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authorBill Wendling <wendling@ncsa.uiuc.edu>2000-10-27 15:58:29 (GMT)
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[svn-r2740] Purpose:
Documentation of TS Library Description: This is the document Chee Wai wrote up about the thread safe version of the HDF5 library. I just put it in HTML format and checked it in... Platforms tested: Netscrape
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
+ "http://www.w3.org/TR/REC-html40/loose.dtd">
+<html lang="en-US">
+<head>
+ <title>Thread Safe Library</title>
+</head>
+
+<body bgcolor=#ffffff>
+
+<center><h1>HDF5 Thread Safe library</h1></center>
+
+<p>
+
+<h1>1. Library header files and conditional compilation</h1>
+
+<p>
+The following code is placed at the beginning of H5private.h:
+</p>
+
+<blockquote>
+ <pre>
+ #ifdef H5_HAVE_THREADSAFE
+ #include <pthread.h>
+ #endif
+ </pre>
+</blockquote>
+
+<p>
+<code>H5_HAVE_THREADSAFE</code> is defined when the HDF-5 library is
+compiled with the --enable-threadsafe configuration option. In general,
+code for the non-threadsafe version of HDF-5 library are placed within
+the <code>#else</code> part of the conditional compilation. The exception
+to this rule are the changes to the <code>FUNC_ENTER</code> (in
+H5private.h), <code>HRETURN</code> and <code>HRETURN_ERROR</code> (in
+H5Eprivate.h) macros (see section 3.2).
+</p>
+
+
+<h1>2. Global variables/structures</h1>
+
+<h2>2.1 Global library initialization variable</h2>
+
+<p>
+In the threadsafe implementation, the global library initialization
+variable <code>H5_libinit_g</code> is changed to a global structure
+consisting of the variable with its associated lock (locks are explained
+in section 4.1):
+</p>
+
+<blockquote>
+ <pre>
+ hbool_t H5_libinit_g = FALSE;
+ </pre>
+</blockquote>
+
+<p>
+becomes
+</p>
+
+<blockquote>
+ <pre>
+ H5_api_t H5_g;
+ </pre>
+</blockquote>
+
+<p>
+where <code>H5_api_t</code> is
+</p>
+
+<blockquote>
+ <pre>
+ typedef struct H5_api_struct {
+ H5_mutex_t init_lock; /* API entrance mutex */
+ hbool_t H5_libinit_g;
+ } H5_api_t;
+ </pre>
+</blockquote>
+
+<p>
+All former references to <code>H5_libinit_g</code> in the library are now
+made using the macro <code>H5_INIT_GLOBAL</code>. If the threadsafe
+library is to be used, the macro is set to <code>H5_g.H5_libinit_g</code>
+instead.
+</p>
+
+<h2>2.2 Global serialization variable</h2>
+
+<p>
+A new global boolean variable <code>H5_allow_concurrent_g</code> is used
+to determine if multiple threads are allowed to an API call
+simultaneously. This is set to <code>FALSE</code>.
+</p>
+
+<p>
+All APIs that are allowed to do so have their own local variable that
+shadows the global variable and is set to <code>TRUE</code>. In phase 1,
+no such APIs exist.
+</p>
+
+<p>
+It is defined in <code>H5.c</code> as follows:
+</p>
+
+<blockquote>
+ <pre>
+ hbool_t H5_allow_concurrent_g = FALSE;
+ </pre>
+</blockquote>
+
+<h2>2.3 Global thread initialization variable</h2>
+
+<p>
+The global variable <code>H5_first_init_g</code> of type
+<code>pthread_once_t</code> is used to allow only the first thread in the
+application process to call an initialization function using
+<code>pthread_once</code>. All subsequent calls to
+<code>pthread_once</code> by any thread are disregarded.
+</p>
+
+<p>
+The call sets up the mutex in the global structure <code>H5_g</code> (see
+section 3.1) via an initialization function
+<code>H5_first_thread_init</code>. The first thread initialization
+function is described in section 4.2.
+</p>
+
+<p>
+<code>H5_first_init_g</code> is defined in <code>H5.c</code> as follows:
+</p>
+
+<blockquote>
+ <pre>
+ pthread_once_t H5_first_init_g = PTHREAD_ONCE_INIT;
+ </pre>
+</blockquote>
+
+<h2>2.4 Global key for per-thread error stacks</h2>
+
+<p>
+A global pthread-managed key <code>H5_errstk_key_g</code> is used to
+allow pthreads to maintain a separate error stack (of type
+<code>H5E_t</code>) for each thread. This is defined in <code>H5.c</code>
+as:
+</p>
+
+<blockquote>
+ <pre>
+ pthread_key_t H5_errstk_key_g;
+ </pre>
+</blockquote>
+
+<p>
+Error stack management is described in section 4.3.
