Le dimanche 29 avril 2007 19:40, John L Fjellstad a écrit :
> "Michael Marsh" <[EMAIL PROTECTED]> writes:
> > If I'm using GNU make, I hardly ever use "=" instead of ":=", unless I
> > really want to define a macro. You're a lot more likely to get what
> > you expect most of the time, and you can use "+=".
>
> What's the difference? My book on Make mentions "=", but not ":=". I
> knew about ":=" but I always thought they were pretty much the same.
>
The difference is that the value of a variable defined with := is expanded
immediately, while with = it is only computed when used. Here's an example:
BAR = $(FOO)
FOO = some value
all:
@echo "BAR = $(BAR)"
the BAR variable will here be expanded to "some value", but if you write
BAR := $(FOO) instead, it will be expanded to an empty string because FOO is
undefined at this time. That's why you can only use += with a variable
defined with := , because it wouldn't make sense in the other case. That's
also why (I guess) Michael says that you're more likely to get what you
expect with := , because it's closer to what happens with other scripting
languages. It's true that the = syntax can be quite tricky and is often the
cause of hard to find bugs, but it's also often useful.
This is documented in the make info page, under section 6.5 "How to use
variables/Setting". I don't know if this is specific to gnu make or not.
--
Cédric Lucantis
