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    <p>Using <strong><code>=</code></strong> causes the variable to be assigned a value. If the variable already had a value, it is replaced. This value will be expanded when it is used. For example:</p> <pre><code>HELLO = world HELLO_WORLD = $(HELLO) world! # This echoes "world world!" echo $(HELLO_WORLD) HELLO = hello # This echoes "hello world!" echo $(HELLO_WORLD) </code></pre> <p>Using <strong><code>:=</code></strong> is similar to using <code>=</code>. However, instead of the value being expanded when it is used, it is expanded during the assignment. For example:</p> <pre><code>HELLO = world HELLO_WORLD := $(HELLO) world! # This echoes "world world!" echo $(HELLO_WORLD) HELLO = hello # Still echoes "world world!" echo $(HELLO_WORLD) HELLO_WORLD := $(HELLO) world! # This echoes "hello world!" echo $(HELLO_WORLD) </code></pre> <p>Using <strong><code>?=</code></strong> assigns the variable a value <em>iff</em> the variable was not previously assigned. If the variable was previously assigned a blank value (<code>VAR=</code>), it is still considered set <em>I think</em>. Otherwise, functions exactly like <code>=</code>.</p> <p>Using <strong><code>+=</code></strong> is like using <code>=</code>, but instead of replacing the value, the value is appended to the current one, with a space in between. If the variable was previously set with <code>:=</code>, it is expanded <em>I think</em>. The resulting value is expanded when it is used <em>I think</em>. For example:</p> <pre><code>HELLO_WORLD = hello HELLO_WORLD += world! # This echoes "hello world!" echo $(HELLO_WORLD) </code></pre> <p>If something like <code>HELLO_WORLD = $(HELLO_WORLD) world!</code> were used, recursion would result, which would most likely end the execution of your Makefile. If <code>A := $(A) $(B)</code> were used, the result would not be the exact same as using <code>+=</code> because <code>B</code> is expanded with <code>:=</code> whereas <code>+=</code> would not cause <code>B</code> to be expanded.</p>
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    1. POWhat is the difference between the GNU Makefile variable assignments =, ?=, := and +=?
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    1. COThanks, this is a really very good explanation!
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      1. USBaiyan Huang
 

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