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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
characters

每个C程序编码在托管执行环境中运行都包含被调用函数的定义(不是原型),该函数main是程序的指定开始。

int main (void) { body }

(1)


int main (int argc, char *argv[]) { body }

(2)


int main (int argc, char *argv[] , other_parameters ) { body }

(3)


/* another implementation-defined signature */

(4)


参数

ARGC

-

非负值,表示从运行程序的环境传递给程序的参数数量。

ARGV

-

指向由空字符结尾的多字节字符串组成的指针数组,这些字符串表示从执行环境(argv0到argvargc-1)传递给程序的参数。argvargc的值保证为0。

body

-

主要功能的主体

other_parameters

-

实现可能允许主函数的其他形式。一个非常常见的扩展是传递char * []类型的第三个参数,指向执行环境变量的指针数组。

名称argcargv是任意的,以及参数类型的表示:int main(int ac, char** av)是同样有效。

返回值

如果使用return语句,则返回值将用作隐式调用的参数exit()(请参阅下面的详细信息)。值为零并EXIT_SUCCESS表示终止成功,该值EXIT_FAILURE表示不成功终止。

说明

main在所有具有静态存储持续时间的对象被初始化之后,程序启动时会调用该函数。它是在托管环境(即操作系统)中执行的程序的指定入口点。任何独立程序(引导加载程序,操作系统内核等)的入口点的名称和类型都是实现定义的。

主函数的双参数形式的参数允许从执行环境传递任意多字节字符串(这些通常称为命令行参数)。指针argv[1] .. argv[argc-1]指向每个字符串中的第一个字符。argv[0]是一个指向空字符结尾多字节字符串的初始字符的指针,它表示用于调用程序本身的名称(或者,如果执行环境不支持它,argv0保证为零)。

如果执行环境无法区分大写字母和小写字母,则命令行参数会转换为小写字母。

这些字符串是可修改的,任何修改都会一直存在,直到程序终止,尽管这些修改不会传播回执行环境:例如,可以使用它们strtok

所指向的数组的大小argv至少为argc+1,最后一个元素argv[argc]保证为空指针。

main函数有几个特殊的属性:

1)该功能的原型不能由程序提供

2)如果主函数的返回类型与之兼容int,那么从初始调用返回到main(但不是从任何后续递归调用返回)相当于执行该exit函数,其值为主函数作为参数返回传递(然后调用注册的函数atexit,刷新并关闭所有流,并删除创建的文件tmpfile,并将控制权返回给执行环境)。

3)如果主函数的返回类型与int(eg void main(void))不兼容,则返回到执行环境的值未指定

4)main函数的主体不需要包含返回语句:如果控制到达main没有遇到return语句的结尾,效果就是执行return 0;

演示如何通知程序哪里可以找到它的输入以及在哪里编写结果。

Invocation: ./a.out indatafile outdatafile.

#include <stdio.h>
 int main(int argc, char *argv[]){    printf("argc = %d\n", argc);
    int ndx = 0;    while (ndx < argc)    {          printf("argv[%d] --> %s\n", ndx,argv[ndx]);          ++ndx;    }    printf("argv[argc] = %s\n", argv[argc]);    return 0;}

可能的输出:

argc = 3argv[0] --> ./a.out
argv[1] --> indatafile
argv[2] --> outdatafile
argv[argc] = (null)

参考

  • C11 standard (ISO/IEC 9899:2011):

    • 5.1.2.2.1 Program startup (p: 13)

  • C99 standard (ISO/IEC 9899:1999):

    • 5.1.2.2.1 Program startup (p: 12)

  • C89/C90 standard (ISO/IEC 9899:1990):

    • 2.1.2.2 Hosted environment

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