从txt文件读取时的文件指针 [英] file pointer while reading from txt file

问题描述

在我的应用程序中，我正在从文本文件中读取，但我需要关注我正在阅读的文件指针或已读取的文件内容。以及从哪里开始下一步。有两种情况我需要这样做。



CASE 1 ::当系统A和B在A写入的同时访问文件F时并且B正在读取文件F.所以系统A需要知道我已经阅读了多少以及从哪里进一步阅读？



案例2 ::当系统A从文件F1中读取并在分析后将文件F1行描述写入文件F2。如果检测到某些特定错误代码，则在读取文件F1时，我需要关闭文件F2并跳过ERROR LINE打开新文件F3，从ERRORLINE + 1开始重新开始F1分析。



您对这两种情况的建议将不胜感激。

感谢您耐心阅读

解决方案

只需跟踪变量中的文件位置即可。文件I / O函数返回已读取或写入的项目数。



高级文件I / O方法通常提供返回当前位置的函数（例如 ftell ）也可以使用。



在多个进程访问文件时必须特别小心同一时间。您应该避免从另一个进程实际打开的文件中读取;特别是当它不仅附加数据而且还插入/覆盖时。所以你应该检查你的案例1是否可以用另一种方式处理。



案例2的伪代码：

 n = 2 
 in = open（F1，read）
 out = open（F2，create_write）
 while（！end_of_file（in））
 {
 line = readline（in）
 description = parseline（line）
 if（error）
 {
 close（out）
n = n + 1 
 out = open（Fn）
} 
 else 
写（out，description）
} 
 close（in）
 close（out）

In my application I am reading from the text file but i need to keep eye on the file pointer where I am reading or how much content of the file i already read. and where to start next. There are TWO situations where i need to do this.

CASE 1:: While 2 system A and B are accessing the file F at the same time where A is writing and B is reading the file F. So system A need to know how much i already read and from where to read further??

CASE 2:: While System A is reading from file F1 and after analyses writing FILE F1 line description into File F2. while reading the file F1 if some specific error code detected then I need to close the file F2 and skip the ERROR LINE open new file F3 to restart the F1 analysis from ERRORLINE+1 onward.

your suggestions to handle both cases would be appreciated.
Thanks for reading with patience

解决方案

Just track the file position in a variable. File I/O functions return the number of items that has been read or written.

High level file I/O methods often provide functions that return the current position (e.g. ftell) than can be used too.

Special care must be taken when files are accessed by multiple processes at the same time. You should avoid to read from a file that is actually opened for writing by another process; especially when that is not only appending data but also inserting / overwriting. So you should check if your case 1 can be handled in another way.

Pseudo code for case 2:

n = 2
in = open(F1, read)
out = open(F2, create_write)
while (!end_of_file(in))
{
    line = readline(in)
    description = parseline(line)
    if (error)
    {
        close(out)
        n = n + 1
        out = open(Fn)
    }
    else
        write(out, description)
}
close(in)
close(out)

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