AlgorithmsAlgorithms（算法）bsearchbsearch_sqsortqsort_sAtomic operationsAtomic operations library（原子操作库）ATOMIC_*_LOCK_FREEatomic_compare_exchange_strongatomic_compare_exchange_strong_explicitatomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_exchangeatomic_exchange_explicitatomic_fetch_addatomic_fetch_add_explicitatomic_fetch_andatomic_fetch_and_explicitatomic_fetch_oratomic_fetch_or_explicitatomic_fetch_subatomic_fetch_sub_explicitatomic_fetch_xoratomic_fetch_xor_explicitatomic_flagatomic_flag_clearatomic_flag_clear_explicitATOMIC_FLAG_INITatomic_flag_test_and_setatomic_flag_test_and_set_explicitatomic_initatomic_is_lock_freeatomic_loadatomic_load_explicitatomic_signal_fenceatomic_storeatomic_store_explicitatomic_thread_fence（线程围栏）ATOMIC_VAR_INITkill_dependencymemory_order（内存排序）C keywordsauto（自动存储）break（跳出循环）C keywords（关键词）casecharconst（常量修饰符）continuedefault（预设运算式）dodouble（双精度浮点型）elseenum（枚举类型）extern（全局变量）float（浮点数）forfortrangoto（goto语句）if（if语句）inline（行内函式）intlong（长整型）register（寄存器变量）restrict（ restrict类型限定符）returnshortsignedsizeof（sizeof运算符）static（静态变量）struct（结构体）switch（switch语句）typedef（typedef关键字）union（联合体）unsigned（无符号）void（空类型）volatile（volatile变量）while（while语句）_Alignas_Alignof_Atomic_Bool_Complex_Generic_Imaginary_Noreturn_Static_assert_Thread_localC language#define directive#elif directive#else directive#endif directive#error directive#if directive#ifdef directive#ifndef directive#include directive#line directive#pragma directivealignas（对齐指定符）Alternative operators and tokens（替代运算符和令牌）AnalyzabilityArithmetic operatorsArithmetic typesArray declaration（数组声明）Array initialization（阵列初始化）ASCII ChartAssignment operators（赋值运算符）types（atomic类型限定符）Basic conceptsBit fields（位域）break statementC languageC Operator Precedencecast operatorcharacter constant（字符字面量）Comments（注释符）Comparison operators（比较运算符）compound literals（符合字面量）Conditional inclusion（条件包含）Conformance（一致性）const type qualifier（const 限定符）Constant expressions（常量表达）continue statementDeclarations（声明）do-while loopEnumerations（枚举类型）Escape sequences（转义字符）Expressions（表达式）External and tentative definitions（外部和暂定的定义）File scope（文件范围）floating constant（浮点常量）for loopFunction declarations（函数声明）Function definitions（函数声明）FunctionsGeneric selection泛型选择goto statementIdentifier（标示符）if statementImplicit conversions（隐式转换）Increment/decrement operators（前置/后置操作符）Initialization（初始化）inline function specifier（内联函式）integer constantLifetime（生命期）Logical operators（逻辑运算符）Lookup and name spacesMain function（主函式）Member access operators（会员接入运营商）Memory modelObjects and alignment（字节对齐）Order of evaluation（评估顺序）Other operatorsPhases of translation（翻译阶段）Pointer declarationPreprocessor（预处理）restrict type qualifier（restrict类型限定符）return statementScalar initialization（标量类型初始化）Scope（范围）sizeof operator（sizeof运算符）Statements（陈述）static assert declaration（静态断言声明）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 statementThread storage duration（线程存储时间）TypeType（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 specifierDate and timeasctime（asctime函数）asctime_sclockCLOCKS_PER_SECclock_tctime（ctime函数）ctime_sDate and time utilities（日期和时间库）difftime（计算两个时间的间隔）gmtimegmtime_slocaltimelocaltime_smktime（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）timetimespectimespec_gettime_ttmwcsftime（格式化时间宽字符）Dynamic memory managementaligned_allocC memory management library（内存管理库）callocfree（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 handlingabort_handler_sassert（断言）constraint_handler_terrno（错误报告）Error handling（错误处理）Error numbers（错误个数）ignore_handler_sset_constraint_handler_sstatic_assertFile input/outputclearerr（清除/复位）fclosefeofferrorfflush（清空文件缓冲区）fgetcfgetposfgetsfgetwcfgetwsFile