From: av568 AT rgfn DOT epcc DOT edu (Beautiful Savior Lutheran Church)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: DJGPP BUG ?
Date: 18 Apr 1997 20:18:47 GMT
Organization: The Rio Grande Free-Net, El Paso Community College, El Paso, TX
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References: <5j5n1j$lph AT news DOT network DOT com>
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To: djgpp AT delorie DOT com
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Mike Collins (e-mail DOT address AT end DOT of DOT text) wrote:
> Am I doing something wrong? Why does the following not work?
> (I already deleted the original, so what I am writing here is 
> not
> compiler tested, but you'll get the idea)

> #include <stdio.h>
> #include <io.h>

> main()
> { FILE *fp;
>   fp = fopen("junk", "wb");
>   fwrite("abcdefghijklmnopqrstuvwxyz", 26, 1, fp);
>   // examination of the file at this point shows that it
>   // contains 26 characters of the alphabet. To this point,
>   // it works

>   printf("%d", (int)(filelength(fileno(fp))));  // prints "0"
>                                                 // - WHY?
>   fclose(fp);
>   fp = fopen("junk", "rb+");
>   printf("%d", (int)(filelength(fileno(fp))));  
>   // prints 26 as expected
>   fwrite("abcdefghijklmnopqrstuvwxyz", 26, 1, fp);
>   // examination of the file at this point shows that it
>   // contains 52 characters of the alphabet - Fine! 

>   printf("%d", (int)(filelength(fileno(fp))));  
>   // still prints 26!
> }

> In my application, a file is written to, then its handle is 
> passed to a function which needs to know the length of the file. 
> This function cannot close and reopen the file, however, because 
> it does not know the filename, which is derived in the first 
> place by a fairly complex process (reading bits of other files) 
> I don't want to have to go through developing the name of the 
> file in the called function.

> This works fine under my  16-bit DOS-based compiler (Power-C by 
> MIX).

I *think* that the problem is that you're not switching to read mode 
correctly, which (AFAICR) must be explicitly done in DJGPP with either an 
fseek(file, 0, SEEK_CUR) or an fflush(file)

> Another thing that works under Power-C but did not under DJGPP 
> was to do with trunkation of a file using chsize(). The 
> returned value was different if I closed the file with 
> fclose(fp) (returned zero) or with close(fileno(fp)) (returned 

fclose() returns zero on success, and EOF on error. close(fileno(file)) 
should (AFAIK) *NOT* be called to close a file opened with fopen().

> something else, but I can't remember what), but the trunkation 
> didn't work in either case. I finally solved it by opening a 
> second file, copying the first file into it up to the trunkation 
> point, then closing both, deleting the original and renaming the 
> new file to the original's name, and that works - but it's 
> cumbersome! 

I wrote a small program to add automatically add entries to the Info dir 
node, and it uses this line to truncate:

ftruncate(fileno(dirfile), ftell(dirfile)-1);

which works perfectly. According to the libc.a docs, chsize() just calls 
ftruncate(), so it is impossible to determine your problem without more 
information. However, you may be forgetting this: you *must* call 
fclose() *immediately* after truncating, or it will *not* work.

--
Beautiful Savior Evangelical Lutheran Church - av568 AT rgfn DOT epcc DOT edu
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