+</p>
+
+<h2>2.5 Global structure and key for thread cancellation prevention</h2>
+
+<p>
+We need to preserve the thread cancellation status of each thread
+individually by using a key <code>H5_cancel_key_g</code>. The status is
+preserved using a structure (of type <code>H5_cancel_t</code>) which
+maintains the cancellability state of the thread before it entered the
+library and a count (which works very much like the recursive lock
+counter) which keeps track of the number of API calls the thread makes
+within the library.
+</p>
+
+<p>
+The structure is defined in <code>H5private.h</code> as:
+</p>
+
+<blockquote>
+ <pre>
+ /* cancelability structure */
+ typedef struct H5_cancel_struct {
+ int previous_state;
+ unsigned int cancel_count;
+ } H5_cancel_t;
+ </pre>
+</blockquote>
+
+<p>
+Thread cancellation is described in section 4.4.
+</p>
+
+
+<h1>3. Changes to Macro expansions</h1>
+
+<h2>3.1 Changes to FUNC_ENTER</h2>
+
+<p>
+The <code>FUNC_ENTER</code> macro is now extended to include macro calls
+to initialize first threads, disable cancellability and wraps a lock
+operation around the checking of the global initialization flag. It
+should be noted that the cancellability should be disabled before
+acquiring the lock on the library. Doing so otherwise would allow the
+possibility that the thread be cancelled just after it has acquired the
+lock on the library and in that scenario, if the cleanup routines are not
+properly set, the library would be permanently locked out.
+</p>
+
+<p>
+The additional macro code and new macro definitions can be found in
+Appendix E.1 to E.5. The changes are made in <code>H5private.h</code>.
+</p>
+
+<h2>3.2 Changes to HRETURN and HRETURN_ERROR</h2>
+
+<p>
+The <code>HRETURN</code> and <code>HRETURN_ERROR</code> macros are the
+counterparts to the <code>FUNC_ENTER</code> macro described in section
+3.1. <code>FUNC_LEAVE</code> makes a macro call to <code>HRETURN</code>,
+so it is also covered here.
+</p>
+
+<p>
+The basic changes to these two macros involve adding macro calls to call
+an unlock operation and re-enable cancellability if necessary. It should
+be noted that the cancellability should be re-enabled only after the
+thread has released the lock to the library. The consequence of doing
+otherwise would be similar to that described in section 3.1.
+</p>
+
+<p>
+The additional macro code and new macro definitions can be found in
+Appendix E.9 to E.9. The changes are made in <code>H5Eprivate.h</code>.
+</p>
+
+<h1>4. Implementation of threadsafe functionality</h1>
+
+<h2>4.1 Recursive Locks</h2>
+
+<p>
+A recursive mutex lock m allows a thread t1 to successfully lock m more
+than once without blocking t1. Another thread t2 will block if t2 tries
+to lock m while t1 holds the lock to m. If t1 makes k lock calls on m,
+then it also needs to make k unlock calls on m before it releases the
+lock.
+</p>
+
+<p>
+Our implementation of recursive locks is built on top of a pthread mutex
+lock (which is not recursive). It makes use of a pthread condition
+variable to have unsuccessful threads wait on the mutex. Waiting threads
+are awaken by a signal from the final unlock call made by the thread
+holding the lock.
+</p>
+
+<p>
+Recursive locks are defined to be the following type
+(<code>H5private.h</code>):
+</p>
+
+<blockquote>
+ <pre>
+ typedef struct H5_mutex_struct {
+ pthread_t owner_thread; /* current lock owner */
+ pthread_mutex_t atomic_lock; /* lock for atomicity of new mechanism */
+ pthread_cond_t cond_var; /* condition variable */
+ unsigned int lock_count;
+ } H5_mutex_t;
+ </pre>
+</blockquote>
+
+<p>
+Detailed implementation code can be found in Appendix A. The
+implementation changes are made in <code>H5TS.c</code>.
