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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Strings (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Strings (GNU C Language Manual)">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Arrays.html" rel="up" title="Arrays">
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| 33 | <link href="Array-Type-Designators.html" rel="next" title="Array Type Designators">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Strings"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Array-Type-Designators.html" accesskey="n" rel="next">Array Type Designators</a>, Previous: <a href="Declaring-an-Array.html" accesskey="p" rel="prev">Declaring an Array</a>, Up: <a href="Arrays.html" accesskey="u" rel="up">Arrays</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Strings-1"></span><h3 class="section">16.3 Strings</h3>
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| 64 | <span id="index-string"></span>
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| 65 |
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| 66 | <p>A string in C is a sequence of elements of type <code>char</code>,
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| 67 | terminated with the null character, the character with code zero.
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| 68 | </p>
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| 69 | <p>Programs often need to use strings with specific, fixed contents. To
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| 70 | write one in a C program, use a <em>string constant</em> such as
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| 71 | <code>"Take me to your leader!"</code>. The data type of a string constant
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| 72 | is <code>char *</code>. For the full syntactic details of writing string
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| 73 | constants, <a href="String-Constants.html">String Constants</a>.
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| 74 | </p>
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| 75 | <p>To declare a place to store a non-constant string, declare an array of
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| 76 | <code>char</code>. Keep in mind that it must include one extra <code>char</code>
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| 77 | for the terminating null. For instance,
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| 78 | </p>
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| 79 | <div class="example">
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| 80 | <pre class="example">char text = { 'H', 'e', 'l', 'l', 'o', 0 };
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>declares an array named ‘<samp>text</samp>’ with six elements—five letters
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| 84 | and the terminating null character. An equivalent way to get the same
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| 85 | result is this,
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">char text = "Hello";
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| 89 | </pre></div>
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| 90 |
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| 91 | <p>which copies the elements of the string constant, including <em>its</em>
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| 92 | terminating null character.
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| 93 | </p>
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| 94 | <div class="example">
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| 95 | <pre class="example">char message[200];
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| 96 | </pre></div>
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| 97 |
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| 98 | <p>declares an array long enough to hold a string of 199 ASCII characters
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| 99 | plus the terminating null character.
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| 100 | </p>
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| 101 | <p>When you store a string into <code>message</code> be sure to check or prove
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| 102 | that the length does not exceed its size. For example,
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| 103 | </p>
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| 104 | <div class="example">
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| 105 | <pre class="example">void
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| 106 | set_message (char *text)
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| 107 | {
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| 108 | int i;
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| 109 | for (i = 0; i < sizeof (message); i++)
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| 110 | {
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| 111 | message[i] = text[i];
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| 112 | if (text[i] == 0)
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| 113 | return;
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| 114 | }
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| 115 | fatal_error ("Message is too long for `message');
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| 116 | }
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| 117 | </pre></div>
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| 118 |
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| 119 | <p>It’s easy to do this with the standard library function
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| 120 | <code>strncpy</code>, which fills out the whole destination array (up to a
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| 121 | specified length) with null characters. Thus, if the last character
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| 122 | of the destination is not null, the string did not fit. Many system
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| 123 | libraries, including the GNU C library, hand-optimize <code>strncpy</code>
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| 124 | to run faster than an explicit <code>for</code>-loop.
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| 125 | </p>
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| 126 | <p>Here’s what the code looks like:
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| 127 | </p>
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| 128 | <div class="example">
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| 129 | <pre class="example">void
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| 130 | set_message (char *text)
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| 131 | {
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| 132 | strncpy (message, text, sizeof (message));
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| 133 | if (message[sizeof (message) - 1] != 0)
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| 134 | fatal_error ("Message is too long for `message');
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| 135 | }
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| 136 | </pre></div>
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| 137 |
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| 138 | <p>See <a href="https://www.gnu.org/software/libc/manual/html_node/String-and-Array-Utilities.html#String-and-Array-Utilities">The GNU C Library</a> in <cite>The GNU C
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| 139 | Library Reference Manual</cite>, for more information about the standard
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| 140 | library functions for operating on strings.
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| 141 | </p>
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| 142 | <p>You can avoid putting a fixed length limit on strings you construct or
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| 143 | operate on by allocating the space for them dynamically.
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| 144 | See <a href="Dynamic-Memory-Allocation.html">Dynamic Memory Allocation</a>.
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| 145 | </p>
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| 146 | <hr>
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| 147 | <div class="header">
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| 148 | <p>
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| 149 | Next: <a href="Array-Type-Designators.html" accesskey="n" rel="next">Array Type Designators</a>, Previous: <a href="Declaring-an-Array.html" accesskey="p" rel="prev">Declaring an Array</a>, Up: <a href="Arrays.html" accesskey="u" rel="up">Arrays</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 150 | </div>
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| 152 |
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