Xref: utzoo comp.lang.c:33354 comp.os.msdos.programmer:1777
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!munnari.oz.au!csource!david
From: david@csource.oz.au (david nugent)
Newsgroups: comp.lang.c,comp.os.msdos.programmer
Subject: Re: How do I SHORTEN a file without rewriting it?
Keywords: truncation
Message-ID: <747@csource.oz.au>
Date: 2 Nov 90 14:32:44 GMT
References: <1162@bilver.UUCP>
Organization: Unique Computing Pty Ltd, Melb, Aust.
Lines: 52

In <1162@bilver.UUCP> alex@bilver.UUCP (Alex Matulich) writes:

>Is there a way to shorten a file, that is, chop some data off the end of
>it, so that it doesn't consume as much physical space on the disk?  The
>file I have is too big to read into memory and write back out again, and
>there is not enough room on the disk to write out a temporary file.

Write zero bytes at that position.

Some C libraries have a chsize() function which does exactly that.
Since those libraries also don't seem to allow writing of zero bytes
you will need to create your own write function.


chsize.c:


  int chsize (int fd, long newsize)
  {
     r = -1;

     if (lseek (fd, newsize, SEEK_SET) != -1L)
          r = _write (fd, NULL, 0);
     return r;
  }

  
_write.asm:

.model c,small

.code

_write PROC, fd:WORD, buf:PTR, count:WORD

   mov bx,[fd]
   mov cx,[count]
   mov dx,[buf]
   mov ah,40H
   int 21H
   jnc .W0
   mov ax,-1
.W0:
   ret

_write ENDP
   
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