1 <html> 2 <head> 3 <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 4 <title>8.DRD: a thread error detector</title> 5 <link rel="stylesheet" type="text/css" href="vg_basic.css"> 6 <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7 <link rel="home" href="index.html" title="Valgrind Documentation"> 8 <link rel="up" href="manual.html" title="Valgrind User Manual"> 9 <link rel="prev" href="hg-manual.html" title="7.Helgrind: a thread error detector"> 10 <link rel="next" href="ms-manual.html" title="9.Massif: a heap profiler"> 11 </head> 12 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13 <div><table class="nav" width="100%" cellspacing="3" cellpadding="3" border="0" summary="Navigation header"><tr> 14 <td width="22px" align="center" valign="middle"><a accesskey="p" href="hg-manual.html"><img src="images/prev.png" width="18" height="21" border="0" alt="Prev"></a></td> 15 <td width="25px" align="center" valign="middle"><a accesskey="u" href="manual.html"><img src="images/up.png" width="21" height="18" border="0" alt="Up"></a></td> 16 <td width="31px" align="center" valign="middle"><a accesskey="h" href="index.html"><img src="images/home.png" width="27" height="20" border="0" alt="Up"></a></td> 17 <th align="center" valign="middle">Valgrind User Manual</th> 18 <td width="22px" align="center" valign="middle"><a accesskey="n" href="ms-manual.html"><img src="images/next.png" width="18" height="21" border="0" alt="Next"></a></td> 19 </tr></table></div> 20 <div class="chapter"> 21 <div class="titlepage"><div><div><h1 class="title"> 22 <a name="drd-manual"></a>8.DRD: a thread error detector</h1></div></div></div> 23 <div class="toc"> 24 <p><b>Table of Contents</b></p> 25 <dl class="toc"> 26 <dt><span class="sect1"><a href="drd-manual.html#drd-manual.overview">8.1. Overview</a></span></dt> 27 <dd><dl> 28 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.mt-progr-models">8.1.1. Multithreaded Programming Paradigms</a></span></dt> 29 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.pthreads-model">8.1.2. POSIX Threads Programming Model</a></span></dt> 30 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.mt-problems">8.1.3. Multithreaded Programming Problems</a></span></dt> 31 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.data-race-detection">8.1.4. Data Race Detection</a></span></dt> 32 </dl></dd> 33 <dt><span class="sect1"><a href="drd-manual.html#drd-manual.using-drd">8.2. Using DRD</a></span></dt> 34 <dd><dl> 35 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.options">8.2.1. DRD Command-line Options</a></span></dt> 36 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.data-races">8.2.2. Detected Errors: Data Races</a></span></dt> 37 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.lock-contention">8.2.3. Detected Errors: Lock Contention</a></span></dt> 38 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.api-checks">8.2.4. Detected Errors: Misuse of the POSIX threads API</a></span></dt> 39 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.clientreqs">8.2.5. Client Requests</a></span></dt> 40 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.C++11">8.2.6. Debugging C++11 Programs</a></span></dt> 41 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.gnome">8.2.7. Debugging GNOME Programs</a></span></dt> 42 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.boost.thread">8.2.8. Debugging Boost.Thread Programs</a></span></dt> 43 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.openmp">8.2.9. Debugging OpenMP Programs</a></span></dt> 44 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.cust-mem-alloc">8.2.10. DRD and Custom Memory Allocators</a></span></dt> 45 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.drd-versus-memcheck">8.2.11. DRD Versus Memcheck</a></span></dt> 46 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.resource-requirements">8.2.12. Resource Requirements</a></span></dt> 47 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.effective-use">8.2.13. Hints and Tips for Effective Use of DRD</a></span></dt> 48 </dl></dd> 49 <dt><span class="sect1"><a href="drd-manual.html#drd-manual.Pthreads">8.3. Using the POSIX Threads API Effectively</a></span></dt> 50 <dd><dl> 51 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.mutex-types">8.3.1. Mutex types</a></span></dt> 52 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.condvar">8.3.2. Condition variables</a></span></dt> 53 <dt><span class="sect2"><a href="drd-manual.html#drd-manual.pctw">8.3.3. pthread_cond_timedwait and timeouts</a></span></dt> 54 </dl></dd> 55 <dt><span class="sect1"><a href="drd-manual.html#drd-manual.limitations">8.4. Limitations</a></span></dt> 56 <dt><span class="sect1"><a href="drd-manual.html#drd-manual.feedback">8.5. Feedback</a></span></dt> 57 </dl> 58 </div> 59 <p>To use this tool, you must specify 60 <code class="option">--tool=drd</code> 61 on the Valgrind command line.</p> 62 <div class="sect1"> 63 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> 64 <a name="drd-manual.overview"></a>8.1.Overview</h2></div></div></div> 65 <p> 66 DRD is a Valgrind tool for detecting errors in multithreaded C and C++ 67 programs. The tool works for any program that uses the POSIX threading 68 primitives or that uses threading concepts built on top of the POSIX threading 69 primitives. 70 </p> 71 <div class="sect2"> 72 <div class="titlepage"><div><div><h3 class="title"> 73 <a name="drd-manual.mt-progr-models"></a>8.1.1.Multithreaded Programming Paradigms</h3></div></div></div> 74 <p> 75 There are two possible reasons for using multithreading in a program: 76 </p> 77 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 78 <li class="listitem"><p> 79 To model concurrent activities. Assigning one thread to each activity 80 can be a great simplification compared to multiplexing the states of 81 multiple activities in a single thread. This is why most server software 82 and embedded software is multithreaded. 