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<h1 class="section">The call Function</h1> <p>The <code>call</code> function is unique in that it can be used to create new parameterized functions. You can write a complex expression as the value of a variable, then use <code>call</code> to expand it with different values. </p> <p>The syntax of the <code>call</code> function is: </p> <div class="example"> <pre class="example">$(call <var>variable</var>,<var>param</var>,<var>param</var>,…)
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</div> <p>When <code>make</code> expands this function, it assigns each <var>param</var> to temporary variables <code>$(1)</code>, <code>$(2)</code>, etc. The variable <code>$(0)</code> will contain <var>variable</var>. There is no maximum number of parameter arguments. There is no minimum, either, but it doesn’t make sense to use <code>call</code> with no parameters. </p> <p>Then <var>variable</var> is expanded as a <code>make</code> variable in the context of these temporary assignments. Thus, any reference to <code>$(1)</code> in the value of <var>variable</var> will resolve to the first <var>param</var> in the invocation of <code>call</code>. </p> <p>Note that <var>variable</var> is the <em>name</em> of a variable, not a <em>reference</em> to that variable. Therefore you would not normally use a ‘<samp>$</samp>’ or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.) </p> <p>If <var>variable</var> is the name of a built-in function, the built-in function is always invoked (even if a <code>make</code> variable by that name also exists). </p> <p>The <code>call</code> function expands the <var>param</var> arguments before assigning them to temporary variables. This means that <var>variable</var> values containing references to built-in functions that have special expansion rules, like <code>foreach</code> or <code>if</code>, may not work as you expect. </p> <p>Some examples may make this clearer. </p> <p>This macro simply reverses its arguments: </p> <div class="example"> <pre class="example">reverse = $(2) $(1)
foo = $(call reverse,a,b)
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</div> <p>Here <code>foo</code> will contain ‘<samp>b a</samp>’. </p> <p>This one is slightly more interesting: it defines a macro to search for the first instance of a program in <code>PATH</code>: </p> <div class="example"> <pre class="example">pathsearch = $(firstword $(wildcard $(addsuffix /$(1),$(subst :, ,$(PATH)))))
LS := $(call pathsearch,ls)
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</div> <p>Now the variable <code>LS</code> contains <code>/bin/ls</code> or similar. </p> <p>The <code>call</code> function can be nested. Each recursive invocation gets its own local values for <code>$(1)</code>, etc. that mask the values of higher-level <code>call</code>. For example, here is an implementation of a <em>map</em> function: </p> <div class="example"> <pre class="example">map = $(foreach a,$(2),$(call $(1),$(a)))
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</div> <p>Now you can <code>map</code> a function that normally takes only one argument, such as <code>origin</code>, to multiple values in one step: </p> <div class="example"> <pre class="example">o = $(call map,origin,o map MAKE)
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</div> <p>and end up with <code>o</code> containing something like ‘<samp>file file default</samp>’. </p> <p>A final caution: be careful when adding whitespace to the arguments to <code>call</code>. As with other functions, any whitespace contained in the second and subsequent arguments is kept; this can cause strange effects. It’s generally safest to remove all extraneous whitespace when providing parameters to <code>call</code>. </p><div class="_attribution">
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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/Call-Function.html" class="_attribution-link">https://www.gnu.org/software/make/manual/html_node/Call-Function.html</a>
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