| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
| 2 | <html> |
| 3 | <head> |
| 4 | <title>SWIG Preprocessor</title> |
| 5 | <link rel="stylesheet" type="text/css" href="style.css"> |
| 6 | </head> |
| 7 | |
| 8 | <body bgcolor="#ffffff"> |
| 9 | <H1><a name="Preprocessor"></a>7 Preprocessing</H1> |
| 10 | <!-- INDEX --> |
| 11 | <div class="sectiontoc"> |
| 12 | <ul> |
| 13 | <li><a href="#Preprocessor_nn2">File inclusion</a> |
| 14 | <li><a href="#Preprocessor_nn3">File imports</a> |
| 15 | <li><a href="#Preprocessor_nn4">Conditional Compilation</a> |
| 16 | <li><a href="#Preprocessor_nn5">Macro Expansion</a> |
| 17 | <li><a href="#Preprocessor_nn6">SWIG Macros</a> |
| 18 | <li><a href="#Preprocessor_nn7">C99 and GNU Extensions</a> |
| 19 | <li><a href="#Preprocessor_nn8">Preprocessing and %{ ... %} blocks</a> |
| 20 | <li><a href="#Preprocessor_nn9">Preprocessing and { ... }</a> |
| 21 | <li><a href="#Preprocessor_nn10">Viewing preprocessor output</a> |
| 22 | </ul> |
| 23 | </div> |
| 24 | <!-- INDEX --> |
| 25 | |
| 26 | |
| 27 | |
| 28 | <p> |
| 29 | SWIG includes its own enhanced version of the C preprocessor. The preprocessor |
| 30 | supports the standard preprocessor directives and macro expansion rules. |
| 31 | However, a number of modifications and enhancements have been made. This |
| 32 | chapter describes some of these modifications. |
| 33 | </p> |
| 34 | |
| 35 | <H2><a name="Preprocessor_nn2"></a>7.1 File inclusion</H2> |
| 36 | |
| 37 | |
| 38 | <p> |
| 39 | To include another file into a SWIG interface, use the <tt>%include</tt> directive |
| 40 | like this: |
| 41 | </p> |
| 42 | |
| 43 | <div class="code"> |
| 44 | <pre> |
| 45 | %include "pointer.i" |
| 46 | </pre> |
| 47 | </div> |
| 48 | |
| 49 | <p> |
| 50 | Unlike, <tt>#include</tt>, <tt>%include</tt> includes each file once (and will not |
| 51 | reload the file on subsequent <tt>%include</tt> declarations). Therefore, it |
| 52 | is not necessary to use include-guards in SWIG interfaces. |
| 53 | </p> |
| 54 | |
| 55 | <p> |
| 56 | By default, the <tt>#include</tt> is ignored unless you run SWIG with the |
| 57 | <tt>-includeall</tt> option. The reason for ignoring traditional includes |
| 58 | is that you often don't want SWIG to try and wrap everything included |
| 59 | in standard header system headers and auxilliary files. |
| 60 | |
| 61 | <H2><a name="Preprocessor_nn3"></a>7.2 File imports</H2> |
| 62 | |
| 63 | |
| 64 | <p> |
| 65 | SWIG provides another file inclusion directive with the <tt>%import</tt> directive. |
| 66 | For example: |
| 67 | </p> |
| 68 | |
| 69 | <div class="code"> |
| 70 | <pre> |
| 71 | %import "foo.i" |
| 72 | </pre> |
| 73 | </div> |
| 74 | |
| 75 | <p> |
| 76 | The purpose of <tt>%import</tt> is to collect certain information from another |
| 77 | SWIG interface file or a header file without actually generating any wrapper code. |
| 78 | Such information generally includes type declarations (e.g., <tt>typedef</tt>) as well as |
| 79 | C++ classes that might be used as base-classes for class declarations in the interface. |
| 80 | The use of <tt>%import</tt> is also important when SWIG is used to generate |
| 81 | extensions as a collection of related modules. This is an advanced topic and is described |
| 82 | in a later chapter. |
| 83 | </p> |
| 84 | |
| 85 | <P> |
| 86 | The <tt>-importall</tt> directive tells SWIG to follow all <tt>#include</tt> statements |
