Newsgroups: comp.os.msdos.djgpp
From: fredex AT fcshome DOT stoneham DOT ma DOT us (fred smith)
Subject: Re: Bug in ftruncate or not?
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Date: Fri, 30 Jul 1999 02:07:10 GMT
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

Pavlos (trash24379 AT usa DOT net) wrote:
: Hello,
: I know that most of 'bug' reports are actually a fault of the user... Do I
: do something wrong here?

I don't know about DJGPP, but on Linux it also fails to truncate. And
I think I know why... Here's an excerpt from the ftruncate man page
on RH 5.2:

       Reads and writes may be intermixed on  read/write  streams
       in any order.  Note that ANSI C requires that a file posi-
       tioning  function  intervene  between  output  and  input,
       unless  an  input  operation  encounters end-of-file.  (If
       this condition is not met,  then  a  read  is  allowed  to
       return  the  result of writes other than the most recent.)
       Therefore it is good practice (and indeed sometimes neces-
       sary  under  Linux)  to  put an fseek or fgetpos operation
       between write and read operations on such a stream.   This
       operation  may  be an apparent no-op (as in fseek(..., 0L,
       SEEK_CUR) called for its synchronizing side effect.

Apparently the ftruncate is considered to be an input function. Does
anybody know why? 

If you add:

	fseek (f, 0L, SEEK_CUR);

immediately before the ftruncate it works fine. If this is the same
behavior you see on DJGPP then I'd guess it's correct since they are
separate implementations. Anybody got a copy of "the standard" handy?

BTW, the second parameter of fseek is type long, so your code is
potentially broken, depending on sizeof(int) on your platform.

Fred

: This simple program will truncate the file only if you comment out #define
: BUGGY.
: At least in my PC...
: If you don't comment #define BUGGY, it won't truncate the file.
: Here is the code.
: When you run it, press '1' the first time to create the file. Then run it
: again and press '2' to truncate it.
: Thank you.

: Pavlos

: <--- cut here --->

: #include <stdio.h>
: #include <stdlib.h>

: #define BUGGY

: int main(int argc, char **argv)
: {
:    FILE *f;
:    char buf[1000];

:    printf("1: Create file (1000 bytes), 2:Truncate file (to 500)\n");
:    printf("Your choice: ");

:    switch(getch())
:    {
:       case  '1':
:          f = fopen("TEST1234.$$$", "wb+");
:          fwrite(buf, 1000, 1, f);
:          return(0);

:       case  '2':
:          f = fopen("TEST1234.$$$", "rb+");
:          fread(buf, 1000, 1, f);
:          #ifdef BUGGY
:          fseek(f, 500, SEEK_SET);
:          fwrite(buf, 500, 1, f);
:          #endif
:          ftruncate(fileno(f), 500);
:          return(0);

:       default:
:          return(0);
:    }
: }

: <--- cut here --->




--
---- Fred Smith -- fredex AT fcshome DOT stoneham DOT ma DOT us -----------------------------
                       I can do all things through Christ 
                              who strengthens me.
------------------------------ Philippians 4:13 -------------------------------