On Wed, 20 May 2009, Toshio Kuratomi wrote:
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.
Locally scoped macros get freed (well, popped) on exit from parametrized
macros - think of local variables in a function. The conditional construct
%{!?bar: %define bar defined}
is the special case where the define ends up on non-global scope but
doesn't get freed immediately as this is not a parametrized macro. Note
"immediately": it will get freed eventually if a "call" to parametrized
macro happens:
---
%prep
%define myecho() echo %1 %2
%{!?bar: %define bar defined}
echo 1: %{bar}
%{myecho 2: %{bar}}
echo 3: %{bar}
---
gives:
1: defined
2: defined
3: %{bar}
but if you use "%{!?bar: %global bar defined}" you get what you expect.
I'm not sure if explaining details of such quirks of rpm macro language
really belongs to packaging guidelines, to put it simply:
Inside %{ } blocks you need to use %global to define global macros.
This is not entirely unlike python variable scoping: by default variables
only exist in the scope (local or global) they are created, unless
"global" statement is used to force glocal scope.
%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::
[snip]
Again I doubt spelling out the crazy details is terribly useful, the rules
are basically:
- Always create parametrized macros with %define.
- Inside %{ } blocks you need to use %global to define global macros.
- %define'd macro is evaluated at time of use, %global macro is evaluated
at time of definition (otherwise it might have references to macros
that have gone out of scope at time of use)
- Panu -
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