| 87 | as imports. This might be useful if you want to extract type definitions from system |
| 88 | header files without generating any wrappers. |
| 89 | |
| 90 | <H2><a name="Preprocessor_nn4"></a>7.3 Conditional Compilation</H2> |
| 91 | |
| 92 | |
| 93 | <p> |
| 94 | SWIG fully supports the use of <tt>#if</tt>, <tt>#ifdef</tt>, |
| 95 | <tt>#ifndef</tt>, <tt>#else</tt>, <tt>#endif</tt> to conditionally |
| 96 | include parts of an interface. The following symbols are predefined |
| 97 | by SWIG when it is parsing the interface: |
| 98 | </p> |
| 99 | |
| 100 | <div class="code"><pre> |
| 101 | SWIG Always defined when SWIG is processing a file |
| 102 | SWIGIMPORTED Defined when SWIG is importing a file with <tt>%import</tt> |
| 103 | SWIGMAC Defined when running SWIG on the Macintosh |
| 104 | SWIGWIN Defined when running SWIG under Windows |
| 105 | SWIG_VERSION Hexadecimal number containing SWIG version, |
| 106 | such as 0x010311 (corresponding to SWIG-1.3.11). |
| 107 | |
| 108 | SWIGCHICKEN Defined when using CHICKEN |
| 109 | SWIGCSHARP Defined when using C# |
| 110 | SWIGGUILE Defined when using Guile |
| 111 | SWIGJAVA Defined when using Java |
| 112 | SWIGMZSCHEME Defined when using Mzscheme |
| 113 | SWIGOCAML Defined when using Ocaml |
| 114 | SWIGPERL Defined when using Perl |
| 115 | SWIGPERL5 Defined when using Perl5 |
| 116 | SWIGPHP Defined when using PHP |
| 117 | SWIGPHP4 Defined when using PHP4 |
| 118 | SWIGPYTHON Defined when using Python |
| 119 | SWIGRUBY Defined when using Ruby |
| 120 | SWIGSEXP Defined when using S-expressions |
| 121 | SWIGTCL Defined when using Tcl |
| 122 | SWIGTCL8 Defined when using Tcl8.0 |
| 123 | SWIGXML Defined when using XML |
| 124 | </pre></div> |
| 125 | |
| 126 | <p> |
| 127 | In addition, SWIG defines the following set of standard C/C++ macros: |
| 128 | </p> |
| 129 | |
| 130 | <div class="code"> |
| 131 | <pre> |
| 132 | __LINE__ Current line number |
| 133 | __FILE__ Current file name |
| 134 | __STDC__ Defined to indicate ANSI C |
| 135 | __cplusplus Defined when -c++ option used |
| 136 | </pre> |
| 137 | </div> |
| 138 | |
| 139 | <p> |
| 140 | Interface files can look at these symbols as necessary to change the |
| 141 | way in which an interface is generated or to mix SWIG directives with |
| 142 | C code. These symbols are also defined within the C code generated by |
| 143 | SWIG (except for the symbol `<tt>SWIG</tt>' which is only defined |
| 144 | within the SWIG compiler). |
| 145 | </p> |
| 146 | |
| 147 | <H2><a name="Preprocessor_nn5"></a>7.4 Macro Expansion</H2> |
| 148 | |
| 149 | |
| 150 | <p> |
| 151 | Traditional preprocessor macros can be used in SWIG interfaces. Be aware that the <tt>#define</tt> statement |
| 152 | is also used to try and detect constants. Therefore, if you have something like this in your file, |
| 153 | </p> |
| 154 | |
| 155 | <div class="code"> |
| 156 | <pre> |
| 157 | #ifndef _FOO_H 1 |
| 158 | #define _FOO_H 1 |
| 159 | ... |
| 160 | #endif |
| 161 | </pre> |
| 162 | </div> |
| 163 | |
| 164 | <p> |
| 165 | you may get some extra constants such as <tt>_FOO_H</tt> showing up in the scripting interface. |
| 166 | </p> |
| 167 | |
| 168 | <p> |
| 169 | More complex macros can be defined in the standard way. For example: |
| 170 | </p> |
| 171 | |
| 172 | <div class="code"> |
| 173 | <pre> |
| 174 | #define EXTERN extern |
