Re: [patch 03/26] Dynamic kernel command-line - arm

Andrew Morton
2007-01-22T19:57:13Z

> On Thu, 18 Jan 2007 15:23:26 +0000 Russell King <rmk+lkml@arm.linux.org.uk> wrote:
> On Thu, Jan 18, 2007 at 04:31:51PM +0100, Tomas Carnecky wrote:
> > Russell King wrote:
> > > On Thu, Jan 18, 2007 at 01:58:52PM +0100, Bernhard Walle wrote:
> > >> -static char command_line[COMMAND_LINE_SIZE];
> > >> +static char __initdata command_line[COMMAND_LINE_SIZE];
> > >
> > > Uninitialised data is placed in the BSS. Adding __initdata to BSS
> > > data causes grief.
> > >
> >
> > Static variables are implicitly initialized to zero. Does that also
> > count as initialization?
>
> No. As I say, they're placed in the BSS. The BSS is zeroed as part of
> the C runtime initialisation.

I don't understand the objection. With the above change, command_line[]
will end up consuming COMMAND_LINE_SIZE bytes of .data.init and will be
reliably initialized to all-zeros by the compiler (won't it?)

> If you want to place a variable in a specific section, it must be
> explicitly initialised. Eg,
>
> static char __initdata command_line[COMMAND_LINE_SIZE] = "";
>
> However, there is a bigger question here: that is the tradeoff between
> making this variable part of the on-disk kernel image, but throw away
> the memory at runtime, or to leave it in the BSS where it will not be
> part of the on-disk kernel image, but will not be thrown away at
> runtime.

Yes, it'll take some space in vmlinux. We could perhaps create a new
__initbss to prevent that, I assume.