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| <title>lockfile API</title> | |
| </head> | |
| <body> | |
| <div id="header"> | |
| <h1>lockfile API</h1> | |
| </div> | |
| <div id="preamble"> | |
| <div class="sectionbody"> | |
| <p>The lockfile API serves two purposes:</p> | |
| <ul> | |
| <li> | |
| <p> | |
| Mutual exclusion. When we write out a new index file, first | |
| we create a new file <tt>$GIT_DIR/index.lock</tt>, write the new | |
| contents into it, and rename it to the final destination | |
| <tt>$GIT_DIR/index</tt>. We try to create the <tt>$GIT_DIR/index.lock</tt> | |
| file with O_EXCL so that we can notice and fail when somebody | |
| else is already trying to update the index file. | |
| </p> | |
| </li> | |
| <li> | |
| <p> | |
| Automatic cruft removal. After we create the "lock" file, we | |
| may decide to <tt>die()</tt>, and we would want to make sure that we | |
| remove the file that has not been committed to its final | |
| destination. This is done by remembering the lockfiles we | |
| created in a linked list and cleaning them up from an | |
| <tt>atexit(3)</tt> handler. Outstanding lockfiles are also removed | |
| when the program dies on a signal. | |
| </p> | |
| </li> | |
| </ul> | |
| </div> | |
| </div> | |
| <h2>The functions</h2> | |
| <div class="sectionbody"> | |
| <dl> | |
| <dt> | |
| hold_lock_file_for_update | |
| </dt> | |
| <dd> | |
| <p> | |
| Take a pointer to <tt>struct lock_file</tt>, the filename of | |
| the final destination (e.g. <tt>$GIT_DIR/index</tt>) and a flag | |
| <tt>die_on_error</tt>. Attempt to create a lockfile for the | |
| destination and return the file descriptor for writing | |
| to the file. If <tt>die_on_error</tt> flag is true, it dies if | |
| a lock is already taken for the file; otherwise it | |
| returns a negative integer to the caller on failure. | |
| </p> | |
| </dd> | |
| <dt> | |
| commit_lock_file | |
| </dt> | |
| <dd> | |
| <p> | |
| Take a pointer to the <tt>struct lock_file</tt> initialized | |
| with an earlier call to <tt>hold_lock_file_for_update()</tt>, | |
| close the file descriptor and rename the lockfile to its | |
| final destination. Returns 0 upon success, a negative | |
| value on failure to close(2) or rename(2). | |
| </p> | |
| </dd> | |
| <dt> | |
| rollback_lock_file | |
| </dt> | |
| <dd> | |
| <p> | |
| Take a pointer to the <tt>struct lock_file</tt> initialized | |
| with an earlier call to <tt>hold_lock_file_for_update()</tt>, | |
| close the file descriptor and remove the lockfile. | |
| </p> | |
| </dd> | |
| <dt> | |
| close_lock_file | |
| </dt> | |
| <dd> | |
| <p> | |
| Take a pointer to the <tt>struct lock_file</tt> initialized | |
| with an earlier call to <tt>hold_lock_file_for_update()</tt>, | |
| and close the file descriptor. Returns 0 upon success, | |
| a negative value on failure to close(2). | |
| </p> | |
| </dd> | |
| </dl> | |
| <p>Because the structure is used in an <tt>atexit(3)</tt> handler, its | |
| storage has to stay throughout the life of the program. It | |
| cannot be an auto variable allocated on the stack.</p> | |
| <p>Call <tt>commit_lock_file()</tt> or <tt>rollback_lock_file()</tt> when you are | |
| done writing to the file descriptor. If you do not call either | |
| and simply <tt>exit(3)</tt> from the program, an <tt>atexit(3)</tt> handler | |
| will close and remove the lockfile.</p> | |
| <p>If you need to close the file descriptor you obtained from | |
| <tt>hold_lock_file_for_update</tt> function yourself, do so by calling | |
| <tt>close_lock_file()</tt>. You should never call <tt>close(2)</tt> yourself! | |
| Otherwise the <tt>struct | |
| lock_file</tt> structure still remembers that the file descriptor | |
| needs to be closed, and a later call to <tt>commit_lock_file()</tt> or | |
| <tt>rollback_lock_file()</tt> will result in duplicate calls to | |
| <tt>close(2)</tt>. Worse yet, if you <tt>close(2)</tt>, open another file | |
| descriptor for completely different purpose, and then call | |
| <tt>commit_lock_file()</tt> or <tt>rollback_lock_file()</tt>, they may close | |
| that unrelated file descriptor.</p> | |
| </div> | |
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| Last updated 08-Jun-2008 01:34:43 UTC | |
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