| 175 | #ifdef __STDC__ |
| 176 | #define _ANSI(args) (args) |
| 177 | #else |
| 178 | #define _ANSI(args) () |
| 179 | #endif |
| 180 | </pre> |
| 181 | </div> |
| 182 | |
| 183 | <p> |
| 184 | The following operators can appear in macro definitions: |
| 185 | </p> |
| 186 | |
| 187 | <ul> |
| 188 | <li><tt>#x</tt><br> |
| 189 | Converts macro argument <tt>x</tt> to a string surrounded by double quotes ("x"). |
| 190 | </li> |
| 191 | |
| 192 | <li><tt>x ## y</tt><br> |
| 193 | Concatenates x and y together to form <tt>xy</tt>. |
| 194 | </li> |
| 195 | |
| 196 | <li><tt>`x`</tt><br> |
| 197 | If <tt>x</tt> is a string surrounded by double quotes, do nothing. Otherwise, turn into a string |
| 198 | like <tt>#x</tt>. This is a non-standard SWIG extension. |
| 199 | </li> |
| 200 | </ul> |
| 201 | |
| 202 | <H2><a name="Preprocessor_nn6"></a>7.5 SWIG Macros</H2> |
| 203 | |
| 204 | |
| 205 | <p> |
| 206 | SWIG provides an enhanced macro capability with the <tt>%define</tt> and <tt>%enddef</tt> directives. |
| 207 | For example: |
| 208 | </p> |
| 209 | |
| 210 | <div class="code"> |
| 211 | <pre> |
| 212 | %define ARRAYHELPER(type,name) |
| 213 | %inline %{ |
| 214 | type *new_ ## name (int nitems) { |
| 215 | return (type *) malloc(sizeof(type)*nitems); |
| 216 | } |
| 217 | void delete_ ## name(type *t) { |
| 218 | free(t); |
| 219 | } |
| 220 | type name ## _get(type *t, int index) { |
| 221 | return t[index]; |
| 222 | } |
| 223 | void name ## _set(type *t, int index, type val) { |
| 224 | t[index] = val; |
| 225 | } |
| 226 | %} |
| 227 | %enddef |
| 228 | |
| 229 | ARRAYHELPER(int, IntArray) |
| 230 | ARRAYHELPER(double, DoubleArray) |
| 231 | </pre> |
| 232 | </div> |
| 233 | |
| 234 | <p> |
| 235 | The primary purpose of <tt>%define</tt> is to define large macros of code. Unlike normal C preprocessor |
| 236 | macros, it is not necessary to terminate each line with a continuation character (\)--the macro definition |
| 237 | extends to the first occurrence of <tt>%enddef</tt>. Furthermore, when such macros are expanded, |
| 238 | they are reparsed through the C preprocessor. Thus, SWIG macros can contain all other preprocessor |
| 239 | directives except for nested <tt>%define</tt> statements. |
| 240 | </p> |
| 241 | |
| 242 | <p> |
| 243 | The SWIG macro capability is a very quick and easy way to generate large amounts of code. In fact, |
| 244 | many of SWIG's advanced features and libraries are built using this mechanism (such as C++ template |
| 245 | support). |
| 246 | </p> |
| 247 | |
| 248 | <H2><a name="Preprocessor_nn7"></a>7.6 C99 and GNU Extensions</H2> |
| 249 | |
| 250 | |
| 251 | <p> |
| 252 | SWIG-1.3.12 and newer releases support variadic preprocessor macros. For example: |
| 253 | </p> |
| 254 | |
| 255 | <div class="code"> |
| 256 | <pre> |
| 257 | #define DEBUGF(fmt,...) fprintf(stderr,fmt,__VA_ARGS__) |
| 258 | </pre> |
| 259 | </div> |
| 260 | |
| 261 | <p> |
| 262 | When used, any extra arguments to <tt>...</tt> are placed into the |
| 263 | special variable <tt>__VA_ARGS__</tt>. This also works with special SWIG |
| 264 | macros defined using <tt>%define</tt>. |
| 265 | </p> |
| 266 | |
| 267 | <p> |
| 268 | SWIG allows a variable number of arguments to be empty. However, this often results |
| 269 | in an extra comma (,) and syntax error in the resulting expansion. For example: |
| 270 | </p> |
| 271 | |
| 272 | <div class="code"> |
