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Algorithms Algorithms（算法）bsearch bsearch_s qsort qsort_s Atomic operations Atomic operations library（原子操作库）ATOMIC_*_LOCK_FREE atomic_compare_exchange_strong atomic_compare_exchange_strong_explicit atomic_compare_exchange_weak atomic_compare_exchange_weak_explicit atomic_exchange atomic_exchange_explicit atomic_fetch_add atomic_fetch_add_explicit atomic_fetch_and atomic_fetch_and_explicit atomic_fetch_or atomic_fetch_or_explicit atomic_fetch_sub atomic_fetch_sub_explicit atomic_fetch_xor atomic_fetch_xor_explicit atomic_flag atomic_flag_clear atomic_flag_clear_explicit ATOMIC_FLAG_INIT atomic_flag_test_and_set atomic_flag_test_and_set_explicit atomic_init atomic_is_lock_free atomic_load atomic_load_explicit atomic_signal_fence atomic_store atomic_store_explicit atomic_thread_fence（线程围栏）ATOMIC_VAR_INIT kill_dependency memory_order（内存排序）C keywords auto（自动存储）break（跳出循环）C keywords（关键词）case char const（常量修饰符）continue default（预设运算式）do double（双精度浮点型）else enum（枚举类型）extern（全局变量）float（浮点数）for fortran goto（goto语句）if（if语句）inline（行内函式）int long（长整型）register（寄存器变量）restrict（ restrict类型限定符）return short signed sizeof（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_local C language #define directive #elif directive #else directive #endif directive #error directive #if directive #ifdef directive #ifndef directive #include directive #line directive #pragma directive alignas（对齐指定符）Alternative operators and tokens（替代运算符和令牌）Analyzability Arithmetic operators Arithmetic types Array declaration（数组声明）Array initialization（阵列初始化）ASCII Chart Assignment operators（赋值运算符）types（atomic类型限定符）Basic concepts Bit fields（位域）break statement C language C Operator Precedence cast operator character 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 statement Declarations（声明）do-while loop Enumerations（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 loop Function declarations（函数声明）Function definitions（函数声明）Functions Generic selection泛型选择 goto statement Identifier（标示符）if statement Implicit 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 constant Lifetime（生命期）Logical operators（逻辑运算符）Lookup and name spaces Main function（主函式）Member access operators（会员接入运营商）Memory model Objects and alignment（字节对齐）Order of evaluation（评估顺序）Other operators Phases of translation（翻译阶段）Pointer declaration Preprocessor（预处理）restrict type qualifier（restrict类型限定符）return statement Scalar 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 statement Thread storage duration（线程存储时间）Type Type（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 specifier Date and time asctime（asctime函数）asctime_s clock CLOCKS_PER_SEC clock_t ctime（ctime函数）ctime_s Date and time utilities（日期和时间库）difftime（计算两个时间的间隔）gmtime gmtime_s localtime localtime_s mktime（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）time timespec timespec_get time_t tm wcsftime（格式化时间宽字符）Dynamic memory management aligned_alloc C memory management library（内存管理库）calloc free（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 handling abort_handler_s assert（断言）constraint_handler_t errno（错误报告）Error handling（错误处理）Error numbers（错误个数）ignore_handler_s set_constraint_handler_s static_assert File input/output clearerr（清除/复位）fclose feof ferror fflush（清空文件缓冲区）fgetc fgetpos fgets fgetwc fgetws File input/output fopen fopen_s fprintf fprintf_s fputc fputs fputwc fputws fread freopen freopen_s fscanf fscanf_s fseek fsetpos ftell fwide fwprintf fwprintf_s fwrite fwscanf fwscanf_s getc getchar gets gets_s getwchar perror printf printf_s putc putchar puts putwc putwchar remove rename rewind scanf scanf_s setbuf setvbuf snprintf sprintf sscanf sscanf_s swprintf swprintf_s swscanf swscanf_s tmpfile tmpfile_s tmpnam tmpnam_s ungetc ungetwc vfprintf vfprintf_s vfscanf vfscanf_s vfwprintf vfwprintf_s vfwscanf vfwscanf_s vprintf vprintf_s vscanf vscanf_s vsnprintf vsprintf vsscanf vsscanf_s vswprintf vswprintf_s vswscanf vswscanf_s vwprintf vwprintf_s vwscanf vwscanf_s wprintf wprintf_s wscanf wscanf_s Localization support lconv LC_ALL LC_COLLATE LC_CTYPE LC_MONETARY LC_NUMERIC LC_TIME localeconv Localization support setlocale 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

文字

在头文件<stdio.h>中定义
	(1)
FILE * fopen（const char * filename，const char * mode）;	（直到C99）
FILE * fopen（const char 限制文件名，const char 限制模式）;	（自C99以来）
errno_t fopen_s（FILE * restrict 限制streamptr，const char 限制文件名，const char *限制模式）;	(2)	（自C11以来）

1）打开由该文件指示的文件filename并返回指向与该文件关联的文件流的指针。mode用于确定文件访问模式。

2）与（1）相同，除了写入指向文件流的指针streamptr以及在运行时检测到以下错误并调用当前安装的约束处理函数：

streamptr 是一个空指针
filename 是一个空指针
mode 是一个空指针

作为所有边界检查函数，fopen_s只有在被__STDC_LIB_EXT1__实现定义__STDC_WANT_LIB_EXT1__并且1在包含之前用户定义为整数常量时才能保证可用<stdio.h>。

