomic uintptr_t
atomic_size_t (C11)	_Atomic size_t
atomic_ptrdiff_t (C11)	_Atomic ptrdiff_t
atomic_intmax_t (C11)	_Atomic intmax_t
atomic_uintmax_t (C11)	_Atomic uintmax_t

Mutual exclusion

Defined in header <threads.h>
mtx_t	mutex identifier
mtx_init (C11)	creates a mutex (function)
mtx_lock (C11)	blocks until locks a mutex (function)
mtx_timedlock (C11)	blocks until locks a mutex or times out (function)
mtx_trylock (C11)	locks a mutex or returns without blocking if already locked (function)
mtx_unlock (C11)	unlocks a mutex (function)
mtx_destroy (C11)	destroys a mutex (function)
mtx_plain mtx_recursive mtx_timed (C11) (C11) (C11)	defines the type of a mutex (enum)
Call once
call_once (C11)	calls a function exactly once (function)

Condition variables

Defined in header <threads.h>
cnd_t	condition variable identifier
cnd_init (C11)	creates a condition variable (function)
cnd_signal (C11)	unblocks one thread blocked on a condition variable (function)
cnd_broadcast (C11)	unblocks all threads blocked on a condition variable (function)
cnd_wait (C11)	blocks on a condition variable (function)
cnd_timedwait (C11)	blocks on a condition variable, with a timeout (function)
cnd_destroy (C11)	destroys a condition variable (function)

Thread-local storage

Defined in header <threads.h>
thread_local (C11) (removed in C23)	convenience macro for storage-class specifier _Thread_local (keyword macro)
tss_t	thread-specific storage pointer
TSS_DTOR_ITERATIONS (C11)	maximum number of times destructors are called (macro constant)
tss_dtor_t (C11)	function pointer type void ( * ) ( void * ) , used for TSS destructor (typedef)
tss_create (C11)	creates thread-specific storage pointer with a given destructor (function)
tss_get (C11)	reads from thread-specific storage (function)
tss_set (C11)	write to thread-specific storage (function)
tss_delete (C11)	releases the resources held by a given thread-specific pointer (function)

Reserved identifiers

In future revisions of the C standard:

• function names, type names, and enumeration constants that begin with either cnd_ , mtx_ , thrd_ , or tss_ , and a lowercase letter may be added to the declarations in the <threads.h> header;
• macros that begin with ATOMIC_ and an uppercase letter may be added to the macros defined in the <stdatomic.h> header;
• typedef names that begin with either atomic_ or memory_ , and a lowercase letter may be added to the declarations in the <stdatomic.h> header;
• enumeration constants that begin with memory_order_ and a lowercase letter may be added to the definition of the memory_order type in the <stdatomic.h> header;
• function names that begin with atomic_ and a lowercase letter may be added to the declarations in the <stdatomic.h> header.

Identifiers reserved for functions names are always potentially (since C23) reserved for use as identifiers with external linkage, while other identifiers list here are potentially (since C23) reserved when <stdatomic.h> is included.

Declaring, defining, or #undef ing such an identifier results in undefined behavior if it is provided by the standard or implementation (since C23) . Portable programs should not use those identifiers.

References

See also

C++ documentation for Concurrency support library

External links

GNU GCC Libc Manual: ISO C Mutexes