RE: [Dev-C++] Re: malloc , extra bytes ! Got even more urgent !

[Josu O. <jon...@ao...>]

Per is absolutely right. If you need to know the number of actual data bytes
inside a buffer, you have to track it on a variable. Never use a strange
thing (and nonportable) such _msize.
Josu

> -----Mensaje original-----
> De:	Per Westermark [SMTP:pw...@ia...]
> Enviado el:	lunes 24 de febrero de 2003 10:04
> Para:	veenurs
> CC:	Danny Roelofs; dev...@li...; ra...@ww...
> Asunto:	Re: [Dev-C++] Re: malloc , extra bytes ! Got even more
> urgent !
> 
> The _msize() you are referring to is part of heap-walk code. They are not
> expected to be used for a normal application, but for debug code to verify
> memory leaks etc.
> 
> There are NO standard method of keeping track of the size of a block of
> memory. When you allocate the block, YOU have to assign the size to a
> separate variable, in case you want to support dynamic resizing.
> 
> However, your malloc() or calloc() call will return your 1000 bytes of
> available buffer space, to be used for whatever you want.
> 
> You may not assume anything about any extra bytes before the returned
> pointer, or after your 1000 bytes of requested data. You may also assume
> that - unless you have buffer overflow problems in the code - that you are
> the sole owner of these 1000 byte of data, i.e. what you put there will
> stay there until your RAM memory fails, or until you put something else
> there.
> 
> The only thing you might assume, is that p+1000 is a valid pointer value,
> even if you may not read/write at that location.
> 
> If these limitations are bothersome, they still represents the hard
> reality.
> 
> /Per W
> 
> 
> On Mon, 24 Feb 2003, veenurs wrote:
> 
> > Hi PW ,
> >  Well , I guessed  it had something to do with the AT Least qualifier
> > .But , I just can't help bothoring abt . the extra bytes !
> >  It's lioke this- the memory created is used to store bytes of formatted
> > data , which are then to be used by another part of application . So ,
> > the extra bytes are screwing up the formatting and making 6the data
> > useless !!
> > The data itself is comming from   decryption  routines , so I cannot be
> > sure of the exact size before decryption ( therefore , I NEED the
> > malloc- realloc thing , would hate to use arrays .)
> >
> > MoreOvere , There is no such problem In VC++ ( I have a learner ed.), or
> > LCC .
> >
> > Thanks , any other solution ?
> >
> > -Rahul.
> >
> > Per Westermark wrote:
> >
> > >Stop bothering about the size of the returned memory block.
> > >
> > >The allocation functions are only required to return AT LEAST the
> required
> > >amount of memory, or a NULL pointer.
> > >
> > >Internally, they normally add 8-16 byte of internal header for
> > >book-keeping on the heap, i.e. to allow the heap manager to walk free
> and
> > >used memory block.
> > >
> > >Besides, most heap managers performs one of the following:
> > >- rounds each block up to the nearest 8 or 16 byte increment.
> > >- allocates the nearest larger 2^n size.
> > >- uses the buddy system and allocates the nearest larger block size.
> > >
> > >As a result - for allocation of very small objects, it might often
> > >be better to allocate a bunch of objects at a time, and keep pool of
> > >evailable objects in a list.
> > >
> > >In the example below, the calloc call MUST use sizeof(BYTE). The
> > >statement calloc(1000,BYTE) doesn't make sense.
> > >
> > >/Per W
> > >
> > >On Mon, 24 Feb 2003, Danny Roelofs wrote:
> > >
> > >
> > >
> > >>Hi,
> > >>
> > >>Well i dont realy know what youre doing with it.. and i never used
> calloc but it seems your allocating memory for 1000 bytes
> > >>and a additional 8 bytes..  sizeof(byte) reserves 8 bytes in my
> opinion. is it perhaps that you have to do this:
> > >>
> > >>calloc(1000,BYTE) instead of calloc(1000,sizeof(BYTE))
> > >>
> > >>Well i actualy dont know.. i am just programming in C for just a month
> or so..
> > >>
> > >>
> > >>  ----- Original Message -----
> > >>  From: veenurs
> > >>  To: dev...@li... ; ra...@ww...
> > >>  Sent: Monday, February 24, 2003 1:16 AM
> > >>  Subject: [Dev-C++] Re: malloc , extra bytes ! Somewhat urgent !
> > >>
> > >>
> > >>
> > >>
> > >>  Hi !
> > >>  It seems too simple .But what is it !?
> > >>  if( (buffer = ( PBYTE )calloc( 1000 ,sizeof(BYTE) )) == NULL ){
> > >>         ------
> > >>        -------
> > >>  }
> > >>    size = _msize( buffer );
> > >>
> > >>  The size I get it is not 1000 , but 1008  !Same for malloc. Any
> subsequent call to realloc  does not add  the 8 bytes,though  .
> > >>
> > >>  [ Dev Cpp Version 4.9.7.0 ]
> > >>  What's happening ?
> > >>  BTW - "Generate debugging info" is OFF .
> > >>  Thanks in advance .
> > >>
> > >>  -Rahul.
> > >>
> > >>
> > >>
> > >>
> > >>
> > >
> > >
> > >
> > >
> >
> >
> 
> 
> 
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