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Algorithms Algorithms(算法) bsearch bsearch_s qsort qsort_s Atomic operations Atomic operations library(原子操作库) ATOMIC_*_LOCK_FREE atomic_compare_exchange_strong atomic_compare_exchange_strong_explicit atomic_compare_exchange_weak atomic_compare_exchange_weak_explicit atomic_exchange atomic_exchange_explicit atomic_fetch_add atomic_fetch_add_explicit atomic_fetch_and atomic_fetch_and_explicit atomic_fetch_or atomic_fetch_or_explicit atomic_fetch_sub atomic_fetch_sub_explicit atomic_fetch_xor atomic_fetch_xor_explicit atomic_flag atomic_flag_clear atomic_flag_clear_explicit ATOMIC_FLAG_INIT atomic_flag_test_and_set atomic_flag_test_and_set_explicit atomic_init atomic_is_lock_free atomic_load atomic_load_explicit atomic_signal_fence atomic_store atomic_store_explicit atomic_thread_fence(线程围栏) ATOMIC_VAR_INIT kill_dependency memory_order(内存排序) C keywords auto(自动存储) break(跳出循环) C keywords(关键词) case char const(常量修饰符) continue default(预设运算式) do double(双精度浮点型) else enum(枚举类型) extern(全局变量) 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Static storage duration(静态存储周期) Storage-class specifiers(存储类说明符) string literals(字符串字面量) Struct and union initialization(结构体与联合体初始化) Struct declaration(结构体声明) switch statement Thread storage duration(线程存储时间) Type Type(类型) Typedef declaration(Typedef声明) Undefined behavior(未定义行为) Union declaration(联合体声明) Value categories(值类别) Variadic arguments(变长参数宏) volatile type qualifier(volatile 类型限定符) while loop _Alignof operator _Noreturn function specifier Date and time asctime(asctime函数) asctime_s clock CLOCKS_PER_SEC clock_t ctime(ctime函数) ctime_s Date and time utilities(日期和时间库) difftime(计算两个时间的间隔) gmtime gmtime_s localtime localtime_s mktime(将时间结构数据转换成经过的秒数的函数) strftime(格式化输出时间函数) time timespec timespec_get time_t tm wcsftime(格式化时间宽字符) Dynamic memory management aligned_alloc C memory management library(内存管理库) calloc free(释放动态分配空间的函数) malloc(动态分配内存空间的函数) realloc(重新分配内存空间的函数) Error handling abort_handler_s assert(断言) constraint_handler_t errno(错误报告) Error handling(错误处理) Error numbers(错误个数) ignore_handler_s set_constraint_handler_s static_assert File input/output clearerr(清除/复位) fclose feof ferror fflush(清空文件缓冲区) fgetc fgetpos fgets fgetwc fgetws File input/output fopen fopen_s fprintf fprintf_s fputc fputs fputwc fputws fread freopen freopen_s fscanf fscanf_s fseek fsetpos ftell fwide fwprintf fwprintf_s fwrite fwscanf fwscanf_s getc getchar gets gets_s getwchar perror printf printf_s putc putchar puts putwc putwchar remove rename rewind scanf scanf_s setbuf setvbuf snprintf sprintf sscanf sscanf_s swprintf swprintf_s swscanf swscanf_s tmpfile tmpfile_s tmpnam tmpnam_s ungetc ungetwc vfprintf vfprintf_s vfscanf vfscanf_s vfwprintf vfwprintf_s vfwscanf vfwscanf_s vprintf vprintf_s vscanf vscanf_s vsnprintf vsprintf vsscanf vsscanf_s vswprintf vswprintf_s vswscanf vswscanf_s vwprintf vwprintf_s vwscanf vwscanf_s wprintf wprintf_s wscanf wscanf_s Localization support lconv LC_ALL LC_COLLATE LC_CTYPE LC_MONETARY LC_NUMERIC LC_TIME localeconv Localization support setlocale Numerics abs acos acosf acosh acoshf acoshl acosl asin asinf asinh asinhf asinhl asinl atan atan2 atan2f atan2l atanf atanh atanhf atanhl atanl cabs cabsf cabsl cacos cacosf cacosh cacoshf cacoshl cacosl carg cargf cargl casin casinf casinh casinhf casinhl casinl catan catanf catanh catanhf catanhl catanl cbrt cbrtf cbrtl ccos ccosf ccosh ccoshf ccoshl ccosl ceil ceilf ceill cexp cexpf cexpl cimag cimagf cimagl clog clogf clogl CMPLX CMPLXF CMPLXL Common mathematical functions complex Complex number arithmetic conj conjf conjl copysign copysignf copysignl cos cosf cosh coshf coshl cosl cpow cpowf cpowl cproj cprojf cprojl creal crealf creall csin csinf csinh csinhf csinhl csinl csqrt csqrtf csqrtl ctan ctanf ctanh ctanhf ctanhl ctanl div double_t erf erfc erfcf erfcl erff erfl exp exp2 exp2f exp2l expf expl expm1 expm1f expm1l fabs fabsf fabsl fdim feclearexcept fegetenv fegetexceptflag fegetround feholdexcept feraiseexcept fesetenv fesetexceptflag fesetround fetestexcept feupdateenv 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
文字

