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| <h5 class="subsubsection">3.2.4.5 Obstack Functions and Macros</h5> |
| |
| <p><a name="index-macros-313"></a> |
| The interfaces for using obstacks may be defined either as functions or |
| as macros, depending on the compiler. The obstack facility works with |
| all C compilers, including both ISO C<!-- /@w --> and traditional C, but there are |
| precautions you must take if you plan to use compilers other than GNU C. |
| |
| <p>If you are using an old-fashioned non-ISO C<!-- /@w --> compiler, all the obstack |
| “functions” are actually defined only as macros. You can call these |
| macros like functions, but you cannot use them in any other way (for |
| example, you cannot take their address). |
| |
| <p>Calling the macros requires a special precaution: namely, the first |
| operand (the obstack pointer) may not contain any side effects, because |
| it may be computed more than once. For example, if you write this: |
| |
| <pre class="smallexample"> obstack_alloc (get_obstack (), 4); |
| </pre> |
| <p class="noindent">you will find that <code>get_obstack</code> may be called several times. |
| If you use <code>*obstack_list_ptr++</code> as the obstack pointer argument, |
| you will get very strange results since the incrementation may occur |
| several times. |
| |
| <p>In ISO C<!-- /@w -->, each function has both a macro definition and a function |
| definition. The function definition is used if you take the address of the |
| function without calling it. An ordinary call uses the macro definition by |
| default, but you can request the function definition instead by writing the |
| function name in parentheses, as shown here: |
| |
| <pre class="smallexample"> char *x; |
| void *(*funcp) (); |
| /* <span class="roman">Use the macro</span>. */ |
| x = (char *) obstack_alloc (obptr, size); |
| /* <span class="roman">Call the function</span>. */ |
| x = (char *) (obstack_alloc) (obptr, size); |
| /* <span class="roman">Take the address of the function</span>. */ |
| funcp = obstack_alloc; |
| </pre> |
| <p class="noindent">This is the same situation that exists in ISO C<!-- /@w --> for the standard library |
| functions. See <a href="Macro-Definitions.html#Macro-Definitions">Macro Definitions</a>. |
| |
| <p><strong>Warning:</strong> When you do use the macros, you must observe the |
| precaution of avoiding side effects in the first operand, even in ISO C<!-- /@w -->. |
| |
| <p>If you use the GNU C compiler, this precaution is not necessary, because |
| various language extensions in GNU C permit defining the macros so as to |
| compute each argument only once. |
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