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memchr memcmp memcpy memcpy_s memmove memmove_s memset memset_s Null-terminated byte strings Null-terminated multibyte strings Null-terminated wide strings strcat strcat_s strchr strcmp strcoll strcpy strcpy_s strcspn strerror strerrorlen_s strerror_s Strings library strlen strncat Thread support call_once cnd_broadcast cnd_destroy cnd_init cnd_signal cnd_timedwait cnd_wait mtx_destroy mtx_init mtx_lock mtx_plain mtx_recursive mtx_timed mtx_timedlock mtx_trylock mtx_unlock once_flag ONCE_FLAG_INIT thrd_busy thrd_create thrd_current thrd_detach thrd_equal thrd_error thrd_exit thrd_join thrd_nomem thrd_sleep thrd_success thrd_timedout thrd_yield Thread support library thread_local tss_create tss_delete TSS_DTOR_ITERATIONS tss_get tss_set Type support Boolean type support library Fixed width integer types FLT_EVAL_METHOD FLT_ROUNDS max_align_t NULL Numeric limits offsetof ptrdiff_t size_t Type support Variadic functions Variadic functions va_arg va_copy va_end va_list va_start
characters

句法

_Atomic ( type-name )

(1)

(since C11)

_Atomic type-name

(2)

(since C11)

1)用作类型说明符; 这指定了一种新的原子类型

2)用作类型限定词; 这指定了类型名称的原子版本。在这个角色中,它可能与 const,volatile 和 restrict混合使用),但与其他限定符不同,type-name 的原子版本可能具有不同的大小,对齐和对象表示。

type-name

-

any type other than array or function. For (1), type-name also cannot be atomic or cvr-qualified

<stdatomic.h>定义37层便于使用的宏,从atomic_boolatomic_uintmax_t,这简化使用这个关键字与内置和库类型的。

_Atomic const int * p1;  // p is a pointer to an atomic const intconst atomic_int * p2;   // sameconst _Atomic(int) * p3; // same

说明

原子类型的对象是唯一没有数据竞争的对象,也就是说,它们可以被两个线程同时修改或修改一个并被另一个线程修改。

每个原子对象都有自己的关联修改顺序,这是对该对象进行修改的总顺序。如果从某个线程的角度来看,A某些原子M的修改发生在修改B相同的原子 M 之前,那么按M的修改顺序,A 在 B 之前发生。

请注意,虽然每个原子对象都有自己的修改顺序,但它不是总顺序; 不同的线程可以观察对不同顺序的不同原子对象的修改。

所有原子操作都有四种连贯性保证:

  • 写 - 写连贯性:如果修改原子对象M的操作A 发生在修改M的操作B 之前,则A按修改顺序M出现在B之前。

  • 读读连贯性:如果原子对象M的值计算A在M的值计算B之前发生,并且A从M上的副作用X取其值,并且由B计算的值是由X存储的值或者是由M上的副作用Y存储的值,其中Y以M的修改顺序比X晚出现。

  • 读写连贯性:如果原子对象M的值计算A在M上的操作B 之前发生,则A从M上的副作用X取其值,其中X以B的修改顺序出现在M之前。

  • 写读连贯性:如果原子对象M上的副作用X 发生在 M 的值计算B 之前,则评估B从X或从修饰顺序为M的X之后出现的副作用Y中获取其值。

一些原子操作也是同步操作; 他们可能会有额外的释放语义,获取语义或顺序一致的语义。看memory_order

内置增量和减量运算符和复合赋值是按顺序一致的顺序进行读 - 修改 - 写原子操作(就像使用一样memory_order_seq_cst)。如果需要较不严格的同步语义,则可以使用标准库函数。

原子属性只对左值表达式有意义。左值到右值转换(将从原子位置读到CPU寄存器的内存建模)与其他限定符一起剥离原子性。

笔记

如果宏常量__STDC_NO_ATOMICS__(C11)由编译器定义,则不提供关键字_Atomic和标题<stdatomic.h>

访问原子结构/联合的成员是未定义的行为。

库类型sig_atomic_t不提供线程间同步或内存排序,只有原子性。

易失性类型不提供线程间同步,内存排序或原子性。

关键词

_Atomic.

#include <stdio.h>#include <threads.h>#include <stdatomic.h>
 atomic_int acnt;int cnt;
 int f(void* thr_data){    for(int n = 0; n < 1000; ++n) {        ++cnt;        ++acnt;        // for this example, relaxed memory order is sufficient, e.g.        // atomic_fetch_add_explicit(&acnt, 1, memory_order_relaxed);    }    return 0;}
 int main(void){
    thrd_t thr[10];    for(int n = 0; n < 10; ++n)        thrd_create(&thr[n], f, NULL);    for(int n = 0; n < 10; ++n)        thrd_join(thr[n], NULL); 
    printf("The atomic counter is %u\n", acnt);    printf("The non-atomic counter is %u\n", cnt);}

可能的输出:

The atomic counter is 10000The non-atomic counter is 8644
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