Re: Zeroed pages returned for heap

[Nagendra Singh Tomar]

On Thu, 9 Jun 2005, Richard B. Johnson wrote:

> On Wed, 8 Jun 2005, Nagendra Singh Tomar wrote:
> 
> The user code can't assume anything about any memory allocated
> by malloc(). The first time a buffer is allocated, it may be
> zero-filled because of the zeroed pages allocated by the kernel
> when the new break address is set. After that, all bets are off
> because once you free a buffer and allocate another one, it
> will probably contain data from malloc()'s previous allocation.
> 
> Even the very first time malloc() returns a pointer, doesn't
> guarantee that the memory will all be cleared. This is because
> many malloc()s use just-obtained memory (via brk) to do some
> house-keeping which may result in some "strange" numbers in
> the memory at some places.
> 
> It is extremely bad coding practice to assume a buffer is
> zero filled when writing user-mode code. That's why we have
> calloc().

My original question was for glibc (as an application) assuming that 
memory it gets from brk()/MAP_ANON is zero filled, and _not_ for an 
application calling malloc() assuming that. glibc does assume that brk() 
memory is zero filled thats why the glibc calloc() implementation does 
_not_ zero it again in user space (Ulrich conformed this). This is what 
was disturbing to me as I  was trying to disable zero-filling for brk() 
pages and to my unpleasant  surprise few applications like gcc/awk were 
breaking. 

Thanx,
Tomar

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