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.

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_successthrd_timedoutthrd_busythrd_nomemthrd_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_FREEATOMIC_CHAR_LOCK_FREEATOMIC_CHAR16_T_LOCK_FREEATOMIC_CHAR32_T_LOCK_FREEATOMIC_WCHAR_T_LOCK_FREEATOMIC_SHORT_LOCK_FREEATOMIC_INT_LOCK_FREEATOMIC_LONG_LOCK_FREEATOMIC_LLONG_LOCK_FREEATOMIC_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_storeatomic_store_explicit (C11)	stores a value in an atomic object (function)
atomic_loadatomic_load_explicit (C11)	reads a value from an atomic object (function)
atomic_exchangeatomic_exchange_explicit (C11)	swaps a value with the value of an atomic object (function)
atomic_compare_exchange_strongatomic_compare_exchange_strong_explicitatomic_compare_exchange_weakatomic_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_addatomic_fetch_add_explicit (C11)	atomic addition (function)
atomic_fetch_subatomic_fetch_sub_explicit (C11)	atomic subtraction (function)
atomic_fetch_oratomic_fetch_or_explicit (C11)	atomic bitwise OR (function)
atomic_fetch_xoratomic_fetch_xor_explicit (C11)	atomic bitwise exclusive OR (function)
atomic_fetch_andatomic_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_setatomic_flag_test_and_set_explicit (C11)	sets an atomic_flag to true and returns the old value (function)
atomic_flag_clearatomic_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_plainmtx_recursivemtx_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 #undefing 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

C23 standard (ISO/IEC 9899:2023):

7.17 Atomics <stdatomic.h> (p: TBD)
7.26 Threads <threads.h> (p: TBD)
7.31.8 Atomics <stdatomic.h> (p: TBD)
7.31.15 Threads <threads.h> (p: TBD)

C17 standard (ISO/IEC 9899:2018):

7.17 Atomics <stdatomic.h> (p: 200-209)
7.26 Threads <threads.h> (p: 274-283)
7.31.8 Atomics <stdatomic.h> (p: 332)
7.31.15 Threads <threads.h> (p: 333)

C11 standard (ISO/IEC 9899:2011):

7.17 Atomics <stdatomic.h> (p: 273-286)
7.26 Threads <threads.h> (p: 376-387)
7.31.8 Atomics <stdatomic.h> (p: 455-456)
7.31.15 Threads <threads.h> (p: 456)

External links

GNU GCC Libc Manual: ISO C Mutexes

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Concurrency support library

Threads

Atomic operations

Operations on atomic types

Flag type and operations

Initialization

Memory synchronization ordering

Convenience type aliases