参数

文件名	-	将文件流关联到的文件名
模式	-	null-terminated character string determining file access mode File access mode string Meaning Explanation Action if file already exists Action if file does not exist "r" read Open a file for reading read from start failure to open "w" write Create a file for writing destroy contents create new "a" append Append to a file write to end create new "r+" read extended Open a file for read/write read from start error "w+" write extended Create a file for read/write destroy contents create new "a+" append extended Open a file for read/write write to end create new File access mode flag "b" can optionally be specified to open a file in binary mode. This flag has effect only on Windows systems. On the append file access modes, data is written to the end of the file regardless of the current position of the file position indicator. When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations. File access mode flag "x" can optionally be appended to "w" or "w+" specifiers. This flag forces the function to fail if the file exists, instead of overwriting it. (C11) When using fopen_s or freopen_s, file access permissions for any file created with "w" or "a" prevents other users from accessing it. File access mode flag "u" can optionally be prepended to any specifier that begins with "w" or "a", to enable the default fopen permissions. (C11)	File access mode string	Meaning	Explanation	Action if file already exists	Action if file does not exist	"r"	read	Open a file for reading	read from start	failure to open	"w"	write	Create a file for writing	destroy contents	create new	"a"	append	Append to a file	write to end	create new	"r+"	read extended	Open a file for read/write	read from start	error	"w+"	write extended	Create a file for read/write	destroy contents	create new	"a+"	append extended	Open a file for read/write	write to end	create new	File access mode flag "b" can optionally be specified to open a file in binary mode. This flag has effect only on Windows systems. On the append file access modes, data is written to the end of the file regardless of the current position of the file position indicator.	When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.	File access mode flag "x" can optionally be appended to "w" or "w+" specifiers. This flag forces the function to fail if the file exists, instead of overwriting it. (C11)	当使用fopen_s或freopen_s时，用“w”或“a”创建的任何文件的文件访问权限会阻止其他用户访问它。文件访问模式标志“u”可以有选择地添加到以“w”或“a”开头的任何说明符中，以启用默认的fopen权限。（C11）
文件访问模式字符串	含义	说明	行动如果文件已经存在	如果文件不存在，则采取行动
“R”	读	打开一个文件以供阅读	从开始阅读	未能打开
“W”	写	创建一个文件写入	破坏内容	创建新的
“一个”	附加	附加到文件	写入结束	创建新的
“R +”	阅读扩展	打开文件进行读取/写入	从开始阅读	错误
“W +”	写入扩展	创建一个用于读/写的文件	破坏内容	创建新的
“A +”	追加扩展	打开文件进行读取/写入	写入结束	创建新的

| 可以选择指定文件访问模式标志“b”以二进制模式打开文件。该标志只对Windows系统有效。在附加文件访问模式下，无论文件位置指示符的当前位置如何，数据都会写入文件末尾。|

| 当以更新模式（'+'作为上述模式参数值列表中的第二个或第三个字符）打开文件时，可以在关联的流上执行输入和输出。然而，输出不应直接跟随输入而没有插入调用fflush函数或文件定位函数（fseek，fsetpos，或倒带），并且输入不应直接跟随输出而没有中间呼叫到一个文件中的定位函数，除非输入操作遇到文件结束。在某些实现中打开（或创建）具有更新模式的文本文件可能会打开（或创建）二进制流。|

| 文件访问模式标志“x”可以选择性地附加到“w”或“w +”说明符。如果文件存在，该标志强制该功能失败，而不是覆盖它。（C11）|

| 当使用fopen_s或freopen_s时，用“w”或“a”创建的任何文件的文件访问权限会阻止其他用户访问它。文件访问模式标志“u”可以有选择地添加到以“w”或“a”开头的任何说明符中，以启用默认的fopen权限。（C11）|

| streamptr | - | 指向函数存储结果的指针（out-parameter）|的指针

返回值

1）如果成功，返回一个指向新文件流的指针。除非filename指向交互式设备，否则该流完全缓冲。出错时，返回一个null pointer。POSIX需要的是errno在这种情况下设置。

2）如果成功，则返回零，并写入指向新文件流的指针*streamptr。出错时，返回一个非零的错误代码并将空指针写入*streamptr（除非streamptr是空指针本身）。

注意

格式filename是实现定义的，并不一定是指文件（例如它可能是控制台或通过文件系统API可访问的其他设备）。在支持它们的平台上，filename可能包括绝对或相对的文件系统路径。

Example

#include <stdio.h>#include <stdlib.h>
 int main(void){
    FILE* fp = fopen("test.txt", "r");    if(!fp) {        perror("File opening failed");        return EXIT_FAILURE;    }
 
    int c; // note: int, not char, required to handle EOF    while ((c = fgetc(fp)) != EOF) { // standard C I/O file reading loop       putchar(c);    } 
    if (ferror(fp))        puts("I/O error when reading");    else if (feof(fp))        puts("End of file reached successfully"); 
    fclose(fp);}

参考

C11标准（ISO / IEC 9899：2011）：

7.21.5.3 fopen函数（p：305-306）
K.3.5.2.1 fopen_s函数（p：588-590）

C99标准（ISO / IEC 9899：1999）：

7.19.5.3 fopen函数（p：271-272）

C89 / C90标准（ISO / IEC 9899：1990）：

4.9.5.3 fopen函数