Re: malloc question

[ratheesh kannoth <ratheesh.ksz@xxxxxxxxx>]

On Tue, Apr 26, 2011 at 4:27 PM, Daniel Baluta <daniel.baluta@xxxxxxxxx> wrote:
> On Tue, Apr 26, 2011 at 1:54 PM, ratheesh kannoth
> <ratheesh.ksz@xxxxxxxxx> wrote:
>> On Tue, Apr 26, 2011 at 4:10 PM, Daniel Baluta <daniel.baluta@xxxxxxxxx> wrote:
>>> On Tue, Apr 26, 2011 at 1:33 PM, Randi Botse <nightdecoder@xxxxxxxxx> wrote:
>>>> Hi All,
>>>>
>>>> I want to ask malloc() behaviour, consider these codes;
>>>>
>>>> ...
>>>> char *ptr = malloc(1);
>>>> strcpy(ptr, "what");
>>>> puts(ptr);
>>>> ....
>>>>
>>>> Confusingly, the strcpy() copied all bytes to ptr, but I just manage
>>>> to allocate ptr only for 1 byte, I guess I will have segfault here,
>>>> why this happen? why the string successfully copied into ptr? , is
>>>> those code legal?
>>>
>>> You didn't get segfault because you were lucky.
>>>
>>> Memory is allocated in multiples of page size (usually 4K).
>>> The memory after your allocated byte is valid in your case.
>>>
>>> thanks,
>>> Daniel.
>>> --
>>
>> U could read a little more about vm_page_struct. ( virtual address
>> space to  physical page ).
>
> Can you elaborate on this?
>
> Daniel.
>

Daniel,

Note: Pls read Linux Kernel internals 2.6.


malloc() and free() works on virtual address space.

malloc(1)           - this allotes a virtual address space of 4k.

strcpy(ptr, "what" ) -    the 4k virtual address is mapped to a 4k
page frame thru page fault exception. Now you have a valid virtual
address of 4k.

free(ptr)              - tells OS that  this virtual address space (
4k )  can be reallotted if there is a need . But  each program has a
virtual address space of 3GB ( 32 bit
                                   ,4GB minus 1GB (kernel) ).  SO ptr
is a valid pointer unless
                                                   1) vitual address
space is realloted.
                                                   2) page frame is
realloted for some other page ( LRU algorithm )
so if ptr is valid , you could do  following

 *(ptr ++ ), *ptr ... etc .


-ratheesh
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