Re: [gpsd-dev] SHM broken

[Gary E. Miller]

Yo Hal!

On Mon, 23 Mar 2015 21:04:33 -0700
Hal Murray <address@hidden> wrote:

> I noticed a SHM warning in a log file.

Yup.

> It's right after the can't find libkern/OSAtomic.h message so
> it looks like SHM depends on a strange file I 've never heard of
> before.

Yup.  gpsd has memory barriers if you have gcc C11 compatibity, or if
you have the OSAtomic.h file.  The header file is only on OS X.

> From the log file:
>   Checking if compiler is C11 ...(cached) no

Your gcc does not support C11

>   Checking for C header file libkern/OSAtomic.h... (cached) no

You are not on a recent OS X.

>   No memory barriers - SHM export and time hinting may not be
> reliable.

Thus no memory barriers, which are a recent addition.

I see that on RasPi running Debian Wheezy.  Gentoo unstable is C11, so
has memory barriers.
 
> Looking at the code in SConstruct
>     # check for C11 or better, and __STDC__NO_ATOMICS__ is no defined
>     # before looking for stdatomic.h
> 
> gpsd_config.h says:
> /* #undef HAVE_STDATOMIC_H */
> /* #undef HAVE_OSATOMIC_H */

Yup, that is correct.

> I've got an up to date Fedora box.
>   gcc (GCC) 4.9.2 20150212 (Red Hat 4.9.2-6)
>   /usr/lib/gcc/i686-redhat-linux/4.9.2/include/stdatomic.h

Well, except RedHat gcc 4.9.2 is not the same a Gnu gcc 4.9.2

RedHat always plays games with version numerbers and upstream version
numbers.

Last I checked RedHat gcc 4.9.2 was not C11 by default, thus no memory
barriers.  GNU gcc 4.9.2 is C11 by default, thus memory barriers.

> google found stuff with "-std=c11" on the command line for gcc.  The
> man page for gcc says the default is gnu89 with lots of fine print.
> grep -w doesn't find any "std" in SConstruct.

Lost me.  Your point is?  Other than RedHat is broken?

RGDS
GARY
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