+</p>
+
+<h2>4.2 First thread initialization</h2>
+
+<p>
+Because the mutex lock associated with a recursive lock cannot be
+statically initialized, a mechanism is required to initialize the
+recursive lock associated with <code>H5_g</code> so that it can be used
+for the first time.
+</p>
+
+<p>
+The pthreads library allows this through the pthread_once call which as
+described in section 3.3 allows only the first thread accessing the
+library in an application to initialize <code>H5_g</code>.
+</p>
+
+<p>
+In addition to initializing <code>H5_g</code>, it also initializes the
+key (see section 3.4) for use with per-thread error stacks (see section
+4.3).
+</p>
+
+<p>
+The first thread initialization mechanism is implemented as the function
+call <code>H5_first_thread_init()</code> in <code>H5TS.c</code>. This is
+described in appendix B.
+</p>
+
+<h2>4.3 Per-thread error stack management</h2>
+
+<p>
+Pthreads allows individual threads to access dynamic and persistent
+per-thread data through the use of keys. Each key is associated with
+a table that maps threads to data items. Keys can be initialized by
+<code>pthread_key_create()</code> in pthreads (see sections 3.4 and 4.2).
+Per-thread data items are accessed using a key through the
+<code>pthread_getspecific()</code> and <code>pthread_setspecific()</code>
+calls to read and write to the association table respectively.
+</p>
+
+<p>
+Per-thread error stacks are accessed through the key
+<code>H5_errstk_key_g</code> which is initialized by the first thread
+initialization call (see section 4.2).
+</p>
+
+<p>
+In the non-threadsafe version of the library, there is a global stack
+variable <code>H5E_stack_g[1]</code> which is no longer defined in the
+threadsafe version. At the same time, the macro call to gain access to
+the error stack <code>H5E_get_my_stack</code> is changed from:
+</p>
+
+<blockquote>
+ <pre>
+ #define H5E_get_my_stack() (H5E_stack_g+0)
+ </pre>
+</blockquote>
+
+<p>
+to:
+</p>
+
+<blockquote>
+ <pre>
+ #define H5E_get_my_stack() H5E_get_stack()
+ </pre>
+</blockquote>
+
+<p>
+where <code>H5E_get_stack()</code> is a surrogate function that does the
+following operations:
+</p>
+
+<ol>
+ <li>if a thread is attempting to get an error stack for the first
+ time, the error stack is dynamically allocated for the thread and
+ associated with <code>H5_errstk_key_g</code> using
+ <code>pthread_setspecific()</code>. The way we detect if it is the
+ first time is through <code>pthread_getspecific()</code> which
+ returns <code>NULL</code> if no previous value is associated with
+ the thread using the key.</li>
+
+ <li>if <code>pthread_getspecific()</code> returns a non-null value,
+ then that is the pointer to the error stack associated with the
+ thread and the stack can be used as usual.</li>
+</ol>
+
+<p>
+A final change to the error reporting routines is as follows; the current
+implementation reports errors to always be detected at thread 0. In the
+threadsafe implementation, this is changed to report the number returned
+by a call to <code>pthread_self()</code>.
+</p>
+
+<p>
+The change in code (reflected in <code>H5Eprint</code> of file
+<code>H5E.c</code>) is as follows:
+</p>
+
+<blockquote>
+ <pre>
+ #ifdef H5_HAVE_THREADSAFE
+ fprintf (stream, "HDF5-DIAG: Error detected in thread %d."
+ ,pthread_self());
+ #else
+ fprintf (stream, "HDF5-DIAG: Error detected in thread 0.");
+ #endif
+ </pre>
+</blockquote>
+
+<p>
+Code for <code>H5E_get_stack()</code> can be found in Appendix C. All the
+above changes were made in <code>H5E.c</code>.
+</p>
+
+<h2>4.4 Thread Cancellation safety</h2>
+
+<p>
+To prevent thread cancellations from killing a thread while it is in the
+library, we maintain per-thread information about the cancellability
+status of the thread before it entered the library so that we can restore
+that same status when the thread leaves the library.
+</p>
+
+<p>
+By <i>enter</i> and <i>leave</i> the library, we mean the points when a
+thread makes an API call from a user application and the time that API
+call returns. Other API or callback function calls made from within that
+API call are considered <i>within</i> the library.
+</p>
+
+<p>
+Because other API calls may be made from within the first API call, we
+need to maintain a counter to determine which was the first and
+correspondingly the last return.
