Gnu Makefile shows two symbols for immediate expansion, as follows:
immediate := immediate
immediate ::= immediate
Is there a difference between these two?
reference: gnu makefile manual section 3.7
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1 Answer
From section 6.2:
Simply expanded variables are defined by lines using ‘:=’ or ‘::=’ (see Setting Variables). Both forms are equivalent in GNU make; however only the ‘::=’ form is described by the POSIX standard (support for ‘::=’ was added to the POSIX standard in 2012, so older versions of make won’t accept this form either).
5 Comments
Lavya
Thanks! Any idea why they (POSIX) decided to chose ::= over simpler := ?
John Marshall
@Lavya: Other make implementations also had their own
:= extensions with various meanings. So POSIX invented new syntax to avoid stepping on any already existing toes. See austingroupbugs.net/view.php?id=330 for details (and dwheeler.com/essays/make.html for the excellent activism that brought this about).Lavya
@JohnMarshall Thanks!
Jonathan Leffler
Interesting assertion about POSIX 2012, but not substantiated by the POSIX 2013 web site (
make — macros). There is no mention of either := or ::= in the POSIX specification that I can see, not even in the Rationale.John Marshall
That bug 330 lists
::= as tagged Issue 8 and resolved as Accepted As Marked in December 2011. The POSIX 2013 web site still reflects the current Issue 7. The Austin Group FAQ doesn't give any clues as to in which decade we might hope to see Issue 8… Seems like the text in the GNU Make manual was hopeful that it would appear within the year when that text was written in January 2012.