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<span class="navbar-brand">Gtk4 tutorial</span>
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<a class="nav-link" href="sec5.html">Prev: section5</a>
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<a class="nav-link" href="sec7.html">Next: section7</a>
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<h1 id="strings-and-memory-management">Strings and memory
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management</h1>
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<p>GtkTextView and GtkTextBuffer have functions that use string
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parameters or return a string. The knowledge of strings and memory
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management is useful to understand how to use these functions.</p>
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<h2 id="string-and-memory">String and memory</h2>
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<p>A String is an array of characters that is terminated with ‘\0’.
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String is not a C type such as char, int, float or double, but a
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character array. It behaves like a string in other languages. So, the
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pointer is often called ‘a string’.</p>
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<p>The following is a sample program.</p>
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<div class="sourceCode" id="cb1"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="dt">char</span> a<span class="op">[</span><span class="dv">10</span><span class="op">],</span> <span class="op">*</span>b<span class="op">;</span></span>
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<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>a<span class="op">[</span><span class="dv">0</span><span class="op">]</span> <span class="op">=</span> <span class="ch">'H'</span><span class="op">;</span></span>
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<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>a<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op">=</span> <span class="ch">'e'</span><span class="op">;</span></span>
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<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a>a<span class="op">[</span><span class="dv">2</span><span class="op">]</span> <span class="op">=</span> <span class="ch">'l'</span><span class="op">;</span></span>
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<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>a<span class="op">[</span><span class="dv">3</span><span class="op">]</span> <span class="op">=</span> <span class="ch">'l'</span><span class="op">;</span></span>
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<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a>a<span class="op">[</span><span class="dv">4</span><span class="op">]</span> <span class="op">=</span> <span class="ch">'o'</span><span class="op">;</span></span>
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<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a>a<span class="op">[</span><span class="dv">5</span><span class="op">]</span> <span class="op">=</span> <span class="ch">'\0'</span><span class="op">;</span></span>
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<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a>b <span class="op">=</span> a<span class="op">;</span></span>
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<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a><span class="co">/* *b is 'H' */</span></span>
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<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a><span class="co">/* *(++b) is 'e' */</span></span></code></pre></div>
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<p>An array <code>a</code> is defined as a <code>char</code> type array
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and its size is ten. The first five elements are ‘H’, ‘e’, ‘l’, ‘l’,
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‘o’. They are character codes. For example, ‘H’ is the same as 0x48 or
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72. The sixth element is ‘\0’, which is the same as zero, and indicates
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that the sequence of the data ends there. The array represents the
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string “Hello”.</p>
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<p>The size of the array is 10, so 4 bytes aren’t used. But it’s OK.
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They are just ignored. (If ‘a’ is defined out of functions or its class
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is static, they are assigned with zero. Otherwise, that is to say, the
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class is auto or register, they are undefined.)</p>
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<p>The variable ‘b’ is a pointer to a character. It is assigned with
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<code>a</code>, so <code>b</code> points the first element of
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<code>a</code> (character ‘H’). The array <code>a</code> is immutable.
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So <code>a=a+1</code> causes syntax error.</p>
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<p>On the other hand, ‘b’ is a pointer type variable, which is mutable.
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So, <code>++b</code>, which increases <code>b</code> by one, is
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allowed.</p>
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<p>If a pointer is NULL, it points nothing. So, the pointer is not a
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string. It is different from empty string. Empty string is a pointer
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points <code>\0</code>.</p>
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<p>There are four cases:</p>
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<ul>
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<li>The string is read only</li>
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<li>The string is in static memory area</li>
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<li>The string is in stack</li>
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<li>The string is in memory allocated from the heap area</li>
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</ul>
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<h2 id="read-only-string">Read only string</h2>
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<p>A string literal is surrounded by double quotes like this:</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="dt">char</span> <span class="op">*</span>s<span class="op">;</span></span>
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<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>s <span class="op">=</span> <span class="st">"Hello"</span></span></code></pre></div>
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<p>“Hello” is a string literal, and is read only. So, the following
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program is illegal.</p>
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<div class="sourceCode" id="cb3"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="op">*(</span>s<span class="op">+</span><span class="dv">1</span><span class="op">)</span> <span class="op">=</span> <span class="ch">'a'</span><span class="op">;</span></span></code></pre></div>
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<p>The result is undefined. Probably a bad thing will happen, for
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example, a segmentation fault.</p>
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<p>NOTE: The memory of the literal string is allocated when the program
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is compiled. It is possible to see the literal strings with
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<code>strings</code> command.</p>
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<pre><code>$ strings src/tvf/a.out
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/lib64/ld-linux-x86-64.so.2
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cN<5
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... ... ...
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... ... ...
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Once upon a time, there was an old man who was called Taketori-no-Okina. It is a japanese word that means a man whose work is making bamboo baskets.
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One day, he went into a mountain and found a shining bamboo. "What a mysterious bamboo it is!," he said. He cut it, then there was a small cute baby girl in it. The girl was shining faintly. He thought this baby girl is a gift from Heaven and took her home.
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His wife was surprized at his story. They were very happy because they had no children.
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... ... ...
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... ... ...</code></pre>
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<p>It tells us that literal strings are embedded in program binary
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codes.</p>
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<h2 id="strings-defined-as-arrays">Strings defined as arrays</h2>
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<p>If a string is defined as an array, it’s stored in static memory area
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or stack. It depends on the class of the array. If the array’s class is
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<code>static</code>, then it’s placed in static memory area. The
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allocated memory lives for the life of the program. This area is
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writable.</p>
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<p>If the array’s class is <code>auto</code>, it’s placed in stack. If
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the array is defined inside a function, its default class is
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<code>auto</code>. The stack area will disappear when the function
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returns to the caller. Arrays defined on the stack are writable.</p>
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<div class="sourceCode" id="cb5"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="dt">char</span> a<span class="op">[]</span> <span class="op">=</span> <span class="op">{</span><span class="ch">'H'</span><span class="op">,</span> <span class="ch">'e'</span><span class="op">,</span> <span class="ch">'l'</span><span class="op">,</span> <span class="ch">'l'</span><span class="op">,</span> <span class="ch">'o'</span><span class="op">,</span> <span class="ch">'\0'</span><span class="op">};</span></span>
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<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
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<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a>print_strings <span class="op">(</span><span class="dt">void</span><span class="op">)</span> <span class="op">{</span></span>
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<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a> <span class="dt">char</span> b<span class="op">[]</span> <span class="op">=</span> <span class="st">"Hello"</span><span class="op">;</span></span>
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<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a> a<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op">=</span> <span class="ch">'a'</span><span class="op">;</span> <span class="co">/* Because the array is static, it's writable. */</span></span>
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<span id="cb5-8"><a href="#cb5-8" aria-hidden="true" tabindex="-1"></a> b<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op">=</span> <span class="ch">'a'</span><span class="op">;</span> <span class="co">/* Because the array is auto, it's writable. */</span></span>
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<span id="cb5-9"><a href="#cb5-9" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb5-10"><a href="#cb5-10" aria-hidden="true" tabindex="-1"></a> printf <span class="op">(</span><span class="st">"%s</span><span class="sc">\n</span><span class="st">"</span><span class="op">,</span> a<span class="op">);</span> <span class="co">/* Hallo */</span></span>
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<span id="cb5-11"><a href="#cb5-11" aria-hidden="true" tabindex="-1"></a> printf <span class="op">(</span><span class="st">"%s</span><span class="sc">\n</span><span class="st">"</span><span class="op">,</span> b<span class="op">);</span> <span class="co">/* Hallo */</span></span>
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<span id="cb5-12"><a href="#cb5-12" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
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<p>The array <code>a</code> is defined out of functions. It is placed in
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the static memory area even if the <code>static</code> class is left
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out. The compiler calculates the number of the elements (six) and
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allocates six bytes in the static memory area. Then, it copies “Hello”
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literal string data to the memory.</p>
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<p>The array <code>b</code> is defined inside the function, so its class
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is <code>auto</code>. The compiler calculates the number of the elements
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in the string literal. It is six because it has ‘\0’ terminator. The
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compiler allocates six bytes in the stack and copies “Hello” litaral
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string to the stack memory.</p>
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<p>Both <code>a</code> and <code>b</code> are writable.</p>
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<p>The memory is allocated and freed by the program automatically so you
|
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don’t need to allocate or free. The array <code>a</code> is alive during
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||
the program’s life time. The array <code>b</code> is alive when the
|
||
function is called until the function returns to the caller.</p>
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<h2 id="strings-in-the-heap-area">Strings in the heap area</h2>
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<p>You can get, use and release memory from the heap area. The standard
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C library provides <code>malloc</code> to get memory and
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<code>free</code> to put back memory. GLib provides the functions
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<code>g_new</code> and <code>g_free</code>. They are similar to
|
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<code>malloc</code> and <code>free</code>.</p>
|
||
<div class="sourceCode" id="cb6"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>g_new <span class="op">(</span>struct_type<span class="op">,</span> n_struct<span class="op">)</span></span></code></pre></div>
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<p><code>g_new</code> is a macro to allocate memory for an array.</p>
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<ul>
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<li><code>struct_type</code> is the type of the element of the
|
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array.</li>
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<li><code>n_struct</code> is the size of the array.</li>
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<li>The return value is a pointer to the array. Its type is a pointer to
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<code>struct_type</code>.</li>
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</ul>
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<p>For example,</p>
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<div class="sourceCode" id="cb7"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="dt">char</span> <span class="op">*</span>s<span class="op">;</span></span>
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<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a>s <span class="op">=</span> g_new <span class="op">(</span><span class="dt">char</span><span class="op">,</span> <span class="dv">10</span><span class="op">);</span></span>
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<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a><span class="co">/* s points an array of char. The size of the array is 10. */</span></span>
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<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb7-5"><a href="#cb7-5" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> tuple <span class="op">{</span><span class="dt">int</span> x<span class="op">,</span> y<span class="op">;}</span> <span class="op">*</span>t<span class="op">;</span></span>
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<span id="cb7-6"><a href="#cb7-6" aria-hidden="true" tabindex="-1"></a>t <span class="op">=</span> g_new <span class="op">(</span><span class="kw">struct</span> tuple<span class="op">,</span> <span class="dv">5</span><span class="op">);</span></span>
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<span id="cb7-7"><a href="#cb7-7" aria-hidden="true" tabindex="-1"></a><span class="co">/* t points an array of struct tuple. */</span></span>
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<span id="cb7-8"><a href="#cb7-8" aria-hidden="true" tabindex="-1"></a><span class="co">/* The size of the array is 5. */</span></span></code></pre></div>
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<p><code>g_free</code> frees memory.</p>
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||
<div class="sourceCode" id="cb8"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
|
||
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a>g_free <span class="op">(</span>gpointer mem<span class="op">);</span></span></code></pre></div>
|
||
<p>If <code>mem</code> is NULL, <code>g_free</code> does nothing.
|
||
<code>gpointer</code> is a type of general pointer. It is the same as
|
||
<code>void *</code>. This pointer can be casted to any pointer type.
