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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每个出现在C程序中的标识符都是可见的（也就是说可以使用），只能在源代码的一些可能不连续的部分称为它的范围。
在一个范围内，只有当实体位于不同的名称空间时，标识符才可以指定多个实体。
C有四种范围：
块范围
文件范围
功能范围
函数原型范围
嵌套范围
如果由同一标识符命名的两个不同实体同时在范围内，并且它们属于同一名称空间，则范围是嵌套的（不允许其他形式的范围重叠），并且出现在内部范围内的声明隐藏出现在外部范围中的声明：
// The name space here is ordinary identifiers.
int a; // file scope of name a begins here void f(void){
int a = 1; // the block scope of the name a begins here; hides file-scope a {
int a = 2; // the scope of the inner a begins here, outer a is hidden printf("%d\n", a); // inner a is in scope, prints 2 } // the block scope of the inner a ends here printf("%d\n", a); // the outer a is in scope, prints 1} // the scope of the outer a ends here void g(int a); // name a has function prototype scope; hides file-scope a块范围
在复合语句中声明的任何标识符的范围，包括函数体，或在if，switch，for，while或do-while语句（自C99以后）中出现的任何表达式，声明或语句中，或者在参数列表中一个函数定义从声明的地方开始，并在声明块或声明结束时结束。
void f(int n) // scope of the function parameter 'n' begins{ // the body of the function begins ++n; // 'n' is in scope and refers to the function parameter// int n = 2; // error: cannot redeclare identifier in the same scope for(int n = 0; n<10; ++n) { // scope of loop-local 'n' begins printf("%d\n", n); // prints 0 1 2 3 4 5 6 7 8 9 } // scope of the loop-local 'n' ends // the function parameter 'n' is back in scope printf("%d\n", n); // prints the value of the parameter} // scope of function parameter 'n' endsint a = n; // Error: name 'n' is not in scope在C99之前，选择和迭代语句没有建立它们自己的块范围（尽管如果在语句中使用复合语句，它具有通常的块范围）：enum {a，b}; int不同（void）{if（sizeof（enum {b，a}）！= sizeof（int））返回a; // a == 1 return b; // C89中的b == 0，C99中的b == 1} | （自C99以来） |
|---|
默认情况下，块范围变量没有链接和自动存储持续时间。请注意，非VLA局部变量的存储时间在输入块时开始，但直到看到声明，该变量不在范围内且无法访问。
文件范围
在任何块或参数列表之外声明的任何标识符的范围从声明点开始，并在翻译单元结束时结束。
int i; // scope of i beginsstatic int g(int a) { return a; } // scope of g begins (note, "a" has block scope)int main(void){
i = g(2); // i and g are in scope}文件范围标识符默认具有外部链接和静态存储持续时间。
功能范围
在函数中声明的标签（只有标签）在该函数中的任何位置，在所有嵌套块中，在它自己的声明之前和之后。注意：通过在任何语句之前的冒号字符之前使用未使用的标识符来隐式声明标签。
void f(){ {
goto label; // label in scope even though declared laterlabel:; }
goto label; // label ignores block scope} void g(){
goto label; // error: label not in scope in g()}函数原型范围
在函数声明的参数列表中引入的不是定义的名称的作用域结束于函数声明器的末尾。
int f(int n, int a[n]); // n is in scope and refers to the first parameter
请注意，如果声明中有多个或嵌套的声明符，则该作用域将在最近的封闭函数声明符的末尾结束：
void f ( // function name 'f' is at file scope
long double f, // the identifier 'f' is now in scope, file-scope 'f' is hidden char (**a)[10 * sizeof f] // 'f' refers to the first parameter, which is in scope); enum{ n = 3 };int (*(*g)(int n))[n]; // the scope of the function parameter 'n' // ends at the end of its function declarator // in the array declarator, global n is in scope// (this declares a pointer to function returning a pointer to an array of 3 int)生，m声明
结构体，联合体和枚举标签的范围在标签出现在声明标签的类型说明符后立即开始。
struct Node {
struct Node* next; // Node is in scope and refers to this struct};枚举常量的范围在枚举器列表中出现其定义枚举器后立即开始。
enum { x = 12 };{ enum { x = x + 1, // new x is not in scope until the comma, x is initialized to 13
y = x + 1 // the new enumerator x is now in scope, y is initialized to 14 };}任何其他标识符的范围都在其声明符结束后并在初始化程序之前（如果有）开始：
int x = 2; // scope of the first 'x' begins{
int x[x]; // scope of the newly declared x begins after the declarator (x[x]). // Within the declarator, the outer 'x' is still in scope. // This declares a VLA array of 2 int.}
unsigned char x = 32; // scope of the outer 'x' begins{
unsigned char x = x; // scope of the inner 'x' begins before the initializer (= x) // this does not initialize the inner 'x' with the value 32, // this initializes the inner 'x' with its own, indeterminate, value}
unsigned long factorial(unsigned long n)// declarator ends, 'factorial' is in scope from this point{ return n<2 ? 1 : n*factorial(n-1); // recursive call}作为一种特殊情况，不是标识符声明的类型名称的范围被认为是在类型名称中出现标识符的地方之后开始的，因为它不会被省略。
注意
在C89之前，即使在块内引入外部链接的标识符也具有文件范围，因此，C89编译器不需要诊断使用超出范围的外部标识符（这种用法是未定义的行为） 。
循环体内的局部变量可以隐藏在C语言的for循环的init子句中声明的变量（它们的作用域是嵌套的），但在C ++中不能这样做。
与C ++不同，C没有结构范围：在struct / union / enum声明中声明的名称与结构声明在同一个范围内（除了数据成员在它们自己的成员名称空间中）：
struct foo {
struct baz {}; enum color {RED, BLUE};};struct baz b; // baz is in scopeenum color x = RED; // color and RED are in scope