OpenEuphoria

1. How to wait for file closing?

• Posted by SnakeCharmer Dec 10, 2012
• 1816 views

I have C++ code, that writes some strings to file. For example:

FILE *file = fopen("yum-yum.lst", "w");
fputs("Hot Fudge Sundae\n", file);
fputs("Muffin\n", file);
fputs("Hash Browns With Maple Syrup\n");
fclose(file);

I need to open and read (but not write) this file by my Euphoria program. How to be convinced of what the file is already closed? Otherwise it can be read not completely. I have no ideas, except adding keyword to end of file. For example, "END". But it isn't so effective because it is necessary to read all file to be convinced that it is full. Maybe, it is superfluous because Euphoria won't open file during writing?

3. Re: How to wait for file closing?

• Posted by SnakeCharmer Dec 10, 2012
• 1762 views

Excuse for my meticulousness, but this problem is too important for me. Let's say two programs (C++ and Euphoria) work in parallel. C++ writes the lines "Hot Fudge Sundae" and "Muffin". Execution of instruction "read_lines" was at this moment. What will occur?

1. read_lines immediately return {"Hot Fudge Sundae", "Muffin"}
2. read_lines will wait for closing of the file and will return {"Hot Fudge Sundae", "Muffin", "Hash Browns With Maple Syrup"}

Thanks for your endurance.

5. Re: How to wait for file closing?

• Posted by ChrisB (moderator) Dec 10, 2012
• 1692 views

Hi

That depends. The only way you can be sure that the eu program will read the program correctly is if the C program has close the file first.

The reason for this is that I believe the OS will 'hold on' to some bytes in a buffer, and this buffer won't be flushed to disk until the calling program closes, or releases, the file handle. In other words (and this is by way of illustration only)

C program

FILE *file = fopen("yum-yum.lst", "w"); 
fputs("Hot Fudge Sundae\n", file); 
fputs("Muffin\n", file); 
fputs("Hash Browns With Maple Syrup\n"); 

(fclose deliberately left off)

whats on the disk 
Hot Fudge Sundae 
Muffin 
Hash Browns Wi        (th Maple Syrup is still in the buffer) 

when fclose is called, th Maple Syrup is flushed, and the file on disk now looks like this

Hot Fudge Sundae 
Muffin 
Hash Browns With Maple Syrup 

Now, some file systems will let you read that partially created disk, and some won't, and I don't know if there is a way to tell that there is an open file handle associated with that file.

If you are writing the C++ program, then I would suggest writing a small 'indicator' file that the eu program checks for before it reads the file, ie

pseudocode

if file 'yumyum.lck' exists then don't read file

The C++ program at the same time creates and deletes yumyum.lck whenever it opens and closes the file

That is how I would do it without having to worry about how the OS handles file locking and releasing.

There are probably better, and more sophisticated ways of doing it though.

Chris

7. Re: How to wait for file closing?

• Posted by useless_ Dec 10, 2012
• 1733 views

You're correct. The biggest problem with using the OS to lock a file is if the app crashes, the file remains locked, and no one can use it until you manually unlock it.

The filesystem can be abused this way tho: any app that wants access to the file can write a filename-req-appname.txt file (or some other more imaginitive name), and then dir(filename-req-*.txt) will list the access queue for the file.

useless

9. Re: How to wait for file closing?

• Posted by DerekParnell (admin) Dec 10, 2012
• 1672 views

I just did a quick test and it appears that lock_file() is not working on Windows platform. It appears that it always reports that it locked a file, even when the file is locked by another lock_file() in a separate application.

11. Re: How to wait for file closing?

• Posted by ChrisB (moderator) Dec 11, 2012
• 1635 views

Hi

That looks a little tangled. Try (pseudocode)

c program

file *fp, *fl 
 
//get data from sensors 
 
fl = fopen("lock_file", "w") 
puts(fl, "~") 
fclose(fl) 
 
fp = fopen("datafile", "w") 
//put all your data in there 
fclose(fp) 
 
unlink("lock_file") 
 

then in the eu program

 
if file_exists("lock_file") then 
    --can't do anything, come back later 
else 
    -- get your data 
    data = read_lines(datafile) 
end if 
 

Note, unless you've got a huge data file to write from the C program, then the chances of the eu program encountering the lock file are low. The other danger of course is that the C program starts to write a new datafile before the eu program has finished reading it - you could always write another don't write anything lock file from the eu program, to be checked by the C program before it starts writing.

Chris

13. Re: How to wait for file closing?

• Posted by mattlewis (admin) Dec 12, 2012
• 1526 views

I think I know why. In be_machine.c, inside of lock_file(), I find:

#if defined(EMINGW) 
	r = 0;  // TODO: FOR NOW! 
#else // defined EMINGW 
	r = lock(fd, first, last - first + 1); 
#endif // defined EMINGW 

...and hg blame tells me that "NOW" was quite a while ago:

jeremy Wed Apr 29 16:24:43 2009 +0000:      r = 0;  // TODO: FOR NOW! 

I suspect we should convert the FILE* to a HANDLE using _get_osfhandle, and then use LockFile on Windows.

Matt

15. Re: How to wait for file closing?

• Posted by FredRansom Dec 12, 2012
• 1561 views

After the last write, just before the fclose, place this statement: fflush (file); That forces Windows to write everything in the buffer to disc right away. Otherwise Windows won't write anything until it gets around to it, when it needs the buffer for something else, or when you shut down. fflush followed by fclose should make the file available for your Euphoria program as soon as you need it.

17. Re: How to wait for file closing?

• Posted by SnakeCharmer Dec 13, 2012
• 1534 views

Thanks!

About pipes: I considered this option, but files are more convenient in my case.