input/outputfopenfopen_sfprintffprintf_sfputcfputsfputwcfputwsfreadfreopenfreopen_sfscanffscanf_sfseekfsetposftellfwidefwprintffwprintf_sfwritefwscanffwscanf_sgetcgetchargetsgets_sgetwcharperrorprintfprintf_sputcputcharputsputwcputwcharremoverenamerewindscanfscanf_ssetbufsetvbufsnprintfsprintfsscanfsscanf_sswprintfswprintf_sswscanfswscanf_stmpfiletmpfile_stmpnamtmpnam_sungetcungetwcvfprintfvfprintf_svfscanfvfscanf_svfwprintfvfwprintf_svfwscanfvfwscanf_svprintfvprintf_svscanfvscanf_svsnprintfvsprintfvsscanfvsscanf_svswprintfvswprintf_svswscanfvswscanf_svwprintfvwprintf_svwscanfvwscanf_swprintfwprintf_swscanfwscanf_sLocalization supportlconvLC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIMElocaleconvLocalization 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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函数定义将函数体（声明和语句序列）与函数名称和参数列表关联起来。与函数声明不同，函数定义只允许在文件范围内（没有嵌套函数）。
C支持两种不同形式的函数定义：
specifiers-and-qualifiers parameter-list-declarator function-body | (1) | |
|---|---|---|
specifiers-and-qualifiers identifier-list-declarator declaration-list function-body | (2) |
其中
符和限定符 | - | 类型说明符的组合，可能被声明器修改，形成返回类型存储类说明符，它确定标识符（静态，外部或无）与inline，noreturn或none |
|---|---|---|
参数列表说明符 | - | 一个函数类型的声明符，它使用参数列表来指定函数参数 |
标识符列表说明符 | - | 一个函数类型的声明符，它使用标识符列表来指定函数参数 |
声明列表 | - | 在identifier-list-declarator中声明每个标识符的声明序列。这些声明不能使用初始化器，唯一允许的存储类说明符是寄存器。 |
功能体 | - | 一个复合语句，它是一个括号括起来的声明和语句序列，每当调用该函数时都会执行 |
可能由声明者修改的类型说明符构成返回类型
存储类说明符，其中确定所述识别符的键（
static，extern或无）函数说明符inline，noreturn或none
parameter-list-declarator - a declarator for a function type which uses a [parameter list](function_declaration) to designate function parameters identifier-list-declarator - a declarator for a function type which uses a [identifier list](function_declaration) to designate function parameters declaration-list - sequence of declarations that declare every identifier in identifier-list-declarator. These declarations cannot use initializers and the only [storage-class specifier](storage_duration) allowed is `register`. function-body - a [compound statement](statements#Compound_statements), that is a brace-enclosed sequence of declarations and statements, that is executed whenever this function is called
1）新式（C89）功能定义。该定义既引入了函数本身，又作为未来函数调用表达式的函数原型，强制从参数表达式转换为声明的参数类型。
int max(int a, int b){ return a>b?a:b;}
double g(void){ return 0.1;}2）旧式（K＆R）功能定义。这个定义不像原型，任何未来的函数调用表达式都会执行默认参数促销。
int max(a, b)int a, b;{ return a>b?a:b;}double g(){ return 0.1;}说明
与函数声明一样，函数的返回类型由声明符中的类型说明符确定，并可能在声明中像通常一样由声明器修改，这些类型必须是完整的非数组对象类型或类型void。
void f(char *s) { puts(s); } // return type is voidint sum(int a, int b) { return a+b: } // return type is intint (*foo(const void *p))[3] { // return type is pointer to array of 3 int return malloc(sizeof(int[3]));}和函数声明一样，参数的类型从函数调整到指针，从数组到指针，为了构造函数类型，它们的顶级限定符被去除。
与函数声明不同，未命名的形式参数是不允许的，即使它们没有在函数中使用，它们也必须被命名。唯一的例外是特殊参数列表(void)。
int f(int, int); // declaration// int f(int, int) { return 7; } // Errorint f(int a, int b) { return 7; } // definitionint g(void) { return 8; } // OK, void doesn't declare a parameter在函数体内，每个参数都是一个左值表达式，它们具有自动存储持续时间和块范围。内存中的参数布局（或者如果它们存储在内存中）未指定：它是调用约定的一部分。
int main(int ac, char **av){
ac = 2; // parameters are lvalues
av = (char *[]){"abc", "def", NULL}; f(ac, av);}有关函数调用机制的其他详细信息，请参阅函数调用运算符，然后返回以从函数返回。
__func__在每个函数体中，可以使用具有块范围和静态存储持续时间的特殊预定义变量__func__，就像在紧跟在大括号之后定义的那样。static const char __func __ [] =“function name”; 这个特殊的标识符有时与预定义的宏常量__FILE__和__LINE__结合使用，例如通过assert。 | （自C99以来） |
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笔记
参数列表必须明确地出现在声明器中，它不能从 typedef 继承。
typedef int p(int q, int r); // p is a function type int(int, int)p f { return q + r; } // Error在C89中，说明符和限定符是可选的，如果省略，函数的返回类型默认为int（可能由声明者修改）。另外，旧式定义不需要声明列表中的每个参数。任何声明缺失的参数都有int类型。max（a，b）// a和b的类型是int，返回类型是int {return a> b？a：b; } | （直到C99） |
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参考
C11 standard (ISO/IEC 9899:2011):
6.9.1 Function definitions (p: 156-158)
C99 standard (ISO/IEC 9899:1999):
6.9.1 Function definitions (p: 141-143)
C89/C90 standard (ISO/IEC 9899:1990):
3.7.1 Function definitions