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characters

在头文件<stdio.h>中定义




(1)


FILE * freopen(const char * filename,const char * mode,FILE * stream);

(直到C99)


FILE * freopen(const char *限制文件名,const char *限制模式,FILE *限制流);

(自C99以来)


errno_t freopen_s(FILE * restrict * restrict newstreamptr,const char * restrict filename,const char * restrict mode,FILE * restrict stream);

(2)

(自C11以来)

1)首先,试图关闭与之相关的文件stream,忽略任何错误。然后,如果filename不为null,则尝试打开通过filename使用modeas 指定的文件fopen,并将该文件与指向的文件流相关联stream。如果filename是空指针,那么函数将尝试重新打开已经关联的文件stream(在此情况下,它是实现定义允许哪些模式更改)。

2)与(1)相同,不同之处在于mode,处理方式如下,fopen_s指向文件流的指针被写入,newstreamptr并且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:

  • newstreamptr 是一个空指针

  • stream 是一个空指针

  • mode 是一个空指针

作为所有的边界检查函数,freopen_s只能保证__STDC_LIB_EXT1__是由实现定义的,并且如果用户在包含之前定义__STDC_WANT_LIB_EXT1__为整数常量。  1<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 new 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”

打开一个文件以供阅读

从开始阅读

未能打开






































“W”

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






































返回值

1)stream成功的值的副本,失败时的空指针。

2)成功时为零(并且stream写入的值的副本,*newstreamptr错误时不为零(并且空指针被写入,*newstreamptr除非newstreamptr它本身是空指针)。

注意

freopen是通过I / O操作或通过I / O操作建立后,改变流的窄/宽方向的唯一方法fwide

以下代码重定向stdout到一个文件。

#include <stdio.h>#include <stdlib.h>
 int main(void){    puts("stdout is printed to console");    if (freopen("redir.txt", "w", stdout) == NULL)    {       perror("freopen() failed");       return EXIT_FAILURE;    }    puts("stdout is redirected to a file"); // this is written to redir.txt    fclose(stdout);}

输出:

stdout is printed to console

参考

  • C11标准(ISO / IEC 9899:2011):

    • 7.21.5.4 freopen函数(p:307)

    • K.3.5.2.2 freopen_s函数(p:590)

  • C99标准(ISO / IEC 9899:1999):

    • 7.19.5.4 freopen函数(p:272-273)

  • C89 / C90标准(ISO / IEC 9899:1990):

    • 4.9.5.4 freopen函数

fopenfopen_s(C11)

打开一个文件(函数)

FCLOSE

关闭文件(函数)

| 用于freopen的C ++文档

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