codeblocks-ep/help/html/programmerHelp/make/Conditional-Syntax.html at master · maifeeulasad/codeblocks-ep · GitHub

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<html lang="en">
<head>
<title>GNU `make'</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="GNU `make'">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home">
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<body>
<div class="node">
<p>
Node:<a name="Conditional%20Syntax">Conditional Syntax</a>,
Next:<a rel="next" accesskey="n" href="Testing-Flags.html#Testing%20Flags">Testing Flags</a>,
Previous:<a rel="previous" accesskey="p" href="Conditional-Example.html#Conditional%20Example">Conditional Example</a>,
Up:<a rel="up" accesskey="u" href="Conditionals.html#Conditionals">Conditionals</a>
<hr><br>
</div>

<h3 class="section">Syntax of Conditionals</h3>

   <p>The syntax of a simple conditional with no <code>else</code> is as follows:

<pre class="example">     <var>conditional-directive</var>
     <var>text-if-true</var>
     endif
     </pre>

<p>The <var>text-if-true</var> may be any lines of text, to be considered as part
of the makefile if the condition is true.  If the condition is false, no
text is used instead.

   <p>The syntax of a complex conditional is as follows:

<pre class="example">     <var>conditional-directive</var>
     <var>text-if-true</var>
     else
     <var>text-if-false</var>
     endif
     </pre>

<p>If the condition is true, <var>text-if-true</var> is used; otherwise,
<var>text-if-false</var> is used instead.  The <var>text-if-false</var> can be any
number of lines of text.

   <p>The syntax of the <var>conditional-directive</var> is the same whether the
conditional is simple or complex.  There are four different directives that
test different conditions.  Here is a table of them:

     <dl>
<dt><code>ifeq (</code><var>arg1</var><code>, </code><var>arg2</var><code>)</code>
     <dd><dt><code>ifeq '</code><var>arg1</var><code>' '</code><var>arg2</var><code>'</code>
     <dd><dt><code>ifeq "</code><var>arg1</var><code>" "</code><var>arg2</var><code>"</code>
     <dd><dt><code>ifeq "</code><var>arg1</var><code>" '</code><var>arg2</var><code>'</code>
     <dd><dt><code>ifeq '</code><var>arg1</var><code>' "</code><var>arg2</var><code>"</code>
     <dd>Expand all variable references in <var>arg1</var> and <var>arg2</var> and
compare them.  If they are identical, the <var>text-if-true</var> is
effective; otherwise, the <var>text-if-false</var>, if any, is effective.

     <p>Often you want to test if a variable has a non-empty value.  When the
value results from complex expansions of variables and functions,
expansions you would consider empty may actually contain whitespace
characters and thus are not seen as empty.  However, you can use the
<code>strip</code> function (see <a href="Text-Functions.html#Text%20Functions">Text Functions</a>) to avoid interpreting
whitespace as a non-empty value.  For example:

     <pre class="example">          ifeq ($(strip $(foo)),)
          <var>text-if-empty</var>
          endif
          </pre>

     <p>will evaluate <var>text-if-empty</var> even if the expansion of
<code>$(foo)</code> contains whitespace characters.

     <br><dt><code>ifneq (</code><var>arg1</var><code>, </code><var>arg2</var><code>)</code>
     <dd><dt><code>ifneq '</code><var>arg1</var><code>' '</code><var>arg2</var><code>'</code>
     <dd><dt><code>ifneq "</code><var>arg1</var><code>" "</code><var>arg2</var><code>"</code>
     <dd><dt><code>ifneq "</code><var>arg1</var><code>" '</code><var>arg2</var><code>'</code>
     <dd><dt><code>ifneq '</code><var>arg1</var><code>' "</code><var>arg2</var><code>"</code>
     <dd>Expand all variable references in <var>arg1</var> and <var>arg2</var> and
compare them.  If they are different, the <var>text-if-true</var> is
effective; otherwise, the <var>text-if-false</var>, if any, is effective.

     <br><dt><code>ifdef </code><var>variable-name</var><code></code>
     <dd>If the variable <var>variable-name</var> has a non-empty value, the
<var>text-if-true</var> is effective; otherwise, the <var>text-if-false</var>,
if any, is effective.  Variables that have never been defined have an
empty value.  The variable <var>variable-name</var> is itself expanded, so
it could be a variable or function that expands to the name of a
variable.

     <p>Note that <code>ifdef</code> only tests whether a variable has a value.  It
does not expand the variable to see if that value is nonempty.
Consequently, tests using <code>ifdef</code> return true for all definitions
except those like <code>foo =</code>.  To test for an empty value, use
<code>ifeq&nbsp;($(foo),)</code>.  For example,

     <pre class="example">          bar =
          foo = $(bar)
          ifdef foo
          frobozz = yes
          else
          frobozz = no
          endif
          </pre>

     <p>sets <code>frobozz</code> to <code>yes</code>, while:

     <pre class="example">          foo =
          ifdef foo
          frobozz = yes
          else
          frobozz = no
          endif
          </pre>

     <p>sets <code>frobozz</code> to <code>no</code>.

     <br><dt><code>ifndef </code><var>variable-name</var><code></code>
     <dd>If the variable <var>variable-name</var> has an empty value, the
<var>text-if-true</var> is effective; otherwise, the <var>text-if-false</var>,
if any, is effective.
</dl>

   <p>Extra spaces are allowed and ignored at the beginning of the conditional
directive line, but a tab is not allowed.  (If the line begins with a tab,
it will be considered a command for a rule.)  Aside from this, extra spaces
or tabs may be inserted with no effect anywhere except within the directive
name or within an argument.  A comment starting with <code>#</code> may appear at
the end of the line.

   <p>The other two directives that play a part in a conditional are <code>else</code>
and <code>endif</code>.  Each of these directives is written as one word, with no
arguments.  Extra spaces are allowed and ignored at the beginning of the
line, and spaces or tabs at the end.  A comment starting with <code>#</code> may
appear at the end of the line.

   <p>Conditionals affect which lines of the makefile <code>make</code> uses.  If
the condition is true, <code>make</code> reads the lines of the
<var>text-if-true</var> as part of the makefile; if the condition is false,
<code>make</code> ignores those lines completely.  It follows that syntactic
units of the makefile, such as rules, may safely be split across the
beginning or the end of the conditional.

   <p><code>make</code> evaluates conditionals when it reads a makefile.
Consequently, you cannot use automatic variables in the tests of
conditionals because they are not defined until commands are run
(see <a href="Automatic.html#Automatic">Automatic Variables</a>).

   <p>To prevent intolerable confusion, it is not permitted to start a
conditional in one makefile and end it in another.  However, you may
write an <code>include</code> directive within a conditional, provided you do
not attempt to terminate the conditional inside the included file.

   </body></html>