| Junio C Hamano | 3dac504 | 2007-12-15 08:40:54 | [diff] [blame] | 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" |
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| 257 | </style> |
| 258 | <title>lockfile API</title> |
| 259 | </head> |
| 260 | <body> |
| 261 | <div id="header"> |
| 262 | <h1>lockfile API</h1> |
| 263 | </div> |
| 264 | <div id="preamble"> |
| 265 | <div class="sectionbody"> |
| Junio C Hamano | 5bd1d59 | 2008-01-17 02:43:10 | [diff] [blame] | 266 | <p>The lockfile API serves two purposes:</p> |
| Junio C Hamano | 3dac504 | 2007-12-15 08:40:54 | [diff] [blame] | 267 | <ul> |
| 268 | <li> |
| 269 | <p> |
| Junio C Hamano | 5bd1d59 | 2008-01-17 02:43:10 | [diff] [blame] | 270 | Mutual exclusion. When we write out a new index file, first |
| 271 | we create a new file <tt>$GIT_DIR/index.lock</tt>, write the new |
| 272 | contents into it, and rename it to the final destination |
| 273 | <tt>$GIT_DIR/index</tt>. We try to create the <tt>$GIT_DIR/index.lock</tt> |
| 274 | file with O_EXCL so that we can notice and fail when somebody |
| 275 | else is already trying to update the index file. |
| Junio C Hamano | 3dac504 | 2007-12-15 08:40:54 | [diff] [blame] | 276 | </p> |
| 277 | </li> |
| 278 | <li> |
| 279 | <p> |
| Junio C Hamano | 5bd1d59 | 2008-01-17 02:43:10 | [diff] [blame] | 280 | Automatic cruft removal. After we create the "lock" file, we |
| 281 | may decide to <tt>die()</tt>, and we would want to make sure that we |
| 282 | remove the file that has not been committed to its final |
| 283 | destination. This is done by remembering the lockfiles we |
| 284 | created in a linked list and cleaning them up from an |
| 285 | <tt>atexit(3)</tt> handler. Outstanding lockfiles are also removed |
| 286 | when the program dies on a signal. |
| Junio C Hamano | 3dac504 | 2007-12-15 08:40:54 | [diff] [blame] | 287 | </p> |
| 288 | </li> |
| 289 | </ul> |
| Junio C Hamano | 3dac504 | 2007-12-15 08:40:54 | [diff] [blame] | 290 | </div> |
| 291 | </div> |
| Junio C Hamano | 5bd1d59 | 2008-01-17 02:43:10 | [diff] [blame] | 292 | <h2>The functions</h2> |
| 293 | <div class="sectionbody"> |
| 294 | <dl> |
| 295 | <dt> |
| 296 | hold_lock_file_for_update |
| 297 | </dt> |
| 298 | <dd> |
| 299 | <p> |
| 300 | Take a pointer to <tt>struct lock_file</tt>, the filename of |
| 301 | the final destination (e.g. <tt>$GIT_DIR/index</tt>) and a flag |
| 302 | <tt>die_on_error</tt>. Attempt to create a lockfile for the |
| 303 | destination and return the file descriptor for writing |
| 304 | to the file. If <tt>die_on_error</tt> flag is true, it dies if |
| 305 | a lock is already taken for the file; otherwise it |
| 306 | returns a negative integer to the caller on failure. |
| 307 | </p> |
| 308 | </dd> |
| 309 | <dt> |
| 310 | commit_lock_file |
| 311 | </dt> |
| 312 | <dd> |
| 313 | <p> |
| 314 | Take a pointer to the <tt>struct lock_file</tt> initialized |
| 315 | with an earlier call to <tt>hold_lock_file_for_update()</tt>, |
| 316 | close the file descriptor and rename the lockfile to its |
| 317 | final destination. Returns 0 upon success, a negative |
| 318 | value on failure to close(2) or rename(2). |
| 319 | </p> |
| 320 | </dd> |
| 321 | <dt> |
| 322 | rollback_lock_file |
| 323 | </dt> |
| 324 | <dd> |
| 325 | <p> |
| 326 | Take a pointer to the <tt>struct lock_file</tt> initialized |
| 327 | with an earlier call to <tt>hold_lock_file_for_update()</tt>, |
| 328 | close the file descriptor and remove the lockfile. |
| 329 | </p> |
| 330 | </dd> |
| 331 | <dt> |
| 332 | close_lock_file |
| 333 | </dt> |
| 334 | <dd> |
| 335 | <p> |
| 336 | Take a pointer to the <tt>struct lock_file</tt> initialized |
| 337 | with an earlier call to <tt>hold_lock_file_for_update()</tt>, |
| 338 | and close the file descriptor. Returns 0 upon success, |
| 339 | a negative value on failure to close(2). |
| 340 | </p> |
| 341 | </dd> |
| 342 | </dl> |
| 343 | <p>Because the structure is used in an <tt>atexit(3)</tt> handler, its |
| 344 | storage has to stay throughout the life of the program. It |
| 345 | cannot be an auto variable allocated on the stack.</p> |
| 346 | <p>Call <tt>commit_lock_file()</tt> or <tt>rollback_lock_file()</tt> when you are |
| 347 | done writing to the file descriptor. If you do not call either |
| 348 | and simply <tt>exit(3)</tt> from the program, an <tt>atexit(3)</tt> handler |
| 349 | will close and remove the lockfile.</p> |
| 350 | <p>If you need to close the file descriptor you obtained from |
| 351 | <tt>hold_lock_file_for_update</tt> function yourself, do so by calling |
| 352 | <tt>close_lock_file()</tt>. You should never call <tt>close(2)</tt> yourself! |
| 353 | Otherwise the <tt>struct |
| 354 | lock_file</tt> structure still remembers that the file descriptor |
| 355 | needs to be closed, and a later call to <tt>commit_lock_file()</tt> or |
| 356 | <tt>rollback_lock_file()</tt> will result in duplicate calls to |
| 357 | <tt>close(2)</tt>. Worse yet, if you <tt>close(2)</tt>, open another file |
| 358 | descriptor for completely different purpose, and then call |
| 359 | <tt>commit_lock_file()</tt> or <tt>rollback_lock_file()</tt>, they may close |
| 360 | that unrelated file descriptor.</p> |
| 361 | </div> |
| Junio C Hamano | 3dac504 | 2007-12-15 08:40:54 | [diff] [blame] | 362 | <div id="footer"> |
| 363 | <div id="footer-text"> |
| Junio C Hamano | fce7c7e | 2008-07-02 03:06:38 | [diff] [blame] | 364 | Last updated 02-Jul-2008 03:02:19 UTC |
| Junio C Hamano | 3dac504 | 2007-12-15 08:40:54 | [diff] [blame] | 365 | </div> |
| 366 | </div> |
| 367 | </body> |
| 368 | </html> |
