[已解决]fopen函数中“a”的用法

wxyMelody

c语言小白想请教大家一下。看到小甲鱼c语言的文件教学视频中有一句话不是很理解。若要向已存在的文件追加信息，只能用“a”方式打开文件，但此时文件必须存在，否则会出错。可是查阅了一些资料，“a”在文件不存在时不是会新建文件嘛，有点迷惑
问题比较基础，谢谢大家的解答 不胜感激！

最佳答案

月排行榜 / 总排行榜

shooan

2020-10-29 12:06:35

filename        -        file name to associate the file stream to
mode        -        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 no effect on POSIX systems, but on Windows, for example, it disables special handling of '\n' and '\x1A'.
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.
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. (C++17)
The behavior is undefined if the mode is not one of the strings listed above. Some implementations define additional supported modes (e.g. Windows).

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wp231957

实测才是王道

shooan

filename        -        file name to associate the file stream to
mode        -        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 no effect on POSIX systems, but on Windows, for example, it disables special handling of '\n' and '\x1A'.
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.
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. (C++17)
The behavior is undefined if the mode is not one of the strings listed above. Some implementations define additional supported modes (e.g. Windows).

shooan

shooan 发表于 2020-10-29 12:06
filename        -        file name to associate the file stream to
mode        -        null-terminated character string determ ...

这里说的比较清楚了，如果文件不存在的话，会创建一个新的文件

wxyMelody

wp231957 发表于 2020-10-29 11:32
实测才是王道

谢谢大佬！确实要实测一下！

wxyMelody

wp231957 发表于 2020-10-29 11:32
实测才是王道

谢谢大佬，确实要实测一下！

wxyMelody

shooan 发表于 2020-10-29 12:07
这里说的比较清楚了，如果文件不存在的话，会创建一个新的文件

谢谢回答！