| <html lang="en"> |
| <head> |
| <title>Directives Within Macro Arguments - The C Preprocessor</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The C Preprocessor"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Macros.html#Macros" title="Macros"> |
| <link rel="prev" href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros" title="Undefining and Redefining Macros"> |
| <link rel="next" href="Macro-Pitfalls.html#Macro-Pitfalls" title="Macro Pitfalls"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, |
| 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008, 2009, 2010 |
| Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.2 or |
| any later version published by the Free Software Foundation. A copy of |
| the license is included in the |
| section entitled ``GNU Free Documentation License''. |
| |
| This manual contains no Invariant Sections. The Front-Cover Texts are |
| (a) (see below), and the Back-Cover Texts are (b) (see below). |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development. |
| --> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Directives-Within-Macro-Arguments"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Macro-Pitfalls.html#Macro-Pitfalls">Macro Pitfalls</a>, |
| Previous: <a rel="previous" accesskey="p" href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros">Undefining and Redefining Macros</a>, |
| Up: <a rel="up" accesskey="u" href="Macros.html#Macros">Macros</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">3.9 Directives Within Macro Arguments</h3> |
| |
| <p><a name="index-macro-arguments-and-directives-71"></a> |
| Occasionally it is convenient to use preprocessor directives within |
| the arguments of a macro. The C and C++ standards declare that |
| behavior in these cases is undefined. |
| |
| <p>Versions of CPP prior to 3.2 would reject such constructs with an |
| error message. This was the only syntactic difference between normal |
| functions and function-like macros, so it seemed attractive to remove |
| this limitation, and people would often be surprised that they could |
| not use macros in this way. Moreover, sometimes people would use |
| conditional compilation in the argument list to a normal library |
| function like ‘<samp><span class="samp">printf</span></samp>’, only to find that after a library upgrade |
| ‘<samp><span class="samp">printf</span></samp>’ had changed to be a function-like macro, and their code |
| would no longer compile. So from version 3.2 we changed CPP to |
| successfully process arbitrary directives within macro arguments in |
| exactly the same way as it would have processed the directive were the |
| function-like macro invocation not present. |
| |
| <p>If, within a macro invocation, that macro is redefined, then the new |
| definition takes effect in time for argument pre-expansion, but the |
| original definition is still used for argument replacement. Here is a |
| pathological example: |
| |
| <pre class="smallexample"> #define f(x) x x |
| f (1 |
| #undef f |
| #define f 2 |
| f) |
| </pre> |
| <p class="noindent">which expands to |
| |
| <pre class="smallexample"> 1 2 1 2 |
| </pre> |
| <p class="noindent">with the semantics described above. |
| |
| </body></html> |
| |
