为什么在更新模式下,在读和写之间总是需要fseek或fflush? [英] Why is fseek or fflush always required between reading and writing in the update modes?
问题描述
Q:我试图通过使用
fopen mode
来更新
,
位置的文件。 r +
阅读某个字符串,并写入
返回一个修改过的字符串,但不是
正在工作。
A:在编写
之前,一定要调用fseek
,这两个函数都要返回字符串开头的
尝试
来覆盖,而因为fseek
或fflush
总是需要的在
阅读和写入
+模式之间。
My问题是为什么 fseek
或 fflush
写+模式?
的第5.2节Andrew Koenig的
C Traps and Pitfalls ( 1989)提到,这是因为一个向后兼容性问题。任何人都可以详细解释一下吗?
库缓冲区输入和输出操作。查看setvbuf()和_IOFBF,_IOLBF参数。 fseek()或fflush()要求库提交缓冲操作。该标准指定一个搜索或刷新操作是强制性的,以允许图书馆的一些捷径;否则,对于每个I / O操作,lib将必须检查前面的操作是否也是读操作(或写操作),并且如果I / O的方向改变,则自己触发刷新。 / p>
Q: I'm trying to update a file in place, by using
fopen
mode"r+"
, reading a certain string, and writing back a modified string, but it's not working.A: Be sure to call
fseek
before you write, both to seek back to the beginning of the string you're trying to overwrite, and because anfseek
orfflush
is always required between reading and writing in the read/write "+" modes.
My question is why fseek
or fflush
is always required between reading and writing in the read/write "+" modes? Section 5.2 of
Andrew Koenig's
C Traps and Pitfalls (1989) mentioned that it is because of a backward compatibility issue. Can anyone explain in detail?
The library buffers input and output operations. Check out setvbuf() and the _IOFBF, _IOLBF parameters to that funktion. fseek() or fflush() require the library to commit buffered operations. The standard specifies a seek or flush operation as mandatory to allow the library some shortcuts; otherwise, for every I/O operation, the lib would have to check if the previous operation was also a read op (or a write op), and trigger a flush by itself if the "direction" of the I/O changed.
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