| 273 | <pre> |
| 274 | DEBUGF("hello"); --> fprintf(stderr,"hello",); |
| 275 | </pre> |
| 276 | </div> |
| 277 | |
| 278 | <p> |
| 279 | To get rid of the extra comma, use <tt>##</tt> like this: |
| 280 | </p> |
| 281 | |
| 282 | <div class="code"> |
| 283 | <pre> |
| 284 | #define DEBUGF(fmt,...) fprintf(stderr,fmt, ##__VA_ARGS__) |
| 285 | </pre> |
| 286 | </div> |
| 287 | |
| 288 | <p> |
| 289 | SWIG also supports GNU-style variadic macros. For example: |
| 290 | </p> |
| 291 | |
| 292 | <div class="code"> |
| 293 | <pre> |
| 294 | #define DEBUGF(fmt, args...) fprintf(stdout,fmt,args) |
| 295 | </pre> |
| 296 | </div> |
| 297 | |
| 298 | <p> |
| 299 | <b>Comment:</b> It's not entirely clear how variadic macros might be useful to |
| 300 | interface building. However, they are used internally to implement a number of |
| 301 | SWIG directives and are provided to make SWIG more compatible with C99 code. |
| 302 | </p> |
| 303 | |
| 304 | <H2><a name="Preprocessor_nn8"></a>7.7 Preprocessing and %{ ... %} blocks</H2> |
| 305 | |
| 306 | |
| 307 | <p> |
| 308 | The SWIG preprocessor does not process any text enclosed in a code block %{ ... %}. Therefore, |
| 309 | if you write code like this, |
| 310 | </p> |
| 311 | |
| 312 | <div class="code"> |
| 313 | <pre> |
| 314 | %{ |
| 315 | #ifdef NEED_BLAH |
| 316 | int blah() { |
| 317 | ... |
| 318 | } |
| 319 | #endif |
| 320 | %} |
| 321 | </pre> |
| 322 | </div> |
| 323 | |
| 324 | <p> |
| 325 | the contents of the <tt>%{ ... %}</tt> block are copied without |
| 326 | modification to the output (including all preprocessor directives). |
| 327 | </p> |
| 328 | |
| 329 | <H2><a name="Preprocessor_nn9"></a>7.8 Preprocessing and { ... }</H2> |
| 330 | |
| 331 | |
| 332 | <p> |
| 333 | SWIG always runs the preprocessor on text appearing inside <tt>{ ... }</tt>. However, |
| 334 | sometimes it is desirable to make a preprocessor directive pass through to the output |
| 335 | file. For example: |
| 336 | </p> |
| 337 | |
| 338 | <div class="code"> |
| 339 | <pre> |
| 340 | %extend Foo { |
| 341 | void bar() { |
| 342 | #ifdef DEBUG |
| 343 | printf("I'm in bar\n"); |
| 344 | #endif |
| 345 | } |
| 346 | } |
| 347 | </pre> |
| 348 | </div> |
| 349 | |
| 350 | <p> |
| 351 | By default, SWIG will interpret the <tt>#ifdef DEBUG</tt> statement. However, if you really wanted that code |
| 352 | to actually go into the wrapper file, prefix the preprocessor directives with <tt>%</tt> like this: |
| 353 | </p> |
| 354 | |
| 355 | <div class="code"> |
| 356 | <pre> |
| 357 | %extend Foo { |
| 358 | void bar() { |
| 359 | %#ifdef DEBUG |
| 360 | printf("I'm in bar\n"); |
| 361 | %#endif |
| 362 | } |
| 363 | } |
| 364 | </pre> |
| 365 | </div> |
| 366 | |
| 367 | <p> |
| 368 | SWIG will strip the extra <tt>%</tt> and leave the preprocessor directive in the code. |
| 369 | </p> |
| 370 | |
| 371 | <H2><a name="Preprocessor_nn10"></a>7.9 Viewing preprocessor output</H2> |
| 372 | |
| 373 | |
| 374 | <p> |
| 375 | Like many compilers, SWIG supports a <tt>-E</tt> command line option to display the output from the preprocessor. |
| 376 | When the <tt>-E</tt> switch is used, SWIG will not generate any wrappers. |
| 377 | Instead the results after the preprocessor has run are displayed. |
| 378 | This might be useful as an aid to debugging and viewing the results of macro expansions. |
| 379 | </p> |
| 380 | |
| 381 | </body> |
| 382 | </html> |