[Fedora-packaging] Correction to guidelines on %define vs %global

[Toshio Kuratomi]

On 05/20/2009 03:23 AM, Panu Matilainen wrote:
>
> Apologies for missing this when the recommendation of %global over
> %define in Fedora guidelines was being discussed, but better late than
> never...
>
> https://fedoraproject.org/wiki/Packaging/Guidelines#.25global_preferred_over_.25define
>
> says "The two macro defining statements behave the same when they are a
> the top level of rpm's nesting level" but this is not correct:
>
> The body of a %global macro is expanded at definition time, whereas the
> body of a %define is expanded only when used. For many uses it doesn't
> make any difference but identical they are not, even on top level - for
> example see the attached example spec:
>
> $ rpmbuild -bp macroex.spec 2>/dev/null|grep ^case
> case1: first - second
> case2: second - %{xxx}
>
> Another example of this difference (in case of parametrized macros):
> https://bugzilla.redhat.com/show_bug.cgi?id=495658
>


Hi Panu, How does this look?  (Note, I haven't tested any of this code 
yet as I'm running out the door.  I wrote it just from how I understand 
your message and bz#495658).  Also note that I don't know what causes 
rpm to clear the %defines that have been defined.  If you can fill that 
in that would be great.  If it's too complex, we can figure out some way 
to rephrase around it.

"""
%global and %define are both used to define macros inside rpm spec 
files.  We generally prefer using %global over %define.

%define created macros when they are used in nested macro expansions 
(like in  %{!?foo: ... }  constructs theoretically only lasts until the 
end brace (local scope), while %global definitions have global scope. 
However, rpm currently doesn't clear the scope for the macros unless 
[Accurate information needed here]. So %define'd macros often last 
throughout a spec file.  However, when they don't, it's often 
non-obvious why the spec file is failing as the failure is caused by 
something changing in another part of the spec file.

%global has another major difference from %define that can cause 
problems when you first write the macro.  %global is evaluated at the 
time that it is written whereas %define is evaluated separately 
everytime it occurs in a spec file.  As an example, if you do the 
following in a spec file::

<pre>
%build

%global foo FOO
%global bar %foo
%define baz %foo
echo %{bar} - %{baz}

%global foo FOOBAR
echo %{bar} - %{baz}

You get this:

FOO - FOO
FOO - FOOBAR
</pre>

One place this becomes apparent is parameterized macros:
<pre>
%build
%global print_arg echo "Hello %1"
%print_arg World
</pre>
because the %1 is evaluated when print_arg is defined, this prints
<code>Hello</code> rather than "Hello World".

Escaping the % fixes this usage:

<pre>
%global print_arg echo "Hello %%1"
</pre>
"""

-Toshio