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supportsetlocaleNumericsabsacosacosfacoshacoshfacoshlacoslasinasinfasinhasinhfasinhlasinlatanatan2atan2fatan2latanfatanhatanhfatanhlatanlcabscabsfcabslcacoscacosfcacoshcacoshfcacoshlcacoslcargcargfcarglcasincasinfcasinhcasinhfcasinhlcasinlcatancatanfcatanhcatanhfcatanhlcatanlcbrtcbrtfcbrtlccosccosfccoshccoshfccoshlccoslceilceilfceillcexpcexpfcexplcimagcimagfcimaglclogclogfcloglCMPLXCMPLXFCMPLXLCommon mathematical functionscomplexComplex number arithmeticconjconjfconjlcopysigncopysignfcopysignlcoscosfcoshcoshfcoshlcoslcpowcpowfcpowlcprojcprojfcprojlcrealcrealfcreallcsincsinfcsinhcsinhfcsinhlcsinlcsqrtcsqrtfcsqrtlctanctanfctanhctanhfctanhlctanldivdouble_terferfcerfcferfclerfferflexpexp2exp2fexp2lexpfexplexpm1expm1fexpm1lfabsfabsffabslfdimfeclearexceptfegetenvfegetexceptflagfegetroundfeholdexceptferaiseexceptfesetenvfesetexceptflagfesetroundfetestexceptfeupdateenvFE_ALL_EXCEPTFE_DFL_ENVFE_DIVBYZEROFE_DOWNWARDFE_INEXACTFE_INVALIDFE_OVERFLOWFE_TONEARESTFE_TOWARDZEROFE_UNDERFLOWFE_UPWARDFloating-point environmentfloat_tfloorfloorffloorlfmafmaffmalfmaxfmaxffmaxlfminfminffminlfmodfmodffmodlfpclassifyFP_INFINITEFP_NANFP_NORMALFP_SUBNORMALFP_ZEROfrexpfrexpffrexplHUGE_VALHUGE_VALFHUGE_VALLhypothypotfhypotlIilogbilogbfilogblimaginaryimaxabsimaxdivINFINITYisfiniteisgreaterisgreaterequalisinfislessislessequalislessgreaterisnanisnormalisunorderedlabsldexpldexpfldexplldivlgammalgammaflgammalllabslldivllrintllrintfllrintlllroundllroundfllroundlloglog10log10flog10llog1plog1pflog1pllog2log2flog2llogblogbflogbllogflogllrintlrintflrintllroundlroundflroundlMATH_ERREXCEPTmath_errhandlingMATH_ERRNOmodfmodffmodflnanNANnanfnanlnearbyintnearbyintfnearbyintlnextafternextafterfnextafterlnexttowardnexttowardfnexttowardlNumericspowpowfpowlPseudo-random number generationrandRAND_MAXremainderremainderfremainderlremquoremquofremquolrintrintfrintlroundroundfroundlscalblnscalblnfscalblnlscalbnscalbnfscalbnlsignbitsinsinfsinhsinhfsinhlsinlsqrtsqrtfsqrtlsrandtantanftanhtanhftanhltanltgammatgammaftgammaltrunctruncftrunclType-generic math_Complex_I_Imaginary_IProgram supportabortatexitat_quick_exitexitEXIT_FAILUREEXIT_SUCCESSgetenvgetenv_sjmp_buflongjmpProgram support utilitiesquick_exitraisesetjmpSIGABRTSIGFPESIGILLSIGINTsignalSIGSEGVSIGTERMsig_atomic_tSIG_DFLSIG_ERRSIG_IGNsystem_ExitStringsatofatoiatolatollbtowcc16rtombc32rtombchar16_tchar32_tisalnumisalphaisblankiscntrlisdigitisgraphislowerisprintispunctisspaceisupperiswalnumiswalphaiswblankiswcntrliswctypeiswdigitiswgraphiswloweriswprintiswpunctiswspaceiswupperiswxdigitisxdigitmblenmbrlenmbrtoc16mbrtoc32mbrtowcmbsinitmbsrtowcsmbsrtowcs_smbstate_tmbstowcsmbstowcs_smbtowcmemchrmemcmpmemcpymemcpy_smemmovememmove_smemsetmemset_sNull-terminated byte stringsNull-terminated multibyte stringsNull-terminated wide stringsstrcatstrcat_sstrchrstrcmpstrcollstrcpystrcpy_sstrcspnstrerrorstrerrorlen_sstrerror_sStrings librarystrlenstrncatThread supportcall_oncecnd_broadcastcnd_destroycnd_initcnd_signalcnd_timedwaitcnd_waitmtx_destroymtx_initmtx_lockmtx_plainmtx_recursivemtx_timedmtx_timedlockmtx_trylockmtx_unlockonce_flagONCE_FLAG_INITthrd_busythrd_createthrd_currentthrd_detachthrd_equalthrd_errorthrd_exitthrd_jointhrd_nomemthrd_sleepthrd_successthrd_timedoutthrd_yieldThread support librarythread_localtss_createtss_deleteTSS_DTOR_ITERATIONStss_gettss_setType supportBoolean type support libraryFixed width integer typesFLT_EVAL_METHODFLT_ROUNDSmax_align_tNULLNumeric limitsoffsetofptrdiff_tsize_tType supportVariadic functionsVariadic functionsva_argva_copyva_endva_listva_start

