freopen function

古城微笑少年丶 2022-12-21 06:27 68阅读 0赞

# **freopen function** #

重定向输入输出流。

`#include <cstdio>`

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

## 1. Reopen `stream` with different `file` or `mode` ##

Reuses `stream` to either open the file specified by `filename` or to change its access `mode`.  
重用 `stream` 来打开 `filename` 指定的文件或更改其访问 `mode`。

If a new `filename` is specified, the function first attempts to close any file already associated with `stream` (third parameter) and disassociates it. Then, independently of whether that `stream` was successfuly closed or not, `freopen` opens the file specified by `filename` and associates it with the `stream` just as `fopen` would do using the specified `mode`.  
如果指定了新的 `filename`，则该函数首先尝试关闭任何已经与 `stream` (第三个参数) 相关联的文件，并将其取消关联。然后，不管是否成功关闭了 `stream`，`freopen` 打开文件名所指定的文件，并将其与 `stream` 关联起来，就像 `fopen` 使用指定的 `mode` 一样。

把一个新的文件名 `filename` 与给定的打开的流 `stream` 关联，同时关闭流中的旧文件。

If `filename` is a null pointer, the function attempts to change the `mode` of the stream. Although a particular library implementation is allowed to restrict the changes permitted, and under which circumstances.  
如果 `filename` 为空指针，则该函数尝试更改流的 `mode`。尽管允许特定的库实现来限制所允许的更改，以及在哪种情况下。

The `error indicator` and `eof indicator` are automatically cleared (as if `clearerr` was called).  
`error indicator` and `eof indicator` 将自动清除 (就像调用了 `clearerr` 一样)。

This function is especially useful for redirecting predefined streams like `stdin`, `stdout` and `stderr` to specific files (see the example below).  
此功能对于将预定义流 (例如 `stdin`, `stdout` and `stderr`) 重定向到特定文件特别有用 (请参见下面的示例)。

## 2. Parameters ##

`filename`  
C string containing the name of the file to be opened.  
C 字符串，包含要打开的文件的名称。

Its value shall follow the file name specifications of the running environment and can include a path (if supported by the system).  
其值应遵循运行环境的文件名规范，并且可以包含路径 (如果系统支持)。

If this parameter is a null pointer, the function attempts to change the mode of the `stream`, as if the file name currently associated with that stream had been used.  
如果此参数为空指针，则该函数将尝试更改 `stream` 的模式，就好像使用了当前与该流关联的文件名一样。

`mode`  
C string containing a file access mode. It can be:  
包含文件访问模式的 C 字符串。

`"r"` read: Open file for input operations. The file must exist.  
打开文件进行读取操作。该文件必须存在。

`"w"` write: Create an empty file for output operations. If a file with the same name already exists, its contents are discarded and the file is treated as a new empty file.  
创建一个空文件进行写入操作。如果已经存在同名文件，则将其内容丢弃，并将该文件视为新的空文件。

`"a"` append: Open file for output at the end of a file. Output operations always write data at the end of the file, expanding it. Repositioning operations (fseek, fsetpos, rewind) are ignored. The file is created if it does not exist.  
打开文件以在文件末尾写入。输出操作始终将数据写入文件的末尾，进行扩展。重新定位操作 (fseek, fsetpos, rewind) 将被忽略。如果文件不存在，则创建该文件。

`"r+"` read/update: Open a file for update (both for input and output). The file must exist.  
打开一个用于更新的文件，可读取也可写入。该文件必须存在。

`"w+"` write/update: Create an empty file and open it for update (both for input and output). If a file with the same name already exists its contents are discarded and the file is treated as a new empty file.  
创建一个空文件并打开以进行更新 (用于输入和输出)。如果已经存在同名文件，则将其内容丢弃，并将该文件视为新的空文件。

`"a+"` append/update: Open a file for update (both for input and output) with all output operations writing data at the end of the file. Repositioning operations (fseek, fsetpos, rewind) affects the next input operations, but output operations move the position back to the end of file. The file is created if it does not exist.  
打开一个文件进行更新 (用于输入和输出)，所有输出操作均在文件末尾写入数据。重新定位操作 (fseek, fsetpos, rewind) 会影响下一个输入操作，但是写入操作会将位置移回文件末尾。如果文件不存在，则创建该文件。