+</p>
+
+<p>
+When a thread makes an API call, the macro <code>H5_API_SET_CANCEL</code>
+calls the worker function <code>H5_cancel_count_inc()</code> which does
+the following:
+</p>
+
+<ol>
+ <li>if this is the first time the thread has entered the library,
+ a new cancellability structure needs to be assigned to it.</li>
+ <li>if the thread is already within the library when the API call is
+ made, then cancel_count is simply incremented. Otherwise, we set
+ the cancellability state to <code>PTHREAD_CANCEL_DISABLE</code>
+ while storing the previous state into the cancellability structure.
+ <code>cancel_count</code> is also incremented in this case.</li>
+</ol>
+
+<p>
+When a thread leaves an API call, the macro
+<code>H5_API_UNSET_CANCEL</code> calls the worker function
+<code>H5_cancel_count_dec()</code> which does the following:
+</p>
+
+<ol>
+ <li>if <code>cancel_count</code> is greater than 1, indicating that the
+ thread is not yet about to leave the library, then
+ <code>cancel_count</code> is simply decremented.</li>
+ <li>otherwise, we reset the cancellability state back to its original
+ state before it entered the library and decrement the count (back
+ to zero).</li>
+</ol>
+
+<p>
+<code>H5_cancel_count_inc</code> and <code>H5_cancel_count_dec</code> are
+described in Appendix D and may be found in <code>H5TS.c</code>.
+</p>
+
+<h1>5. Test programs</h1>
+
+<p>
+Except where stated, all tests involve 16 simultaneous threads that make
+use of HDF-5 API calls without any explicit synchronization typically
+required in a non-threadsafe environment.
+</p>
+
+<h2>5.1 Data set create and write</h2>
+
+<p>
+The test program sets up 16 threads to simultaneously create 16
+different datasets named from <i>zero</i> to <i>fifteen</i> for a single
+file and then writing an integer value into that dataset equal to the
+dataset's named value.
+</p>
+
+<p>
+The main thread would join with all 16 threads and attempt to match the
+resulting HDF-5 file with expected results - that each dataset contains
+the correct value (0 for <i>zero</i>, 1 for <i>one</i> etc ...) and all
+datasets were correctly created.
+</p>
+
+<p>
+The test is implemented in the file <code>ttsafe_dcreate.c</code>.
+</p>
+
+<h2>5.2 Test on error stack</h2>
+
+<p>
+The error stack test is one in which 16 threads simultaneously try to
+create datasets with the same name. The result, when properly serialized,
+should be equivalent to 16 attempts to create the dataset with the same
+name.
+</p>
+
+<p>
+The error stack implementation runs correctly if it reports 15 instances
+of the dataset name conflict error and finally generates a correct HDF-5
+containing that single dataset. Each thread should report its own stack
+of errors with a thread number associated with it.
+</p>
+
+<p>
+The test is implemented in the file <code>ttsafe_error.c</code>.
+</p>
+
+<h2>5.3 Test on cancellation safety</h2>
+
+<p>
+The main idea in thread cancellation safety is as follows; a child thread
+is spawned to create and write to a dataset. Following that, it makes a
+<code>H5Diterate</code> call on that dataset which activates a callback
+function.
+</p>
+
+<p>
+A deliberate barrier is invoked at the callback function which waits for
+both the main and child thread to arrive at that point. After that
+happens, the main thread proceeds to make a thread cancel call on the
+child thread while the latter sleeps for 3 seconds before proceeding to
+write a new value to the dataset.
+</p>
+
+<p>
+After the iterate call, the child thread logically proceeds to wait
+another 3 seconds before writing another newer value to the dataset.
+</p>
+
+<p>
+The test is correct if the main thread manages to read the second value
+at the end of the test. This means that cancellation did not take place
+until the end of the iteration call despite of the 3 second wait within
+the iteration callback and the extra dataset write operation.
+Furthermore, the cancellation should occur before the child can proceed
+to write the last value into the dataset.
+</p>
+
+<h2>5.4 Test on attribute creation</h2>
+
+<p>
+A main thread makes 16 threaded calls to <code>H5Acreate</code> with a
+generated name for each attribute. Sixteen attributes should be created
+for the single dataset in random (chronological) order and receive values
+depending on its generated attribute name (e.g. <i>attrib010</i> would
+receive the value 10).
+</p>
+
+<p>
+After joining with all child threads, the main thread proceeds to read
+each attribute by generated name to see if the value tallies. Failure is
+detected if the attribute name does not exist (meaning they were never
+created) or if the wrong values were read back.
