Regarding the p_align Elf_Phdr structure variable.

Regarding the p_align Elf_Phdr structure variable.

[vamsi krishna]

Dear All,

I have a question regarding the program header structure variable p_align.

o If the program header states the segment type as PT_LOAD and
phr.p_vaddr and phdr.p_paddr both set to a virtual address
'0x0000ac00' and I have a global variable(say a char) whose virtual
address is '0x0000ac0f'  (assume this is program header which is
having the .data section).

o Now if the value of the p_align is set to some 2^x will it change
the virtual address my variable after the loader does the mmaping? of
the segment??

When do we need to set the p_align flag? I'am trying to create a elf
file with out p_align ,  does it have any impact on loaders
performance, I dont find any details on how OS loader uses this
p_align while mmaping my program header.

Really thankful for you comments.

Best,
Vamsi.

Re: Regarding the p_align Elf_Phdr structure variable.

[Alan Modra]

On Mon, May 29, 2006 at 12:23:32AM +0530, vamsi krishna wrote:
> Dear All,
> 
> I have a question regarding the program header structure variable p_align.
> 
> o If the program header states the segment type as PT_LOAD and
> phr.p_vaddr and phdr.p_paddr both set to a virtual address
> '0x0000ac00' and I have a global variable(say a char) whose virtual
> address is '0x0000ac0f'  (assume this is program header which is
> having the .data section).
> 
> o Now if the value of the p_align is set to some 2^x will it change
> the virtual address my variable after the loader does the mmaping? of
> the segment??

No, but changing p_align independently of other segment header fields
may result in an executable that gives loader errors.  You must have
p_offset mod p_align == p_vaddr mod p_align to support demand paging.

> When do we need to set the p_align flag? I'am trying to create a elf
> file with out p_align ,  does it have any impact on loaders
> performance, I dont find any details on how OS loader uses this
> p_align while mmaping my program header.

p_align is used to calculate the base address of the executable or
shared library, and in a similar calculation for TLS segments.   p_align
must be at least as large as disk and memory page size to support demand
paged executables.  See glibc sources for all the details.

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre