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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="string-and-memory-management">String and memory 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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Strings are not a C type such as char, int, float or double, but exist
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as a pointer to a character array. They behaves like a string type which
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you may be familiar from other languages. So, this pointer is often
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called ‘a string’.</p>
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<p>In the following, <code>a</code> and <code>b</code> defined as
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character arrays, and are strings.</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>The array <code>a</code> has <code>char</code> elements and the size
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of ten. The first six elements are ‘H’, ‘e’, ‘l’, ‘l’, ‘o’ and ‘\0’.
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This array represents the string “Hello”. The first five elements are
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character codes that correspond to the characters. The sixth element is
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‘\0’, which is the same as zero, and indicates that the string ends
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there. The size of the array is 10, so 4 bytes aren’t used, but that’s
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OK, they are just ignored.</p>
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<p>The variable ‘b’ is a pointer to a character. Because <code>b</code>
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is assigned to be <code>a</code>, <code>a</code> and <code>b</code>
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point the same character (‘H’). The variable <code>a</code> is defined
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as an array and it can’t be changed. It always point the top address of
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the array. On the other hand, ‘b’ is a pointer, which is mutable, so
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<code>b</code> can be change. It is then possible to write statements
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like <code>++b</code>, which means take the value in b (n address),
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increase it by one, and store that back in <code>b</code>.</p>
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<p>If a pointer is NULL, it points to nothing. So, the pointer is not a
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string. A NULL string on the other hand will be a pointer which points
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to a location that contains <code>\0</code>, which is a string of length
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0 (or ““). Programs that use strings will include bugs if you aren’t
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careful when using NULL pointers.</p>
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<p>Another annoying problem is the memory that a string is allocated.
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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; and</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 in a C program is surrounded by double quotes and
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written as the following:</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 stored in program memory. A
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string literal is read only. In the program above, <code>s</code> points
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the string literal.</p>
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<p>So, the following 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 view all the literal strings defined in
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your program by using the <code>string</code> command.</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 in either stored in the
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static memory area or stack. This depends on the class of the array. If
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the array’s class is <code>static</code>, then it’s placed in static
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memory area. This allocation and memory address is fixed for the life of
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the program. This area can be changed and is writable.</p>
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<p>If the array’s class is <code>auto</code>, then it’s placed in stack.
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If 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 exits
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and returns to the caller. Arrays defined on the stack are writable.</p>
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<div class="sourceCode" id="cb4"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb4-2"><a href="#cb4-2" 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="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
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<span id="cb4-5"><a href="#cb4-5" 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="cb4-6"><a href="#cb4-6" 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="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb4-8"><a href="#cb4-8" 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="cb4-9"><a href="#cb4-9" 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="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb4-11"><a href="#cb4-11" 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> a<span class="op">);</span> <span class="co">/* Hallo */</span></span>
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<span id="cb4-12"><a href="#cb4-12" 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="cb4-13"><a href="#cb4-13" 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 externally to a function and is
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global in its scope. Such variables are placed in static memory area
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even if the <code>static</code> class is left out. The compiler
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calculates the number of the elements in the right hand side (six), and
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then creates code that allocates six bytes in the static memory area and
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||
copies the data to this 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 has six elements as the zero termination
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||
character is also included. The compiler creates code which allocates
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six bytes memory in the stack and copies the data to the 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 managed by the executable program. You don’t need your
|
||
program to allocate or free the memory for <code>a</code> and
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<code>b</code>. The array <code>a</code> is created then the program is
|
||
first run and remains for the life of the program. The array
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||
<code>b</code> is created on the stack then the function is called,
|
||
disappears when the function returns.</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 also get, use and release memory from the heap area. The
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standard 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> to do the same thing, with
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||
support for some additional GLib functionality.</p>
|
||
<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>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="cb6"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb6-1"><a href="#cb6-1" 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="cb6-2"><a href="#cb6-2" 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="cb6-3"><a href="#cb6-3" 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="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb6-5"><a href="#cb6-5" 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="cb6-6"><a href="#cb6-6" 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="cb6-7"><a href="#cb6-7" aria-hidden="true" tabindex="-1"></a><span class="co">/* t points an array of struct tuple. */</span></span>
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<span id="cb6-8"><a href="#cb6-8" 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="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">void</span></span>
|
||
<span id="cb7-2"><a href="#cb7-2" 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="cb8"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a>g_free <span class="op">(</span>s<span class="op">);</span></span>
|
||
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a><span class="co">/* Frees the memory allocated to s. */</span></span>
|
||
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a></span>
|
||
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a>g_free <span class="op">(</span>t<span class="op">);</span></span>
|
||
<span id="cb8-5"><a href="#cb8-5" 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="cb9"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a><span class="dt">char</span> <span class="op">*</span>s<span class="op">;</span></span>
|
||
<span id="cb9-2"><a href="#cb9-2" 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="cb9-3"><a href="#cb9-3" 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 it. <code>g_strdup</code> gets 6 bytes from the heap area and
|
||
copies the string to the memory. <code>s</code> is assigned the top
|
||
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>The function reference will describe if the returned value needs to
|
||
be freed. If you forget to free the allocated memory it will remain
|
||
allocated. Repeated use will cause more memory to be allocated to the
|
||
program, which will grow over time. This is called a memory leak, and
|
||
the only way to address this bug is to close the program (and restart
|
||
it), which will automatically release all of the programs memory back to
|
||
the system.</p>
|
||
<p>Some GLib functions return a string which mustn’t be freed by the
|
||
caller.</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">const</span> <span class="dt">char</span> <span class="op">*</span></span>
|
||
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a>g_quark_to_string <span class="op">(</span>GQuark quark<span class="op">);</span></span></code></pre></div>
|
||
<p>This function returns <code>const char*</code> type. The qualifier
|
||
<code>const</code> means that the returned value is immutable. The
|
||
characters pointed by the returned value aren’t be allowed to be changed
|
||
or freed.</p>
|
||
<p>If a variable is qualified with <code>const</code>, the variable
|
||
can’t be assigned except during initialization.</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>
|
||
</div>
|
||
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