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| 22 | <title>Complete Explanation (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Complete-Program.html" rel="up" title="Complete Program">
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| 33 | <link href="Complete-Line_002dby_002dLine.html" rel="next" title="Complete Line-by-Line">
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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="Complete-Explanation"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Complete-Line_002dby_002dLine.html" accesskey="n" rel="next">Complete Line-by-Line</a>, Previous: <a href="Complete-Example.html" accesskey="p" rel="prev">Complete Example</a>, Up: <a href="Complete-Program.html" accesskey="u" rel="up">Complete Program</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="Complete-Program-Explanation"></span><h3 class="section">2.2 Complete Program Explanation</h3>
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| 64 |
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| 65 | <p>Here’s the explanation of the code of the example in the
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| 66 | previous section.
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| 67 | </p>
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| 68 | <p>This sample program prints a message that shows the value of <code>fib
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| 69 | (20)</code>, and exits with code 0 (which stands for successful execution).
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| 70 | </p>
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| 71 | <p>Every C program is started by running the function named <code>main</code>.
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| 72 | Therefore, the example program defines a function named <code>main</code> to
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| 73 | provide a way to start it. Whatever that function does is what the
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| 74 | program does. See <a href="The-main-Function.html">The main Function</a>.
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| 75 | </p>
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| 76 | <p>The <code>main</code> function is the first one called when the program
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| 77 | runs, but it doesn’t come first in the example code. The order of the
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| 78 | function definitions in the source code makes no difference to the
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| 79 | program’s meaning.
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| 80 | </p>
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| 81 | <p>The initial call to <code>main</code> always passes certain arguments, but
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| 82 | <code>main</code> does not have to pay attention to them. To ignore those
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| 83 | arguments, define <code>main</code> with <code>void</code> as the parameter list.
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| 84 | (<code>void</code> as a function’s parameter list normally means “call with
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| 85 | no arguments,” but <code>main</code> is a special case.)
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| 86 | </p>
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| 87 | <p>The function <code>main</code> returns 0 because that is
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| 88 | the conventional way for <code>main</code> to indicate successful execution.
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| 89 | It could instead return a positive integer to indicate failure, and
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| 90 | some utility programs have specific conventions for the meaning of
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| 91 | certain numeric <em>failure codes</em>. See <a href="Values-from-main.html">Values from main</a>.
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| 92 | </p>
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| 93 | <span id="index-printf"></span>
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| 94 | <p>The simplest way to print text in C is by calling the <code>printf</code>
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| 95 | function, so here we explain what that does.
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| 96 | </p>
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| 97 | <span id="index-standard-output"></span>
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| 98 | <p>The first argument to <code>printf</code> is a <em>string constant</em>
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| 99 | (see <a href="String-Constants.html">String Constants</a>) that is a template for output. The
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| 100 | function <code>printf</code> copies most of that string directly as output,
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| 101 | including the newline character at the end of the string, which is
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| 102 | written as ‘<samp>\n</samp>’. The output goes to the program’s <em>standard
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| 103 | output</em> destination, which in the usual case is the terminal.
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| 104 | </p>
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| 105 | <p>‘<samp>%</samp>’ in the template introduces a code that substitutes other text
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| 106 | into the output. Specifically, ‘<samp>%d</samp>’ means to take the next
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| 107 | argument to <code>printf</code> and substitute it into the text as a decimal
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| 108 | number. (The argument for ‘<samp>%d</samp>’ must be of type <code>int</code>; if it
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| 109 | isn’t, <code>printf</code> will malfunction.) So the output is a line that
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| 110 | looks like this:
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| 111 | </p>
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| 112 | <div class="example">
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| 113 | <pre class="example">Fibonacci series item 20 is 6765
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| 114 | </pre></div>
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| 115 |
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| 116 | <p>This program does not contain a definition for <code>printf</code> because
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| 117 | it is defined by the C library, which makes it available in all C
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| 118 | programs. However, each program does need to <em>declare</em>
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| 119 | <code>printf</code> so it will be called correctly. The <code>#include</code>
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| 120 | line takes care of that; it includes a <em>header file</em> called
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| 121 | <samp>stdio.h</samp> into the program’s code. That file is provided by the
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| 122 | operating system and it contains declarations for the many standard
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| 123 | input/output functions in the C library, one of which is
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| 124 | <code>printf</code>.
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| 125 | </p>
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| 126 | <p>Don’t worry about header files for now; we’ll explain them later in
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| 127 | <a href="Header-Files.html">Header Files</a>.
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| 128 | </p>
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| 129 | <p>The first argument of <code>printf</code> does not have to be a string
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| 130 | constant; it can be any string (see <a href="Strings.html">Strings</a>). However, using a
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| 131 | constant is the most common case.
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| 132 | </p>
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| 133 | <p>To learn more about <code>printf</code> and other facilities of the C
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| 134 | library, see <a href="https://www.gnu.org/software/libc/manual/html_node/index.html#Top">The GNU C Library</a> in <cite>The GNU C Library
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| 135 | Reference Manual</cite>.
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| 136 | </p>
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| 137 | <hr>
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| 138 | <div class="header">
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| 139 | <p>
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| 140 | Next: <a href="Complete-Line_002dby_002dLine.html" accesskey="n" rel="next">Complete Line-by-Line</a>, Previous: <a href="Complete-Example.html" accesskey="p" rel="prev">Complete Example</a>, Up: <a href="Complete-Program.html" accesskey="u" rel="up">Complete Program</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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| 141 | </div>
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