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| 22 | <title>Environment Variables (GNU C Language Manual)</title>
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| 24 | <meta name="description" content="Environment Variables (GNU C Language Manual)">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="The-main-Function.html" rel="up" title="The main Function">
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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="Environment-Variables"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Command_002dline-Parameters.html" accesskey="p" rel="prev">Command-line Parameters</a>, Up: <a href="The-main-Function.html" accesskey="u" rel="up">The main Function</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="Accessing-Environment-Variables"></span><h4 class="subsection">22.6.3 Accessing Environment Variables</h4>
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| 64 | <span id="index-environment-variables"></span>
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| 65 |
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| 66 | <p>You can optionally include a third parameter to <code>main</code>, another
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| 67 | array of strings, to capture the environment variables available to
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| 68 | the program. Unlike what happens with <code>argv</code>, there is no
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| 69 | additional parameter for the count of environment variables; rather,
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| 70 | the array of environment variables concludes with a null pointer.
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">#include <stdio.h> /* <span class="roman">Declares <code>printf</code>.</span> */
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| 74 |
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| 75 | int
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| 76 | main (int argc, char *argv[], char *envp[])
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| 77 | {
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| 78 | /* <span class="roman">Print out all environment variables.</span> */
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| 79 | int i = 0;
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| 80 | while (envp[i])
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| 81 | {
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| 82 | printf ("%s\n", envp[i]);
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| 83 | i++;
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| 84 | }
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| 85 | }
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| 86 | </pre></div>
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| 87 |
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| 88 | <p>Another method of retrieving environment variables is to use the
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| 89 | library function <code>getenv</code>, which is defined in <code>stdlib.h</code>.
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| 90 | Using <code>getenv</code> does not require defining <code>main</code> to accept the
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| 91 | <code>envp</code> pointer. For example, here is a program that fetches and prints
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| 92 | the user’s home directory (if defined):
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| 93 | </p>
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| 94 | <div class="example">
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| 95 | <pre class="example">#include <stdlib.h> /* <span class="roman">Declares <code>getenv</code>.</span> */
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| 96 | #include <stdio.h> /* <span class="roman">Declares <code>printf</code>.</span> */
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| 97 |
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| 98 | int
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| 99 | main (void)
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| 100 | {
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| 101 | char *home_directory = getenv ("HOME");
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| 102 | if (home_directory)
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| 103 | printf ("My home directory is: %s\n", home_directory);
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| 104 | else
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| 105 | printf ("My home directory is not defined!\n");
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| 106 | }
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| 107 | </pre></div>
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| 108 |
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| 109 |
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| 110 |
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| 111 |
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| 112 | </body>
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