Implementing confstr()

[Chris Johns]

On 08/08/2017 00:38, Joel Sherrill wrote:
> Hi
> 
> Taking a private discussion public. Aditya is implementing the POSIX
> method confstr() next. This email is my current set of thoughts on the
> implementation of this method for RTEMS.
> 
> First, I think confstr() belongs in the RTEMS source tree. It 
> is very similar to sysconf() but has to take the following "names" as
> those to be able to retrieve:
> 
> _CS_PATH
> _CS_POSIX_V7_ILP32_OFF32_CFLAGS
> _CS_POSIX_V7_ILP32_OFF32_LDFLAGS
> _CS_POSIX_V7_ILP32_OFF32_LIBS
> _CS_POSIX_V7_ILP32_OFFBIG_CFLAGS
> _CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS
> _CS_POSIX_V7_ILP32_OFFBIG_LIBS
> _CS_POSIX_V7_LP64_OFF64_CFLAGS
> _CS_POSIX_V7_LP64_OFF64_LDFLAGS
> _CS_POSIX_V7_LP64_OFF64_LIBS
> _CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS
> _CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS
> _CS_POSIX_V7_LPBIG_OFFBIG_LIBS
> _CS_POSIX_V7_THREADS_CFLAGS
> _CS_POSIX_V7_THREADS_LDFLAGS
> _CS_POSIX_V7_WIDTH_RESTRICTED_ENVS
> _CS_V7_ENV
> 
> This is per http://pubs.opengroup.org/onlinepubs/9699919799/functions/confstr.html
> 

Does the Open Group define the purpose for each of these values? It looks x86
and Linux specific, ie how to build a 32bit app.

What is the purpose of 'POSIX_V7' in the label? I see FreeBSD 11.1 has
'_CS_POSIX_V6_ILP32_OFF32_CFLAGS' etc and the Linux page you provided has
'_CS_XBS5_ILP32_OFF32_CFLAGS' and I think glibc has 'POSIX_V7'.

Looking at the rationale and playing with the command on FreeBSD I see:

$ command -p getconf _CS_PATH
getconf: no such configuration parameter `_CS_PATH'
$ command -p getconf PATH
/usr/bin:/bin:/usr/sbin:/sbin
$ command -p getconf POSIX_V6_LPBIG_OFFBIG_CFLAGS
unsupported programming environment
$ command -p getconf POSIX_V6_WIDTH_RESTRICTED_ENVS
_POSIX_V6_LP64_OFF64
$ command -p getconf _POSIX_V6_LP64_OFF64




1

I find this a little confusing.

> The value for those #define's should likely match what is in the Linux Software
> Base: https://refspecs.linuxfoundation.org/LSB_1.2.0/gLSB.html That much goes
> in newlib.

For the numbering I see:

$ uname
FreeBSD
$ grep -r _CS_PATH /usr/include/
/usr/include/unistd.h:#define   _CS_PATH                1       /* default value
of PATH */

I would prefer we follow the FreeBSD lead here for numbering.

> 
> The confstr() implementation will be a switch on those values and 
> I honestly don't even know what most of those should be in an 
> RTEMS environment.
> 
> But those are the names and we have to answer the question of what
> is the appropriate value to return. A first implementation could return
> something like getenv("PATH") for __CS_PATH, and take a swing at
> the "obvious ones".  
> 
> The ones with CFLAGS, LDFLAGS, and LIBS would first have
> to evaluated to see if the name makes sense to provide a value
> for. And then return the value. I am tending to lean to returning
> empty string for all of those.
> 
> Looks like Linux doesn't support most of them: 
> 
> http://man7.org/linux/man-pages/man3/confstr.3.html
> 
> I don't think this is a commonly used method so I would be perfectly
> happy to support it in a minimal, but compliant manner.
> 
> Any other thoughts?
> 

If no one has a common set of names I wonder how it is to be managed in a
portable way?

Chris