|
||
Conversely, any pointer type can be casted to <code>gpointer</code>.</p>
|
||
<div class="sourceCode" id="cb9"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a>g_free <span class="op">(</span>s<span class="op">);</span></span>
|
||
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a><span class="co">/* Frees the memory allocated to s. */</span></span>
|
||
<span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a></span>
|
||
<span id="cb9-4"><a href="#cb9-4" aria-hidden="true" tabindex="-1"></a>g_free <span class="op">(</span>t<span class="op">);</span></span>
|
||
<span id="cb9-5"><a href="#cb9-5" aria-hidden="true" tabindex="-1"></a><span class="co">/* Frees the memory allocated to t. */</span></span></code></pre></div>
|
||
<p>If the argument doesn’t point allocated memory it will cause an
|
||
error, specifically, a segmentation fault.</p>
|
||
<p>Some GLib functions allocate memory. For example,
|
||
<code>g_strdup</code> allocates memory and copies a string given as an
|
||
argument.</p>
|
||
<div class="sourceCode" id="cb10"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="dt">char</span> <span class="op">*</span>s<span class="op">;</span></span>
|
||
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a>s <span class="op">=</span> g_strdup <span class="op">(</span><span class="st">"Hello"</span><span class="op">);</span></span>
|
||
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a>g_free <span class="op">(</span>s<span class="op">);</span></span></code></pre></div>
|
||
<p>The string literal “Hello” has 6 bytes because the string has ‘\0’ at
|
||
the end. <code>g_strdup</code> gets 6 bytes from the heap area and
|
||
copies the string to the memory. <code>s</code> is assigned the start
|
||
address of the memory. <code>g_free</code> returns the memory to the
|
||
heap area.</p>
|
||
<p><code>g_strdup</code> is described in <a
|
||
href="https://docs.gtk.org/glib/func.strdup.html">GLib API
|
||
Reference</a>. The following is extracted from the reference.</p>
|
||
<blockquote>
|
||
<p>The returned string should be freed with <code>g_free()</code> when
|
||
no longer needed.</p>
|
||
</blockquote>
|
||
<p>If you forget to free the allocated memory it will remain until the
|
||
program ends. Repeated allocation and no freeing cause memory leak. It
|
||
is a bug and may bring a serious problem.</p>
|
||
<h2 id="const-qualifier">const qualifier</h2>
|
||
<p>A <code>const</code> qualified variable can be assigned to initialize
|
||
it. Once it is initialized, it is never allowed to change or free.</p>
|
||
<div class="sourceCode" id="cb11"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="dt">const</span> <span class="dt">int</span> x <span class="op">=</span> <span class="dv">10</span><span class="op">;</span> <span class="co">/* initialization is OK. */</span></span>
|
||
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a></span>
|
||
<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a>x <span class="op">=</span> <span class="dv">20</span><span class="op">;</span> <span class="co">/* This is illegal because x is qualified with const */</span></span></code></pre></div>
|
||
<p>If a function returns <code>const char*</code> type, the string can’t
|
||
be changed or freed. If a function has a <code>const char *</code> type
|
||
parameter, it ensures that the parameter is not changed in the
|
||
function.</p>
|
||
<div class="sourceCode" id="cb12"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb12-1"><a href="#cb12-1" aria-hidden="true" tabindex="-1"></a><span class="co">// You never change or free the returned string.</span></span>
|
||
<span id="cb12-2"><a href="#cb12-2" aria-hidden="true" tabindex="-1"></a><span class="dt">const</span> <span class="dt">char</span><span class="op">*</span></span>
|
||
<span id="cb12-3"><a href="#cb12-3" aria-hidden="true" tabindex="-1"></a>gtk_label_get_text <span class="op">(</span></span>
|
||
<span id="cb12-4"><a href="#cb12-4" aria-hidden="true" tabindex="-1"></a> GtkLabel<span class="op">*</span> self</span>
|
||
<span id="cb12-5"><a href="#cb12-5" aria-hidden="true" tabindex="-1"></a><span class="op">)</span></span>
|
||
<span id="cb12-6"><a href="#cb12-6" aria-hidden="true" tabindex="-1"></a></span>
|
||
<span id="cb12-7"><a href="#cb12-7" aria-hidden="true" tabindex="-1"></a><span class="co">// Str keeps itself during the function runs</span></span>
|
||
<span id="cb12-8"><a href="#cb12-8" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
|
||
<span id="cb12-9"><a href="#cb12-9" aria-hidden="true" tabindex="-1"></a>gtk_label_set_text <span class="op">(</span></span>
|
||
<span id="cb12-10"><a href="#cb12-10" aria-hidden="true" tabindex="-1"></a> GtkLabel<span class="op">*</span> self<span class="op">,</span></span>
|
||
<span id="cb12-11"><a href="#cb12-11" aria-hidden="true" tabindex="-1"></a> <span class="dt">const</span> <span class="dt">char</span><span class="op">*</span> str</span>
|
||
<span id="cb12-12"><a href="#cb12-12" aria-hidden="true" tabindex="-1"></a><span class="op">)</span></span></code></pre></div>
|
||
</div>
|
||
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|
||
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|
||
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