+</p>
+
+<h1>A. Recursive Lock implementation code</h1>
+
+<blockquote>
+ <pre>
+ void H5_mutex_init(H5_mutex_t *H5_mutex)
+ {
+ H5_mutex-&gt;owner_thread = NULL;
+ pthread_mutex_init(&amp;H5_mutex-&gt;atomic_lock, NULL);
+ pthread_cond_init(&amp;H5_mutex-&gt;cond_var, NULL);
+ H5_mutex-&gt;lock_count = 0;
+ }
+
+ void H5_mutex_lock(H5_mutex_t *H5_mutex)
+ {
+ pthread_mutex_lock(&amp;H5_mutex-&gt;atomic_lock);
+
+ if (pthread_equal(pthread_self(), H5_mutex-&gt;owner_thread)) {
+ /* already owned by self - increment count */
+ H5_mutex-&gt;lock_count++;
+ } else {
+ if (H5_mutex-&gt;owner_thread == NULL) {
+ /* no one else has locked it - set owner and grab lock */
+ H5_mutex-&gt;owner_thread = pthread_self();
+ H5_mutex-&gt;lock_count = 1;
+ } else {
+ /* if already locked by someone else */
+ while (1) {
+ pthread_cond_wait(&amp;H5_mutex-&gt;cond_var, &amp;H5_mutex-&gt;atomic_lock);
+
+ if (H5_mutex-&gt;owner_thread == NULL) {
+ H5_mutex-&gt;owner_thread = pthread_self();
+ H5_mutex-&gt;lock_count = 1;
+ break;
+ } /* else do nothing and loop back to wait on condition*/
+ }
+ }
+ }
+
+ pthread_mutex_unlock(&amp;H5_mutex-&gt;atomic_lock);
+ }
+
+ void H5_mutex_unlock(H5_mutex_t *H5_mutex)
+ {
+ pthread_mutex_lock(&amp;H5_mutex-&gt;atomic_lock);
+ H5_mutex-&gt;lock_count--;
+
+ if (H5_mutex-&gt;lock_count == 0) {
+ H5_mutex-&gt;owner_thread = NULL;
+ pthread_cond_signal(&amp;H5_mutex-&gt;cond_var);
+ }
+ pthread_mutex_unlock(&amp;H5_mutex-&gt;atomic_lock);
+ }
+ </pre>
+</blockquote>
+
+<h1>B. First thread initialization</h1>
+
+<blockquote>
+ <pre>
+ void H5_first_thread_init(void)
+ {
+ /* initialize global API mutex lock */
+ H5_g.H5_libinit_g = FALSE;
+ H5_g.init_lock.owner_thread = NULL;
+ pthread_mutex_init(&amp;H5_g.init_lock.atomic_lock, NULL);
+ pthread_cond_init(&amp;H5_g.init_lock.cond_var, NULL);
+ H5_g.init_lock.lock_count = 0;
+
+ /* initialize key for thread-specific error stacks */
+ pthread_key_create(&amp;H5_errstk_key_g, NULL);
+
+ /* initialize key for thread cancellability mechanism */
+ pthread_key_create(&amp;H5_cancel_key_g, NULL);
+ }
+ </pre>
+</blockquote>
+
+
+<h1>C. Per-thread error stack acquisition</h1>
+
+<blockquote>
+ <pre>
+ H5E_t *H5E_get_stack(void)
+ {
+ H5E_t *estack;
+
+ if (estack = pthread_getspecific(H5_errstk_key_g)) {
+ return estack;
+ } else {
+ /* no associated value with current thread - create one */
+ estack = (H5E_t *)malloc(sizeof(H5E_t));
+ pthread_setspecific(H5_errstk_key_g, (void *)estack);
+ return estack;
+ }
+ }
+ </pre>
+</blockquote>
+
+<h1>D. Thread cancellation mechanisms</h1>
+
+<blockquote>
+ <pre>
+ void H5_cancel_count_inc(void)
+ {
+ H5_cancel_t *cancel_counter;
+
+ if (cancel_counter = pthread_getspecific(H5_cancel_key_g)) {
+ /* do nothing here */
+ } else {
+ /*
