Re: [Qemu-devel] [PATCH 1/7] util: Add UUID API

[Jeff Cody]

On Wed, Aug 03, 2016 at 10:36:40AM +0800, Fam Zheng wrote:
> On Tue, 08/02 15:45, Paolo Bonzini wrote:
> > 
> > 
> > ----- Original Message -----
> > > From: "Fam Zheng" <address@hidden>
> > > To: address@hidden
> > > Cc: address@hidden, address@hidden, address@hidden, address@hidden, 
> > > address@hidden,
> > > address@hidden, address@hidden, address@hidden, address@hidden
> > > Sent: Tuesday, August 2, 2016 11:18:32 AM
> > > Subject: [PATCH 1/7] util: Add UUID API
> > > 
> > > A number of different places across the code base use CONFIG_UUID. Some
> > > of them are soft dependency, some are not built if libuuid is not
> > > available, some come with dummy fallback, some throws runtime error.
> > > 
> > > It is hard to maintain, and hard to reason for users.
> > > 
> > > Since UUID is a simple standard with only a small number of operations,
> > > it is cleaner to have a central support in libqemuutil. This patch adds
> > > qemu_uuid_* the functions so that all uuid users in the code base can
> > > rely on. Except for qemu_uuid_generate which is new code, all other
> > > functions are just copy from existing fallbacks from other files.
> > 
> > How is g_random_* seeded?
> 
> According to glib doc:
> 
> > GLib changed the seeding algorithm for the pseudo-random number generator
> > Mersenne Twister, as used by GRand.
> 
> The urandom source is /dev/urandom (or time based if unavailable).
> 
> (RFC 4122 explicitly accepts pseudo-random.)
> 
> Fam
>

To piggyback on Fam's answer:

It is as if qemu called g_rand_new() [1] for a global static GRand struct.

The g_random_* functions use the glib default global GRand struct. If
you don't set the global seed yourself with g_random_set_seed(), then the
first call into one of the g_random_ functions will create the
global GRand struct seeded from /dev/urandom (if available), or the current
time (if /dev/urandom is not available).

[1] https://developer.gnome.org/glib/stable/glib-Random-Numbers.html#g-rand-new


-Jeff