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| author | Craig Jennings <c@cjennings.net> | 2024-04-07 13:41:34 -0500 |
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| committer | Craig Jennings <c@cjennings.net> | 2024-04-07 13:41:34 -0500 |
| commit | 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 (patch) | |
| tree | f1190704f78f04a2b0b4c977d20fe96a828377f1 /devdocs/gnu_make/undefine-directive.html | |
new repository
Diffstat (limited to 'devdocs/gnu_make/undefine-directive.html')
| -rw-r--r-- | devdocs/gnu_make/undefine-directive.html | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/devdocs/gnu_make/undefine-directive.html b/devdocs/gnu_make/undefine-directive.html new file mode 100644 index 00000000..81879a20 --- /dev/null +++ b/devdocs/gnu_make/undefine-directive.html @@ -0,0 +1,17 @@ + <h1 class="section">Undefining Variables</h1> <p>If you want to clear a variable, setting its value to empty is usually sufficient. Expanding such a variable will yield the same result (empty string) regardless of whether it was set or not. However, if you are using the <code>flavor</code> (see <a href="flavor-function">Flavor Function</a>) and <code>origin</code> (see <a href="origin-function">Origin Function</a>) functions, there is a difference between a variable that was never set and a variable with an empty value. In such situations you may want to use the <code>undefine</code> directive to make a variable appear as if it was never set. For example: </p> <div class="example"> <pre class="example">foo := foo +bar = bar + +undefine foo +undefine bar + +$(info $(origin foo)) +$(info $(flavor bar)) +</pre> +</div> <p>This example will print “undefined” for both variables. </p> <p>If you want to undefine a command-line variable definition, you can use the <code>override</code> directive together with <code>undefine</code>, similar to how this is done for variable definitions: </p> <div class="example"> <pre class="example">override undefine CFLAGS +</pre> +</div><div class="_attribution"> + <p class="_attribution-p"> + Copyright © 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022 Free Software Foundation, Inc. <br>Licensed under the GNU Free Documentation License.<br> + <a href="https://www.gnu.org/software/make/manual/html_node/Undefine-Directive.html" class="_attribution-link">https://www.gnu.org/software/make/manual/html_node/Undefine-Directive.html</a> + </p> +</div> |