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FE_ALL_EXCEPT FE_DFL_ENV FE_DIVBYZERO FE_DOWNWARD FE_INEXACT FE_INVALID FE_OVERFLOW FE_TONEAREST FE_TOWARDZERO FE_UNDERFLOW FE_UPWARD Floating-point environment float_t floor floorf floorl fma fmaf fmal fmax fmaxf fmaxl fmin fminf fminl fmod fmodf fmodl fpclassify FP_INFINITE FP_NAN FP_NORMAL FP_SUBNORMAL FP_ZERO frexp frexpf frexpl HUGE_VAL HUGE_VALF HUGE_VALL hypot hypotf hypotl I ilogb ilogbf ilogbl imaginary imaxabs imaxdiv INFINITY isfinite isgreater isgreaterequal isinf isless islessequal islessgreater isnan isnormal isunordered labs ldexp ldexpf ldexpl ldiv lgamma lgammaf lgammal llabs lldiv llrint llrintf llrintl llround llroundf llroundl log log10 log10f log10l log1p log1pf log1pl log2 log2f log2l logb logbf logbl logf logl lrint lrintf lrintl lround lroundf lroundl MATH_ERREXCEPT math_errhandling MATH_ERRNO modf modff modfl nan NAN nanf nanl nearbyint nearbyintf nearbyintl nextafter nextafterf nextafterl nexttoward nexttowardf nexttowardl Numerics pow powf powl Pseudo-random number generation rand RAND_MAX remainder remainderf remainderl remquo remquof remquol rint rintf rintl round roundf roundl scalbln scalblnf scalblnl scalbn scalbnf scalbnl signbit sin sinf sinh sinhf sinhl sinl sqrt sqrtf sqrtl srand tan tanf tanh tanhf tanhl tanl tgamma tgammaf tgammal trunc truncf truncl Type-generic math _Complex_I _Imaginary_I Program support abort atexit at_quick_exit exit EXIT_FAILURE EXIT_SUCCESS getenv getenv_s jmp_buf longjmp Program support utilities quick_exit raise setjmp SIGABRT SIGFPE SIGILL SIGINT signal SIGSEGV SIGTERM sig_atomic_t SIG_DFL SIG_ERR SIG_IGN system _Exit Strings atof atoi atol atoll btowc c16rtomb c32rtomb char16_t char32_t isalnum isalpha isblank iscntrl isdigit isgraph islower isprint ispunct isspace isupper iswalnum iswalpha iswblank iswcntrl iswctype iswdigit iswgraph iswlower iswprint iswpunct iswspace iswupper iswxdigit isxdigit mblen mbrlen mbrtoc16 mbrtoc32 mbrtowc mbsinit mbsrtowcs mbsrtowcs_s mbstate_t mbstowcs mbstowcs_s mbtowc 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
テキスト

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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