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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
personnages

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(staticextern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)

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)

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

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