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Concurrency support library

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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.

Contents

[edit] Threads

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

[edit] Atomic operations

Defined in header <stdatomic.h>
Operations on atomic types
indicates that the given atomic type is lock-free
(macro constant) [edit]
indicates whether the atomic object is lock-free
(function) [edit]
stores a value in an atomic object
(function) [edit]
reads a value from an atomic object
(function) [edit]
swaps a value with the value of an atomic object
(function) [edit]
swaps a value with an atomic object if the old value is what is expected, otherwise reads the old value
(function) [edit]
atomic addition
(function) [edit]
atomic subtraction
(function) [edit]
atomic bitwise OR
(function) [edit]
atomic bitwise exclusive OR
(function) [edit]
atomic bitwise AND
(function) [edit]
Flag type and operations
lock-free atomic boolean flag
(struct)[edit]
sets an atomic_flag to true and returns the old value
(function) [edit]
sets an atomic_flag to false
(function) [edit]
Initialization
initializes an existing atomic object
(function) [edit]
(C11)(deprecated in C17)(removed in C23)
initializes a new atomic object
(function macro) [edit]
initializes a new atomic_flag
(macro constant) [edit]
Memory synchronization ordering
defines memory ordering constraints
(enum) [edit]
breaks a dependency chain for memory_order_consume
(function macro) [edit]
generic memory order-dependent fence synchronization primitive
(function) [edit]
fence between a thread and a signal handler executed in the same thread
(function) [edit]
Convenience type aliases
Typedef name Full type name
atomic_bool (C11) _Atomic _Bool(until C23)_Atomic bool(since C23)
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

[edit] Mutual exclusion

Defined in header <threads.h>
mtx_t mutex identifier [edit]
(C11)
creates a mutex
(function) [edit]
(C11)
blocks until locks a mutex
(function) [edit]
blocks until locks a mutex or times out
(function) [edit]
locks a mutex or returns without blocking if already locked
(function) [edit]
unlocks a mutex
(function) [edit]
destroys a mutex
(function) [edit]
defines the type of a mutex
(enum) [edit]
Call once
calls a function exactly once
(function) [edit]

[edit] Condition variables

Defined in header <threads.h>
cnd_t condition variable identifier
(C11)
creates a condition variable
(function) [edit]
unblocks one thread blocked on a condition variable
(function) [edit]
unblocks all threads blocked on a condition variable
(function) [edit]
(C11)
blocks on a condition variable
(function) [edit]
blocks on a condition variable, with a timeout
(function) [edit]
destroys a condition variable
(function) [edit]

[edit] Thread-local storage

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

[edit] 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.

[edit] 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)

[edit] See also

C++ documentation for Concurrency support library

[edit] External links

GNU GCC Libc Manual: ISO C Mutexes
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