联合是由存储重叠的成员序列组成的类型(与结构相对,结构是一种由存储按有序序列分配的成员序列组成的类型)。至多一个成员的价值可以在任何时候存储在工会中。

struct除了使用的关键字之外,联合的类型说明符与   类型说明符相同:

句法

联合体名称(可选){结构声明列表}

(1)


工会名称

(2)


name

-

正在定义的联盟的名称

结构声明列表

-

任何数量的变量声明,位域声明和静态断言声明。不完整类型的成员和函数类型的成员是不允许的。

说明

工会的规模只有持有其最大成员所需的那么大(另外还可以添加未命名的追尾填充)。其他成员以相同字节分配,作为该最大成员的一部分。

指向联合的指针可以转换为指向其每个成员的指针(如果联合有位字段成员,指向联合的指针可以转换为指向位字段的基础类型的指针)。同样,可以将指向任何联合成员的指针转换为指向包围联合的指针。

如果用于访问联合的内容的成员与上次用于存储值的成员不相同,则存储的值的对象表示将被重新解释为新类型的对象表示(这称为类型打孔)。如果新类型的大小大于最后写入类型的大小,则超出字节的内容未指定(并且可能是陷阱表示)。

与struct类似,类型为无名称的联合的联合的未命名成员称为匿名联合。匿名联盟的每个成员都被认为是封闭结构或联盟的成员。如果封闭的结构或联合也是匿名的,则这将递归应用。struct v {union {//匿名联合结构{int i,j; }; //匿名结构struct {long k,l; } w; }; int m; } v1; v1.i = 2; //有效v1.k = 3; //无效:内部结构不是匿名的v1.wk = 5; // valid类似于struct,如果union没有任何命名成员(包括通过匿名嵌套结构或联合获得的)定义,则程序的行为是未定义的。

(自C11以来)

关键词

union.

笔记

有关结构和联合初始化的规则,请参阅结构初始化。

#include <stdio.h>#include <stdint.h>#include <assert.h>
 int main(void){
    union S {
        uint32_t u32;
        uint16_t u16[2];
        uint8_t  u8;    } s = {0x12345678}; // s.u32 is now the active member    printf("Union S has size %zu and holds %x\n", sizeof s, s.u32);
    s.u16[0] = 0x0011;  // s.u16 is now the active member    // reading from s.u32 or from s.u8 reinterprets the object representation//  printf("s.u8 is now %x\n", s.u8); // unspecified, typically 11 or 00//  printf("s.u32 is now %x\n", s.u32); // unspecified, typically 12340011 or 00115678 
    // pointers to all members of a union compare equal to themselves and the union    assert((uint8_t*)&s == &s.u8); 
    // this union has 3 bytes of trailing padding
    union pad {
       char  c[5];   // occupies 5 bytes
       float f;      // occupies 4 bytes, imposes alignment 4    } p = {.f = 1.23}; // the size is 8 to satisfy float's alignment    printf("size of union of char[5] and float is %zu\n", sizeof p);}

输出:

Union S has size 4 and holds 12345678size of union of char[5] and float is 8

每个成员都被分配好像它是唯一的成员,这就是为什么s.c在上面的例子中第一个字节是别名s.s[0]

参考

  • C11标准(ISO / IEC 9899:2011):

    • 6.7.2.1结构和联合说明符(p:112-117)

  • C99标准(ISO / IEC 9899:1999):

    • 6.7.2.1结构和联合说明符(p:101-104)

  • C89 / C90标准(ISO / IEC 9899:1990):

    • 3.5.2.1结构和联合说明符

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