With the `mode` specifiers above the file is open as a `text` file. In order to open a file as a binary file, a `"b"` character has to be included in the `mode` string. This additional `"b"` character can either be appended at the end of the string (thus making the following compound modes: `"rb"`, `"wb"`, `"ab"`, `"r+b"`, `"w+b"`, `"a+b"`) or be inserted between the letter and the `"+"` sign for the mixed modes (`"rb+"`, `"wb+"`, `"ab+"`).  
使用 `mode` 说明符后，该文件将作为 `text` 文件打开。为了以二进制文件形式打开文件，必须在 `mode` 字符串中包含一个 `"b"` 字符。可以在字符串的末尾附加这个附加的 `"b"` 字符 (从而形成以下复合模式：`"rb"`, `"wb"`, `"ab"`, `"r+b"`, `"w+b"`, `"a+b"`) 或插入字母和 `"+"` 符号之间的混合模式 (`"rb+"`, `"wb+"`, `"ab+"`)。

The new C standard (C2011, which is not part of C++) adds a new standard subspecifier (`"x"`), that can be appended to any `"w"` specifier (to form `"wx"`, `"wbx"`, `"w+x"` or `"w+bx"`/`"wb+x"`). This subspecifier forces the function to fail if the file exists, instead of overwriting it.  
新的 C 标准 (C2011, which is not part of C++) 添加了新的标准子说明符 (`"x"`)，可以将其附加到任何 `"w"` 说明符 (以形成 `"wx"`, `"wbx"`, `"w+x"` or `"w+bx"`/`"wb+x"`)。如果文件存在，此子说明符将强制该函数失败，而不是覆盖它。

If additional characters follow the sequence, the behavior depends on the library implementation: some implementations may ignore additional characters so that for example an additional `"t"` (sometimes used to explicitly state a `text file`) is accepted.  
如果遵循此顺序，则其他行为取决于库的实现：某些实现可能会忽略其他字符，例如，可以接受其他 `"t"` (有时用于显式声明 `text file`)。

On some library implementations, opening or creating a text file with update mode may treat the stream instead as a binary file.  
在某些库实现中，以更新模式打开或创建文本文件可能会将流视为二进制文件。

`stream`  
pointer to a FILE object that identifies the stream to be reopened.  
指向一个标识要重新打开的流的 FILE 对象的指针。

## 3. Return Value ##

If the file is successfully reopened, the function returns the pointer passed as parameter `stream`, which can be used to identify the reopened stream.  
如果文件成功重新打开，该函数将返回作为参数 `stream` 传递的指针，该指针可用于标识重新打开的流。

Otherwise, a null pointer is returned.  
否则，将返回空指针。

On most library implementations, the `errno` variable is also set to a system-specific error code on failure.  
在大多数库实现中，`errno` 变量也会在失败时设置为特定于系统的错误代码。

如果文件成功打开，则函数返回一个指针，指向用于标识流的对象。否则，返回空指针。

## 4. Example ##

//============================================================================
    // Name        : freopen
    // Author      : Yongqiang Cheng
    // Version     : Version 1.0.0
    // Copyright   : Copyright (c) 2019 Yongqiang Cheng
    // Description : Hello World in C++, Ansi-style
    //============================================================================
    
    #ifndef _CRT_SECURE_NO_WARNINGS
    #define _CRT_SECURE_NO_WARNINGS
    #endif
    
    /* freopen example: redirecting stdout */
    #include <stdio.h>
    
    int main()
    {
    	freopen("output.txt", "w", stdout);
    	printf("This sentence is redirected to a file.\n");
    	fclose(stdout);
    
    	return 0;
    }

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This sample code redirects the output that would normally go to the standard output to a file called `output.txt`, that after this program is executed contains:  
此示例代码将通常会从标准输出重定向到名为 `output.txt` 的文件，该文件在执行此程序后包含：  
`This sentence is redirected to a file.`

### 4.1 Sample ###

//============================================================================
    // Name        : freopen
    // Author      : Yongqiang Cheng
    // Version     : Version 1.0.0
    // Copyright   : Copyright (c) 2019 Yongqiang Cheng
    // Description : Hello World in C++, Ansi-style
    //============================================================================
    
    #ifndef _CRT_SECURE_NO_WARNINGS
    #define _CRT_SECURE_NO_WARNINGS
    #endif
    
    /* freopen example: redirecting stdout */
    #include <stdio.h>
    
    int main()
    {
    	FILE *fp;
    	printf("yongqiang.\n");
    
    	fp = freopen("output.txt", "w", stdout);
    	printf("This sentence is redirected to a file.\n");
    
    	fclose(fp);
    
    	return 0;
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5neXExMTY_size_16_color_FFFFFF_t_70_pic_center 2]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5neXExMTY_size_16_color_FFFFFF_t_70_pic_center 3]

## References ##

[http://www.cplusplus.com/reference/cstdio/freopen/][http_www.cplusplus.com_reference_cstdio_freopen]

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[http_www.cplusplus.com_reference_cstdio_freopen]: http://www.cplusplus.com/reference/cstdio/freopen/