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| <h3 class="section">3.3 Macro Arguments</h3> |
| |
| <p><a name="index-arguments-46"></a><a name="index-macros-with-arguments-47"></a><a name="index-arguments-in-macro-definitions-48"></a> |
| Function-like macros can take <dfn>arguments</dfn>, just like true functions. |
| To define a macro that uses arguments, you insert <dfn>parameters</dfn> |
| between the pair of parentheses in the macro definition that make the |
| macro function-like. The parameters must be valid C identifiers, |
| separated by commas and optionally whitespace. |
| |
| <p>To invoke a macro that takes arguments, you write the name of the macro |
| followed by a list of <dfn>actual arguments</dfn> in parentheses, separated |
| by commas. The invocation of the macro need not be restricted to a |
| single logical line—it can cross as many lines in the source file as |
| you wish. The number of arguments you give must match the number of |
| parameters in the macro definition. When the macro is expanded, each |
| use of a parameter in its body is replaced by the tokens of the |
| corresponding argument. (You need not use all of the parameters in the |
| macro body.) |
| |
| <p>As an example, here is a macro that computes the minimum of two numeric |
| values, as it is defined in many C programs, and some uses. |
| |
| <pre class="smallexample"> #define min(X, Y) ((X) < (Y) ? (X) : (Y)) |
| x = min(a, b); ==> x = ((a) < (b) ? (a) : (b)); |
| y = min(1, 2); ==> y = ((1) < (2) ? (1) : (2)); |
| z = min(a + 28, *p); ==> z = ((a + 28) < (*p) ? (a + 28) : (*p)); |
| </pre> |
| <p class="noindent">(In this small example you can already see several of the dangers of |
| macro arguments. See <a href="Macro-Pitfalls.html#Macro-Pitfalls">Macro Pitfalls</a>, for detailed explanations.) |
| |
| <p>Leading and trailing whitespace in each argument is dropped, and all |
| whitespace between the tokens of an argument is reduced to a single |
| space. Parentheses within each argument must balance; a comma within |
| such parentheses does not end the argument. However, there is no |
| requirement for square brackets or braces to balance, and they do not |
| prevent a comma from separating arguments. Thus, |
| |
| <pre class="smallexample"> macro (array[x = y, x + 1]) |
| </pre> |
| <p class="noindent">passes two arguments to <code>macro</code>: <code>array[x = y</code> and <code>x + |
| 1]</code>. If you want to supply <code>array[x = y, x + 1]</code> as an argument, |
| you can write it as <code>array[(x = y, x + 1)]</code>, which is equivalent C |
| code. |
| |
| <p>All arguments to a macro are completely macro-expanded before they are |
| substituted into the macro body. After substitution, the complete text |
| is scanned again for macros to expand, including the arguments. This rule |
| may seem strange, but it is carefully designed so you need not worry |
| about whether any function call is actually a macro invocation. You can |
| run into trouble if you try to be too clever, though. See <a href="Argument-Prescan.html#Argument-Prescan">Argument Prescan</a>, for detailed discussion. |
| |
| <p>For example, <code>min (min (a, b), c)</code> is first expanded to |
| |
| <pre class="smallexample"> min (((a) < (b) ? (a) : (b)), (c)) |
| </pre> |
| <p class="noindent">and then to |
| |
| <pre class="smallexample"> ((((a) < (b) ? (a) : (b))) < (c) |
| ? (((a) < (b) ? (a) : (b))) |
| : (c)) |
| </pre> |
| <p class="noindent">(Line breaks shown here for clarity would not actually be generated.) |
| |
| <p><a name="index-empty-macro-arguments-49"></a>You can leave macro arguments empty; this is not an error to the |
| preprocessor (but many macros will then expand to invalid code). |
| You cannot leave out arguments entirely; if a macro takes two arguments, |
| there must be exactly one comma at the top level of its argument list. |
| Here are some silly examples using <code>min</code>: |
| |
| <pre class="smallexample"> min(, b) ==> (( ) < (b) ? ( ) : (b)) |
| min(a, ) ==> ((a ) < ( ) ? (a ) : ( )) |
| min(,) ==> (( ) < ( ) ? ( ) : ( )) |
| min((,),) ==> (((,)) < ( ) ? ((,)) : ( )) |
| |
| min() error--> macro "min" requires 2 arguments, but only 1 given |
| min(,,) error--> macro "min" passed 3 arguments, but takes just 2 |
| </pre> |
| <p>Whitespace is not a preprocessing token, so if a macro <code>foo</code> takes |
| one argument, <code>foo ()<!-- /@w --></code> and <code>foo ( )<!-- /@w --></code> both supply it an |
| empty argument. Previous GNU preprocessor implementations and |
| documentation were incorrect on this point, insisting that a |
| function-like macro that takes a single argument be passed a space if an |
| empty argument was required. |
| |
| <p>Macro parameters appearing inside string literals are not replaced by |
| their corresponding actual arguments. |
| |
| <pre class="smallexample"> #define foo(x) x, "x" |
| foo(bar) ==> bar, "x" |
| </pre> |
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