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| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "DBM_Filter 3" |
| 132 | .TH DBM_Filter 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | DBM_Filter \-\- Filter DBM keys/values |
| 135 | .SH "SYNOPSIS" |
| 136 | .IX Header "SYNOPSIS" |
| 137 | .Vb 2 |
| 138 | \& use DBM_Filter ; |
| 139 | \& use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File |
| 140 | .Ve |
| 141 | .PP |
| 142 | .Vb 1 |
| 143 | \& $db = tie %hash, ... |
| 144 | .Ve |
| 145 | .PP |
| 146 | .Vb 2 |
| 147 | \& $db->Filter_Push(Fetch => sub {...}, |
| 148 | \& Store => sub {...}); |
| 149 | .Ve |
| 150 | .PP |
| 151 | .Vb 2 |
| 152 | \& $db->Filter_Push('my_filter1'); |
| 153 | \& $db->Filter_Push('my_filter2', params...); |
| 154 | .Ve |
| 155 | .PP |
| 156 | .Vb 2 |
| 157 | \& $db->Filter_Key_Push(...) ; |
| 158 | \& $db->Filter_Value_Push(...) ; |
| 159 | .Ve |
| 160 | .PP |
| 161 | .Vb 2 |
| 162 | \& $db->Filter_Pop(); |
| 163 | \& $db->Filtered(); |
| 164 | .Ve |
| 165 | .PP |
| 166 | .Vb 1 |
| 167 | \& package DBM_Filter::my_filter1; |
| 168 | .Ve |
| 169 | .PP |
| 170 | .Vb 2 |
| 171 | \& sub Store { ... } |
| 172 | \& sub Fetch { ... } |
| 173 | .Ve |
| 174 | .PP |
| 175 | .Vb 1 |
| 176 | \& 1; |
| 177 | .Ve |
| 178 | .PP |
| 179 | .Vb 1 |
| 180 | \& package DBM_Filter::my_filter2; |
| 181 | .Ve |
| 182 | .PP |
| 183 | .Vb 8 |
| 184 | \& sub Filter |
| 185 | \& { |
| 186 | \& my @opts = @_; |
| 187 | \& ... |
| 188 | \& return ( |
| 189 | \& sub Store { ... }, |
| 190 | \& sub Fetch { ... } ); |
| 191 | \& } |
| 192 | .Ve |
| 193 | .PP |
| 194 | .Vb 1 |
| 195 | \& 1; |
| 196 | .Ve |
| 197 | .SH "DESCRIPTION" |
| 198 | .IX Header "DESCRIPTION" |
| 199 | This module provides an interface that allows filters to be applied |
| 200 | to tied Hashes associated with \s-1DBM\s0 files. It builds on the \s-1DBM\s0 Filter |
| 201 | hooks that are present in all the *DB*_File modules included with the |
| 202 | standard Perl source distribution from version 5.6.1 onwards. In addition |
| 203 | to the *DB*_File modules distributed with Perl, the BerkeleyDB module, |
| 204 | available on \s-1CPAN\s0, supports the \s-1DBM\s0 Filter hooks. See perldbmfilter |
| 205 | for more details on the \s-1DBM\s0 Filter hooks. |
| 206 | .SH "What is a DBM Filter?" |
| 207 | .IX Header "What is a DBM Filter?" |
| 208 | A \s-1DBM\s0 Filter allows the keys and/or values in a tied hash to be modified |
| 209 | by some user-defined code just before it is written to the \s-1DBM\s0 file and |
| 210 | just after it is read back from the \s-1DBM\s0 file. For example, this snippet |
| 211 | of code |
| 212 | .PP |
| 213 | .Vb 1 |
| 214 | \& $some_hash{"abc"} = 42; |
| 215 | .Ve |
| 216 | .PP |
| 217 | could potentially trigger two filters, one for the writing of the key |
| 218 | \&\*(L"abc\*(R" and another for writing the value 42. Similarly, this snippet |
| 219 | .PP |
| 220 | .Vb 1 |
| 221 | \& my ($key, $value) = each %some_hash |
| 222 | .Ve |
| 223 | .PP |
| 224 | will trigger two filters, one for the reading of the key and one for |
| 225 | the reading of the value. |
| 226 | .PP |
| 227 | Like the existing \s-1DBM\s0 Filter functionality, this module arranges for the |
| 228 | \&\f(CW$_\fR variable to be populated with the key or value that a filter will |
| 229 | check. This usually means that most \s-1DBM\s0 filters tend to be very short. |
| 230 | .Sh "So what's new?" |
| 231 | .IX Subsection "So what's new?" |
| 232 | The main enhancements over the standard \s-1DBM\s0 Filter hooks are: |
