Read a file to the end, new data is appended, no more is read?

[Ali Çehreli]

On 06/20/2013 07:10 AM, Gary Willoughby wrote:
> I have a simple problem, I want to open a file and read data from it.
> When the file changes, i will read more data. The trouble is i only want
> to open the file once.
>
> Here's the simplified code:
>
> import core.thread;
> import std.stdio;
>
> void main(string[] args)
> {
>      auto file = File("file.txt", "r");
>
>      string line;
>
>      writeln("First Read:");
>
>      while ((line = file.readln()) !is null)
>      {
>          write(line);
>      }
>
>      Thread.sleep(dur!("seconds")(5)); // <-- More data is appended to
> the file while waiting here.
>
>      writeln("Second Read:");
>
>      while ((line = file.readln()) !is null) // <-- Error! New data is
> not read!
>      {
>          write(line);
>      }
> }
>
> I read the file, then when its paused, i add new lines to the opened
> file. When the program resumes it totally ignores the new lines. doh!
>
> I guess this is the result of buffering somewhere so i've tried all
> sorts to try and escape this behaviour, including:
>
> file.clearerr()
> file.seek()
> file.flush()
>
> Nothing seems to work. What am i missing? It must be something simple.
> Any ideas?

This must be platform-dependent. Your program works as expected under my 
Scientific Linux distribution. (Same as Red Hat.)

I have tried four kinds of modifications to the file:

1) Append a new line: works as expected

     echo new line >> file.txt

2) Overwrite the same file: works as expected

     echo new content > file.txt

3) Delete the file and recreate: works as expected

     rm file.txt; echo new content > file.txt

4) Rename the file: works better than expected :)

     mv file.txt file2.txt; echo new line >> file2.txt

In any case, there must be lots to learn from the 'tail' utility. Its 
'-f' command line switch achieves what you need, and '-F' achieves more 
than what you need:

   http://git.savannah.gnu.org/cgit/coreutils.git/tree/src/tail.c

Ali