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| 22 | <title>Function Call Semantics (GNU C Language Manual)</title>
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| 58 | <span id="Function-Call-Semantics"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Function-Pointers.html" accesskey="n" rel="next">Function Pointers</a>, Previous: <a href="Function-Calls.html" accesskey="p" rel="prev">Function Calls</a>, Up: <a href="Functions.html" accesskey="u" rel="up">Functions</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="Function-Call-Semantics-1"></span><h3 class="section">22.4 Function Call Semantics</h3>
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| 64 | <span id="index-function-call-semantics"></span>
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| 65 | <span id="index-semantics-of-function-calls"></span>
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| 66 | <span id="index-call_002dby_002dvalue"></span>
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| 67 |
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| 68 | <p>The meaning of a function call is to compute the specified argument
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| 69 | expressions, convert their values according to the function’s
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| 70 | declaration, then run the function giving it copies of the converted
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| 71 | values. (This method of argument passing is known as
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| 72 | <em>call-by-value</em>.) When the function finishes, the value it
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| 73 | returns becomes the value of the function-call expression.
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| 74 | </p>
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| 75 | <p>Call-by-value implies that an assignment to the function argument
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| 76 | variable has no direct effect on the caller. For instance,
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| 77 | </p>
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| 78 | <div class="example">
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| 79 | <pre class="example">#include <stdlib.h> /* <span class="roman">Defines <code>EXIT_SUCCESS</code>.</span> */
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| 80 | #include <stdio.h> /* <span class="roman">Declares <code>printf</code>.</span> */
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| 81 |
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| 82 | void
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| 83 | subroutine (int x)
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| 84 | {
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| 85 | x = 5;
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| 86 | }
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| 87 |
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| 88 | void
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| 89 | main (void)
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| 90 | {
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| 91 | int y = 20;
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| 92 | subroutine (y);
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| 93 | printf ("y is %d\n", y);
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| 94 | return EXIT_SUCCESS;
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| 95 | }
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| 96 | </pre></div>
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| 97 |
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| 98 | <p>prints ‘<samp>y is 20</samp>’. Calling <code>subroutine</code> initializes <code>x</code>
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| 99 | from the value of <code>y</code>, but this does not establish any other
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| 100 | relationship between the two variables. Thus, the assignment to
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| 101 | <code>x</code>, inside <code>subroutine</code>, changes only <em>that</em> <code>x</code>.
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| 102 | </p>
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| 103 | <p>If an argument’s type is specified by the function’s declaration, the
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| 104 | function call converts the argument expression to that type if
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| 105 | possible. If the conversion is impossible, that is an error.
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| 106 | </p>
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| 107 | <p>If the function’s declaration doesn’t specify the type of that
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| 108 | argument, then the <em>default argument promotions</em> apply.
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| 109 | See <a href="Argument-Promotions.html">Argument Promotions</a>.
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| 110 | </p>
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