| 233 | .IP "\(bu" 4 |
| 234 | A cleaner interface. |
| 235 | .IP "\(bu" 4 |
| 236 | The ability to easily apply multiple filters to a single \s-1DBM\s0 file. |
| 237 | .IP "\(bu" 4 |
| 238 | The ability to create \*(L"canned\*(R" filters. These allow commonly used filters |
| 239 | to be packaged into a stand-alone module. |
| 240 | .SH "METHODS" |
| 241 | .IX Header "METHODS" |
| 242 | This module will arrange for the following methods to be available via |
| 243 | the object returned from the \f(CW\*(C`tie\*(C'\fR call. |
| 244 | .Sh "$db\->\fIFilter_Push()\fP" |
| 245 | .IX Subsection "$db->Filter_Push()" |
| 246 | .Sh "$db\->\fIFilter_Key_Push()\fP" |
| 247 | .IX Subsection "$db->Filter_Key_Push()" |
| 248 | .Sh "$db\->\fIFilter_Value_Push()\fP" |
| 249 | .IX Subsection "$db->Filter_Value_Push()" |
| 250 | Add a filter to filter stack for the database, \f(CW$db\fR. The three formats |
| 251 | vary only in whether they apply to the \s-1DBM\s0 key, the \s-1DBM\s0 value or both. |
| 252 | .IP "Filter_Push" 5 |
| 253 | .IX Item "Filter_Push" |
| 254 | The filter is applied to \fIboth\fR keys and values. |
| 255 | .IP "Filter_Key_Push" 5 |
| 256 | .IX Item "Filter_Key_Push" |
| 257 | The filter is applied to the key \fIonly\fR. |
| 258 | .IP "Filter_Value_Push" 5 |
| 259 | .IX Item "Filter_Value_Push" |
| 260 | The filter is applied to the value \fIonly\fR. |
| 261 | .Sh "$db\->\fIFilter_Pop()\fP" |
| 262 | .IX Subsection "$db->Filter_Pop()" |
| 263 | Removes the last filter that was applied to the \s-1DBM\s0 file associated with |
| 264 | \&\f(CW$db\fR, if present. |
| 265 | .Sh "$db\->\fIFiltered()\fP" |
| 266 | .IX Subsection "$db->Filtered()" |
| 267 | Returns \s-1TRUE\s0 if there are any filters applied to the \s-1DBM\s0 associated |
| 268 | with \f(CW$db\fR. Otherwise returns \s-1FALSE\s0. |
| 269 | .SH "Writing a Filter" |
| 270 | .IX Header "Writing a Filter" |
| 271 | Filters can be created in two main ways |
| 272 | .Sh "Immediate Filters" |
| 273 | .IX Subsection "Immediate Filters" |
| 274 | An immediate filter allows you to specify the filter code to be used |
| 275 | at the point where the filter is applied to a dbm. In this mode the |
| 276 | Filter_*_Push methods expects to receive exactly two parameters. |
| 277 | .PP |
| 278 | .Vb 3 |
| 279 | \& my $db = tie %hash, 'SDBM_File', ... |
| 280 | \& $db->Filter_Push( Store => sub { }, |
| 281 | \& Fetch => sub { }); |
| 282 | .Ve |
| 283 | .PP |
| 284 | The code reference associated with \f(CW\*(C`Store\*(C'\fR will be called before any |
| 285 | key/value is written to the database and the code reference associated |
| 286 | with \f(CW\*(C`Fetch\*(C'\fR will be called after any key/value is read from the |
| 287 | database. |
| 288 | .PP |
| 289 | For example, here is a sample filter that adds a trailing \s-1NULL\s0 character |
| 290 | to all strings before they are written to the \s-1DBM\s0 file, and removes the |
| 291 | trailing \s-1NULL\s0 when they are read from the \s-1DBM\s0 file |
| 292 | .PP |
| 293 | .Vb 3 |
| 294 | \& my $db = tie %hash, 'SDBM_File', ... |
| 295 | \& $db->Filter_Push( Store => sub { $_ .= "\ex00" ; }, |
| 296 | \& Fetch => sub { s/\ex00$// ; }); |
| 297 | .Ve |
| 298 | .PP |
| 299 | Points to note: |
| 300 | .IP "1." 5 |
| 301 | Both the Store and Fetch filters manipulate \f(CW$_\fR. |
| 302 | .Sh "Canned Filters" |
| 303 | .IX Subsection "Canned Filters" |
| 304 | Immediate filters are useful for one-off situations. For more generic |
