Re: Understanding Linux addr space, malloc, and heap

[Arjan van de Ven <arjan@infradead.org>]

On Fri, 2005-10-21 at 08:46 -0400, Vincent W. Freeh wrote:
> I am trying to understand the Linux addr space.  I figured someone might 
> be able to shed some light on it.  Or at least point me to some sources 
> that will help.
> 
> I don't understand what is happening with malloc and the heap in my 
> process. According to /proc/<pid>/maps the memory from heap to stack 
> initially looks like that.  I only show the four "maps" from the heap 
> and above.  (This is a slightly altered form consisting of start_addr, 
> end_addr, size_in_pgs, permissions, and path_if_one):
> 
> 0x08d42000 - 0x08d63000 (33 pgs) rw-p   path `[heap]'
> 0xb7ef8000 - 0xb7ef9000 (1 pgs) rw-p
> 0xb7f09000 - 0xb7f0b000 (2 pgs) rw-p
> 0xbfaf5000 - 0xbfb0b000 (22 pgs) rw-p   path `[stack]'
> 
> First, please fix any erroneous statements/assumptions above.  Next I 
> have many questions.  A few follow.
> 
> * How does the heap work?  I learned/teach that heap is a contiguous 
> chunk of memory that holds dynamically-allocated memory.  Doesn't appear 
> to be the case.

that's the old school 1970's stuff

the "heap" is still brk in linux, however there is no 1:1 relation
between heap and malloc. malloc in glibc is implemented both using brk
and mmap, depending on the size of your allocation.


> 
> * Man pg says can only mprotect mmap-able pages.  But what are these? 
> How can I tell?

you need to mmap these yourself to be sure.. eg you cannot mprotect the
output of malloc, at least not reliably. Only of mmap.

> 
> * Why does mprotect silently fail?

no it has sideeffects; eg it most likely affects more memory than just
your malloc()'d part


> * I thought brk indicated the top of the heap and that all dynamic 
> memory would be between bss end and brk.  That's not true.  What is brk 
> for then?

see definition of heap vs malloc above