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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_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_CHAR8_T_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)(C11)(C23)(C11)(C11)(C11)(C11)(C11)(C11)(C11)(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(until C23)_Atomicbool(since C23)
atomic_char (C11) _Atomicchar
atomic_schar (C11) _Atomicsignedchar
atomic_uchar (C11) _Atomicunsignedchar
atomic_short (C11) _Atomicshort
atomic_ushort (C11) _Atomicunsignedshort
atomic_int (C11) _Atomicint
atomic_uint (C11) _Atomicunsignedint
atomic_long (C11) _Atomiclong
atomic_ulong (C11) _Atomicunsignedlong
atomic_llong (C11) _Atomiclonglong
atomic_ullong (C11) _Atomicunsignedlonglong
atomic_char8_t (C23) _Atomicchar8_t
atomic_char16_t (C11) _Atomicchar16_t
atomic_char32_t (C11) _Atomicchar32_t
atomic_wchar_t (C11) _Atomicwchar_t
atomic_int_least8_t (C11) _Atomicint_least8_t
atomic_uint_least8_t (C11) _Atomicuint_least8_t
atomic_int_least16_t (C11) _Atomicint_least16_t
atomic_uint_least16_t (C11) _Atomicuint_least16_t
atomic_int_least32_t (C11) _Atomicint_least32_t
atomic_uint_least32_t (C11) _Atomicuint_least32_t
atomic_int_least64_t (C11) _Atomicint_least64_t
atomic_uint_least64_t (C11) _Atomicuint_least64_t
atomic_int_fast8_t (C11) _Atomicint_fast8_t
atomic_uint_fast8_t (C11) _Atomicuint_fast8_t
atomic_int_fast16_t (C11) _Atomicint_fast16_t
atomic_uint_fast16_t (C11) _Atomicuint_fast16_t
atomic_int_fast32_t (C11) _Atomicint_fast32_t
atomic_uint_fast32_t (C11) _Atomicuint_fast32_t
atomic_int_fast64_t (C11) _Atomicint_fast64_t
atomic_uint_fast64_t (C11) _Atomicuint_fast64_t
atomic_intptr_t (C11) _Atomicintptr_t
atomic_uintptr_t (C11) _Atomicuintptr_t
atomic_size_t (C11) _Atomicsize_t
atomic_ptrdiff_t (C11) _Atomicptrdiff_t
atomic_intmax_t (C11) _Atomicintmax_t
atomic_uintmax_t (C11) _Atomicuintmax_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 9899:2024):
    • 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)

See also

C++ documentation for Concurrency support library

External links

GNU GCC Libc Manual: ISO C Mutexes

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https://en.cppreference.com/c/thread

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