| 305 | problems it can be useful to package the filter up in its own module. |
| 306 | .PP |
| 307 | The usage is for a canned filter is: |
| 308 | .PP |
| 309 | .Vb 1 |
| 310 | \& $db->Filter_Push("name", params) |
| 311 | .Ve |
| 312 | .PP |
| 313 | where |
| 314 | .ie n .IP """name""" 5 |
| 315 | .el .IP "``name''" 5 |
| 316 | .IX Item "name" |
| 317 | is the name of the module to load. If the string specified does not |
| 318 | contain the package separator characters \*(L"::\*(R", it is assumed to refer to |
| 319 | the full module name \*(L"DBM_Filter::name\*(R". This means that the full names |
| 320 | for canned filters, \*(L"null\*(R" and \*(L"utf8\*(R", included with this module are: |
| 321 | .Sp |
| 322 | .Vb 2 |
| 323 | \& DBM_Filter::null |
| 324 | \& DBM_Filter::utf8 |
| 325 | .Ve |
| 326 | .IP "params" 5 |
| 327 | .IX Item "params" |
| 328 | any optional parameters that need to be sent to the filter. See the |
| 329 | encode filter for an example of a module that uses parameters. |
| 330 | .PP |
| 331 | The module that implements the canned filter can take one of two |
| 332 | forms. Here is a template for the first |
| 333 | .PP |
| 334 | .Vb 1 |
| 335 | \& package DBM_Filter::null ; |
| 336 | .Ve |
| 337 | .PP |
| 338 | .Vb 2 |
| 339 | \& use strict; |
| 340 | \& use warnings; |
| 341 | .Ve |
| 342 | .PP |
| 343 | .Vb 4 |
| 344 | \& sub Store |
| 345 | \& { |
| 346 | \& # store code here |
| 347 | \& } |
| 348 | .Ve |
| 349 | .PP |
| 350 | .Vb 4 |
| 351 | \& sub Fetch |
| 352 | \& { |
| 353 | \& # fetch code here |
| 354 | \& } |
| 355 | .Ve |
| 356 | .PP |
| 357 | .Vb 1 |
| 358 | \& 1; |
| 359 | .Ve |
| 360 | .PP |
| 361 | Notes: |
| 362 | .IP "1." 5 |
| 363 | The package name uses the \f(CW\*(C`DBM_Filter::\*(C'\fR prefix. |
| 364 | .IP "2." 5 |
| 365 | The module \fImust\fR have both a Store and a Fetch method. If only one is |
| 366 | present, or neither are present, a fatal error will be thrown. |
| 367 | .PP |
| 368 | The second form allows the filter to hold state information using a |
| 369 | closure, thus: |
| 370 | .PP |
| 371 | .Vb 1 |
| 372 | \& package DBM_Filter::encoding ; |
| 373 | .Ve |
| 374 | .PP |
| 375 | .Vb 2 |
| 376 | \& use strict; |
| 377 | \& use warnings; |
| 378 | .Ve |
| 379 | .PP |
| 380 | .Vb 3 |
| 381 | \& sub Filter |
| 382 | \& { |
| 383 | \& my @params = @_ ; |
| 384 | .Ve |
| 385 | .PP |
| 386 | .Vb 6 |
| 387 | \& ... |
| 388 | \& return { |
| 389 | \& Store => sub { $_ = $encoding->encode($_) }, |
| 390 | \& Fetch => sub { $_ = $encoding->decode($_) } |
| 391 | \& } ; |
| 392 | \& } |
| 393 | .Ve |
| 394 | .PP |
| 395 | .Vb 1 |
| 396 | \& 1; |
| 397 | .Ve |
| 398 | .PP |
| 399 | In this instance the \*(L"Store\*(R" and \*(L"Fetch\*(R" methods are encapsulated inside a |
| 400 | \&\*(L"Filter\*(R" method. |
| 401 | .SH "Filters Included" |
| 402 | .IX Header "Filters Included" |
| 403 | A number of canned filers are provided with this module. They cover a |
| 404 | number of the main areas that filters are needed when interfacing with |
| 405 | \&\s-1DBM\s0 files. They also act as templates for your own filters. |
| 406 | .PP |
| 407 | The filter included are: |
| 408 | .IP "* utf8" 5 |
| 409 | .IX Item "utf8" |
| 410 | This module will ensure that all data written to the \s-1DBM\s0 will be encoded |
| 411 | in \s-1UTF\-8\s0. |
| 412 | .Sp |
| 413 | This module needs the Encode module. |
| 414 | .IP "* encode" 5 |
| 415 | .IX Item "encode" |
| 416 | Allows you to choose the character encoding will be store in the \s-1DBM\s0 file. |