+ * first time thread calls library - create new counter and
+ * associate with key
+ */
+ cancel_counter = (H5_cancel_t *)malloc(sizeof(H5_cancel_t));
+ cancel_counter-&gt;cancel_count = 0;
+ pthread_setspecific(H5_cancel_key_g, (void *)cancel_counter);
+ }
+
+ if (cancel_counter-&gt;cancel_count == 0) {
+ /* thread entering library */
+ pthread_setcancelstate(PTHREAD_CANCEL_DISABLE,
+ &amp;(cancel_counter-&gt;previous_state));
+ }
+
+ cancel_counter-&gt;cancel_count++;
+ }
+
+ void H5_cancel_count_dec(void)
+ {
+ H5_cancel_t *cancel_counter = pthread_getspecific(H5_cancel_key_g);
+
+ if (cancel_counter-&gt;cancel_count == 1)
+ pthread_setcancelstate(cancel_counter-&gt;previous_state, NULL);
+
+ cancel_counter-&gt;cancel_count--;
+ }
+ </pre>
+</blockquote>
+
+<h1>E. Macro expansion codes</h1>
+
+<h2>E.1 <code>FUNC_ENTER</code></h2>
+
+<blockquote>
+ <pre>
+ /* Initialize the library */ \
+ H5_FIRST_THREAD_INIT \
+ H5_API_UNSET_CANCEL \
+ H5_API_LOCK_BEGIN \
+ if (!(H5_INIT_GLOBAL)) { \
+ H5_INIT_GLOBAL = TRUE; \
+ if (H5_init_library() &lt; 0) { \
+ HRETURN_ERROR (H5E_FUNC, H5E_CANTINIT, err, \
+ "library initialization failed"); \
+ } \
+ } \
+ H5_API_LOCK_END \
+ :
+ :
+ :
+ </pre>
+</blockquote>
+
+<h2>E.2 <code>H5_FIRST_THREAD_INIT</code></h2>
+
+<blockquote>
+ <pre>
+ /* Macro for first thread initialization */
+ #define H5_FIRST_THREAD_INIT \
+ pthread_once(&amp;H5_first_init_g, H5_first_thread_init);
+ </pre>
+</blockquote>
+
+
+<h2>E.3 <code>H5_API_UNSET_CANCEL</code></h2>
+
+<blockquote>
+ <pre>
+ #define H5_API_UNSET_CANCEL \
+ if (H5_IS_API(FUNC)) { \
+ H5_cancel_count_inc(); \
+ }
+ </pre>
+</blockquote>
+
+
+<h2>E.4 <code>H5_API_LOCK_BEGIN</code></h2>
+
+<blockquote>
+ <pre>
+ #define H5_API_LOCK_BEGIN \
+ if (H5_IS_API(FUNC)) { \
+ H5_mutex_lock(&amp;H5_g.init_lock);
+ </pre>
+</blockquote>
+
+
+<h2>E.5 <code>H5_API_LOCK_END</code></h2>
+
+<blockquote>
+ <pre>
+ #define H5_API_LOCK_END }
+ </pre>
+</blockquote>
+
+
+<h2>E.6 <code>HRETURN</code> and <code>HRETURN_ERROR</code></h2>
+
+<blockquote>
+ <pre>
+ :
+ :
+ H5_API_UNLOCK_BEGIN \
+ H5_API_UNLOCK_END \
+ H5_API_SET_CANCEL \
+ return ret_val; \
+ }
+ </pre>
+</blockquote>
+
+<h2>E.7 <code>H5_API_UNLOCK_BEGIN</code></h2>
+
+<blockquote>
+ <pre>
+ #define H5_API_UNLOCK_BEGIN \
+ if (H5_IS_API(FUNC)) { \
+ H5_mutex_unlock(&amp;H5_g.init_lock);
+ </pre>
+</blockquote>
+
+<h2>E.8 <code>H5_API_UNLOCK_END</code></h2>
+
+<blockquote>
+ <pre>
+ #define H5_API_UNLOCK_END }
+ </pre>
+</blockquote>
+
+
+<h2>E.9 <code>H5_API_SET_CANCEL</code></h2>
+
+<blockquote>
+ <pre>
+ #define H5_API_SET_CANCEL \
+ if (H5_IS_API(FUNC)) { \
+ H5_cancel_count_dec(); \
+ }
+ </pre>
+</blockquote>
+
+<h2>By Chee Wai Lee</h2>
+<h4>By Bill Wendling</h4>
+<h4>27. October 2000</h4>
+
+</body>
+</html>