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| 22 | <title>String Constants (GNU C Language Manual)</title>
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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="Constants.html" rel="up" title="Constants">
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| 33 | <link href="UTF_002d8-String-Constants.html" rel="next" title="UTF-8 String Constants">
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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="String-Constants"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="UTF_002d8-String-Constants.html" accesskey="n" rel="next">UTF-8 String Constants</a>, Previous: <a href="Character-Constants.html" accesskey="p" rel="prev">Character Constants</a>, Up: <a href="Constants.html" accesskey="u" rel="up">Constants</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="String-Constants-1"></span><h3 class="section">12.7 String Constants</h3>
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| 64 | <span id="index-string-constants"></span>
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| 65 | <span id="index-constants_002c-string"></span>
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| 66 |
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| 67 | <p>A <em>string constant</em> represents a series of characters. It starts
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| 68 | with ‘<samp>"</samp>’ and ends with ‘<samp>"</samp>’; in between are the contents of
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| 69 | the string. Quoting special characters such as ‘<samp>"</samp>’, ‘<samp>\</samp>’ and
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| 70 | newline in the contents works in string constants as in character
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| 71 | constants. In a string constant, ‘<samp>'</samp>’ does not need to be quoted.
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| 72 | </p>
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| 73 | <p>A string constant defines an array of characters which contains the
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| 74 | specified characters followed by the null character (code 0). Using
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| 75 | the string constant is equivalent to using the name of an array with
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| 76 | those contents. In simple cases, the length in bytes of the string
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| 77 | constant is one greater than the number of characters written in it.
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| 78 | </p>
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| 79 | <p>As with any array in C, using the string constant in an expression
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| 80 | converts the array to a pointer (see <a href="Pointers.html">Pointers</a>) to the array’s
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| 81 | first element (see <a href="Accessing-Array-Elements.html">Accessing Array Elements</a>). This pointer will
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| 82 | have type <code>char *</code> because it points to an element of type
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| 83 | <code>char</code>. <code>char *</code> is an example of a type designator for a
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| 84 | pointer type (see <a href="Pointer-Type-Designators.html">Pointer Type Designators</a>). That type is used
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| 85 | for strings generally, not just the strings expressed as constants
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| 86 | in a program.
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| 87 | </p>
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| 88 | <p>Thus, the string constant <code>"Foo!"</code> is almost
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| 89 | equivalent to declaring an array like this
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| 90 | </p>
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| 91 | <div class="example">
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| 92 | <pre class="example">char string_array_1[] = {'F', 'o', 'o', '!', '\0' };
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| 93 | </pre></div>
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| 94 |
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| 95 | <p>and then using <code>string_array_1</code> in the program. There
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| 96 | are two differences, however:
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| 97 | </p>
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| 98 | <ul>
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| 99 | <li> The string constant doesn’t define a name for the array.
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| 100 |
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| 101 | </li><li> The string constant is probably stored in a read-only area of memory.
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| 102 | </li></ul>
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| 103 |
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| 104 | <p>Newlines are not allowed in the text of a string constant. The motive
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| 105 | for this prohibition is to catch the error of omitting the closing
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| 106 | ‘<samp>"</samp>’. To put a newline in a constant string, write it as
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| 107 | ‘<samp>\n</samp>’ in the string constant.
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| 108 | </p>
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| 109 | <p>A real null character in the source code inside a string constant
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| 110 | causes a warning. To put a null character in the middle of a string
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| 111 | constant, write ‘<samp>\0</samp>’ or ‘<samp>\000</samp>’.
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| 112 | </p>
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| 113 | <p>Consecutive string constants are effectively concatenated. Thus,
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| 114 | </p>
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| 115 | <div class="example">
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| 116 | <pre class="example">"Fo" "o!" <span class="roman">is equivalent to</span> "Foo!"
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| 117 | </pre></div>
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| 118 |
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| 119 | <p>This is useful for writing a string containing multiple lines,
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| 120 | like this:
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| 121 | </p>
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| 122 | <div class="example">
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| 123 | <pre class="example">"This message is so long that it needs more than\n"
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| 124 | "a single line of text. C does not allow a newline\n"
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| 125 | "to represent itself in a string constant, so we have to\n"
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| 126 | "write \\n to put it in the string. For readability of\n"
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| 127 | "the source code, it is advisable to put line breaks in\n"
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| 128 | "the source where they occur in the contents of the\n"
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| 129 | "constant.\n"
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| 130 | </pre></div>
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| 131 |
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| 132 | <p>The sequence of a backslash and a newline is ignored anywhere
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| 133 | in a C program, and that includes inside a string constant.
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| 134 | Thus, you can write multi-line string constants this way:
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| 135 | </p>
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| 136 | <div class="example">
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| 137 | <pre class="example">"This is another way to put newlines in a string constant\n\
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| 138 | and break the line after them in the source code."
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| 139 | </pre></div>
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| 140 |
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| 141 | <p>However, concatenation is the recommended way to do this.
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| 142 | </p>
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| 143 | <p>You can also write perverse string constants like this,
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| 144 | </p>
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| 145 | <div class="example">
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| 146 | <pre class="example">"Fo\
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| 147 | o!"
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| 148 | </pre></div>
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| 149 |
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| 150 | <p>but don’t do that—write it like this instead:
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| 151 | </p>
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| 152 | <div class="example">
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| 153 | <pre class="example">"Foo!"
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| 154 | </pre></div>
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| 155 |
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| 156 | <p>Be careful to avoid passing a string constant to a function that
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| 157 | modifies the string it receives. The memory where the string constant
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| 158 | is stored may be read-only, which would cause a fatal <code>SIGSEGV</code>
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| 159 | signal that normally terminates the function (see <a href="Signals.html">Signals</a>. Even
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| 160 | worse, the memory may not be read-only. Then the function might
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| 161 | modify the string constant, thus spoiling the contents of other string
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| 162 | constants that are supposed to contain the same value and are unified
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| 163 | by the compiler.
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| 164 | </p>
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| 165 | <hr>
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| 166 | <div class="header">
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| 167 | <p>
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| 168 | Next: <a href="UTF_002d8-String-Constants.html" accesskey="n" rel="next">UTF-8 String Constants</a>, Previous: <a href="Character-Constants.html" accesskey="p" rel="prev">Character Constants</a>, Up: <a href="Constants.html" accesskey="u" rel="up">Constants</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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| 169 | </div>
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| 170 |
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| 171 |
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| 172 |
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| 173 | </body>
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