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在头文件<fenv.h>中定义



#define FE_DOWNWARD     /*implementation defined*/


(自 C99)

#define FE_TONEAREST    /*implementation defined*/


(自 C99)

#define FE_TOWARDZERO   /*implementation defined*/


(自 C99)

#define FE_UPWARD       /*implementation defined*/


(自 C99)

这些宏常量中的每一个都扩展为一个非负整数常量表达式，它可以与fesetround和fegetround一起使用来指示支持的浮点舍入模式之一。 该实现可以在<fenv.h>中定义附加的舍入模式常量，它们都应以FE_开头，后跟至少一个大写字母。 每个宏只在被支持时才被定义。

在大多数实现中，这些宏常量扩展为等于FLT_ROUNDS和float_round_style值的值。

常量

说明

FE_DOWNWARD

趋于负无穷

FE_TONEAREST

向最接近的整数舍入

FE_TOWARDZERO

四舍五入到零

FE_UPWARD

向正无穷的方向前进

实现可能会支持其他舍入模式。

当前舍入模式会影响以下内容：

  • 浮点算术运算符在常量表达式之外的结果

double x = 1;x/10; // 0.09999999999999999167332731531132594682276248931884765625   // or 0.1000000000000000055511151231257827021181583404541015625

  • 标准库数学函数的结果

sqrt(2); // 1.41421356237309492343001693370752036571502685546875      // or 1.4142135623730951454746218587388284504413604736328125

  • 浮点到浮点隐式转换和转换

double d = 1 + DBL_EPSILON;float f = d; //  1.00000000000000000000000           // or 1.00000011920928955078125

  • 字符串转换，如strtod或printf

strtof("0.1", NULL); // 0.0999999940395355224609375                  // or 0.100000001490116119384765625

  • 库的四舍五入功能nearbyint, rint, lrint

lrint(2.1); // 2 or 3

当前舍入模式不会影响以下内容：

  • 浮点到整数隐式转换和强制转换（总是趋近于零）

  • 浮点算术运算符在常量表达式中的结果（总是最近）

  • 库的功能是round, lround, ceil, floor, trunc

与任何浮点环境功能一样，只有在设置了#pragma STDC FENV_ACCESS ON时才能保证舍入。

例

#include <stdio.h>#include <stdlib.h>#include <fenv.h>#include <math.h>int main(){#pragma STDC FENV_ACCESS ON    fesetround(FE_DOWNWARD);    puts("rounding down: ");    printf("           pi = %.22f\n", acosf(-1));    printf("strtof(\"1.1\") = %.22f\n", strtof("1.1", NULL));    printf("    rint(2.1) = %.22f\n\n", rintf(2.1));    fesetround(FE_UPWARD);    puts("rounding up: ");    printf("           pi = %.22f\n", acosf(-1));    printf("strtof(\"1.1\") = %.22f\n", strtof("1.1", NULL));    printf("    rint(2.1) = %.22f\n", rintf(2.1));}

输出：

rounding down: 
           pi = 3.1415925025939941406250strtof("1.1") = 1.0999999046325683593750    rint(2.1) = 2.0000000000000000000000
 rounding up: 
           pi = 3.1415927410125732421875strtof("1.1") = 1.1000000238418579101563    rint(2.1) = 3.0000000000000000000000

参考

  • C11标准（ISO / IEC 9899：2011）：

    • 7.6 / 8浮点环境<fenv.h>（p：207）

  • C99标准（ISO / IEC 9899：1999）：

    • 7.6 / 7浮点环境<fenv.h>（p：188）

扩展内容

fegetroundfesetround（C99）（C99）

获取或设置舍入方向（功能）

| 用于浮点舍入宏的C ++文档 |

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