| 417 | .IP "* compress" 5 |
| 418 | .IX Item "compress" |
| 419 | This filter will compress all data before it is written to the database |
| 420 | and uncompressed it on reading. |
| 421 | .Sp |
| 422 | This module needs Compress::Zlib. |
| 423 | .IP "* int32" 5 |
| 424 | .IX Item "int32" |
| 425 | This module is used when interoperating with a C/\*(C+ application that |
| 426 | uses a C int as either the key and/or value in the \s-1DBM\s0 file. |
| 427 | .IP "* null" 5 |
| 428 | .IX Item "null" |
| 429 | This module ensures that all data written to the \s-1DBM\s0 file is null |
| 430 | terminated. This is useful when you have a perl script that needs |
| 431 | to interoperate with a \s-1DBM\s0 file that a C program also uses. A fairly |
| 432 | common issue is for the C application to include the terminating null |
| 433 | in a string when it writes to the \s-1DBM\s0 file. This filter will ensure that |
| 434 | all data written to the \s-1DBM\s0 file can be read by the C application. |
| 435 | .SH "NOTES" |
| 436 | .IX Header "NOTES" |
| 437 | .Sh "Maintain Round Trip Integrity" |
| 438 | .IX Subsection "Maintain Round Trip Integrity" |
| 439 | When writing a \s-1DBM\s0 filter it is \fIvery\fR important to ensure that it is |
| 440 | possible to retrieve all data that you have written when the \s-1DBM\s0 filter |
| 441 | is in place. In practice, this means that whatever transformation is |
| 442 | applied to the data in the Store method, the \fIexact\fR inverse operation |
| 443 | should be applied in the Fetch method. |
| 444 | .PP |
| 445 | If you don't provide an exact inverse transformation, you will find that |
| 446 | code like this will not behave as you expect. |
| 447 | .PP |
| 448 | .Vb 4 |
| 449 | \& while (my ($k, $v) = each %hash) |
| 450 | \& { |
| 451 | \& ... |
| 452 | \& } |
| 453 | .Ve |
| 454 | .PP |
| 455 | Depending on the transformation, you will find that one or more of the |
| 456 | following will happen |
| 457 | .IP "1" 5 |
| 458 | .IX Item "1" |
| 459 | The loop will never terminate. |
| 460 | .IP "2" 5 |
| 461 | .IX Item "2" |
| 462 | Too few records will be retrieved. |
| 463 | .IP "3" 5 |
| 464 | .IX Item "3" |
| 465 | Too many will be retrieved. |
| 466 | .IP "4" 5 |
| 467 | .IX Item "4" |
| 468 | The loop will do the right thing for a while, but it will unexpectedly fail. |
| 469 | .Sh "Don't mix filtered & non-filtered data in the same database file." |
| 470 | .IX Subsection "Don't mix filtered & non-filtered data in the same database file." |
| 471 | This is just a restatement of the previous section. Unless you are |
| 472 | completely certain you know what you are doing, avoid mixing filtered & |
| 473 | non-filtered data. |
| 474 | .SH "EXAMPLE" |
| 475 | .IX Header "EXAMPLE" |
| 476 | Say you need to interoperate with a legacy C application that stores |
| 477 | keys as C ints and the values and null terminated \s-1UTF\-8\s0 strings. Here |
| 478 | is how you would set that up |
| 479 | .PP |
| 480 | .Vb 1 |
| 481 | \& my $db = tie %hash, 'SDBM_File', ... |
| 482 | .Ve |
| 483 | .PP |
| 484 | .Vb 1 |
| 485 | \& $db->Filter_Key_Push('int32') ; |
| 486 | .Ve |
| 487 | .PP |
| 488 | .Vb 2 |
| 489 | \& $db->Filter_Value_Push('utf8'); |
| 490 | \& $db->Filter_Value_Push('null'); |
| 491 | .Ve |
| 492 | .SH "SEE ALSO" |
| 493 | .IX Header "SEE ALSO" |
| 494 | <DB_File>, GDBM_File, NDBM_File, ODBM_File, SDBM_File, perldbmfilter |
| 495 | .SH "AUTHOR" |
| 496 | .IX Header "AUTHOR" |
| 497 | Paul Marquess <pmqs@cpan.org> |