83 </p></li> 84 <li class="listitem"><p> 85 To use multiple CPU cores simultaneously for speeding up 86 computations. This is why many High Performance Computing (HPC) 87 applications are multithreaded. 88 </p></li> 89 </ul></div> 90 <p> 91 </p> 92 <p> 93 Multithreaded programs can use one or more of the following programming 94 paradigms. Which paradigm is appropriate depends e.g. on the application type. 95 Some examples of multithreaded programming paradigms are: 96 </p> 97 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 98 <li class="listitem"><p> 99 Locking. Data that is shared over threads is protected from concurrent 100 accesses via locking. E.g. the POSIX threads library, the Qt library 101 and the Boost.Thread library support this paradigm directly. 102 </p></li> 103 <li class="listitem"><p> 104 Message passing. No data is shared between threads, but threads exchange 105 data by passing messages to each other. Examples of implementations of 106 the message passing paradigm are MPI and CORBA. 107 </p></li> 108 <li class="listitem"><p> 109 Automatic parallelization. A compiler converts a sequential program into 110 a multithreaded program. The original program may or may not contain 111 parallelization hints. One example of such parallelization hints is the 112 OpenMP standard. In this standard a set of directives are defined which 113 tell a compiler how to parallelize a C, C++ or Fortran program. OpenMP 114 is well suited for computational intensive applications. As an example, 115 an open source image processing software package is using OpenMP to 116 maximize performance on systems with multiple CPU 117 cores. GCC supports the 118 OpenMP standard from version 4.2.0 on. 119 </p></li> 120 <li class="listitem"><p> 121 Software Transactional Memory (STM). Any data that is shared between 122 threads is updated via transactions. After each transaction it is 123 verified whether there were any conflicting transactions. If there were 124 conflicts, the transaction is aborted, otherwise it is committed. This 125 is a so-called optimistic approach. There is a prototype of the Intel C++ 126 Compiler available that supports STM. Research about the addition of 127 STM support to GCC is ongoing. 128 </p></li> 129 </ul></div> 130 <p> 131 </p> 132 <p> 133 DRD supports any combination of multithreaded programming paradigms as 134 long as the implementation of these paradigms is based on the POSIX 135 threads primitives. DRD however does not support programs that use 136 e.g. Linux' futexes directly. Attempts to analyze such programs with 137 DRD will cause DRD to report many false positives. 138 </p> 139 </div> 140 <div class="sect2"> 141 <div class="titlepage"><div><div><h3 class="title"> 142 <a name="drd-manual.pthreads-model"></a>8.1.2.POSIX Threads Programming Model</h3></div></div></div> 143 <p> 144 POSIX threads, also known as Pthreads, is the most widely available 145 threading library on Unix systems. 146 </p> 147 <p> 148 The POSIX threads programming model is based on the following abstractions: 149 </p> 150 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 151 <li class="listitem"><p> 152 A shared address space. All threads running within the same 153 process share the same address space. All data, whether shared or 154 not, is identified by its address. 155 </p></li> 156 <li class="listitem"><p> 157 Regular load and store operations, which allow to read values 158 from or to write values to the memory shared by all threads 159 running in the same process. 160 </p></li> 161 <li class="listitem"><p> 162 Atomic store and load-modify-store operations. While these are 163 not mentioned in the POSIX threads standard, most 164 microprocessors support atomic memory operations. 165 </p></li> 166 <li class="listitem"><p> 167 Threads. Each thread represents a concurrent activity. 168 </p></li> 169 <li class="listitem"><p> 170 Synchronization objects and operations on these synchronization 171 objects. The following types of synchronization objects have been 172 defined in the POSIX threads standard: mutexes, condition variables, 173 semaphores, reader-writer synchronization objects, barriers and 174 spinlocks. 175 </p></li> 176 </ul></div> 177 <p> 178 </p> 179 <p> 180 Which source code statements generate which memory accesses depends on 181 the <span class="emphasis"><em>memory model</em></span> of the programming language being 182 used. There is not yet a definitive memory model for the C and C++ 183 languages. For a draft memory model, see also the document 184 <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2338.html" target="_top"> 185 WG21/N2338: Concurrency memory model compiler consequences</a>. 186 </p> 187 <p> 188 For more information about POSIX threads, see also the Single UNIX 189 Specification version 3, also known as 190 <a class="ulink" href="http://www.opengroup.org/onlinepubs/000095399/idx/threads.html" target="_top"> 191 IEEE Std 1003.1</a>. 192 </p> 193 </div> 194 <div class="sect2"> 195 <div class="titlepage"><div><div><h3 class="title"> 196 <a name="drd-manual.mt-problems"></a>8.1.3.Multithreaded Programming Problems</h3></div></div></div> 197 <p> 198 Depending on which multithreading paradigm is being used in a program, 199 one or more of the following problems can occur: 200 </p> 201 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 202 <li class="listitem"><p> 203 Data races. One or more threads access the same memory location without 204 sufficient locking. Most but not all data races are programming errors 205 and are the cause of subtle and hard-to-find bugs. 206 </p></li> 207 <li class="listitem"><p> 208 Lock contention. One thread blocks the progress of one or more other 209 threads by holding a lock too long. 210 </p></li> 211 <li class="listitem"><p> 212 Improper use of the POSIX threads API. Most implementations of the POSIX 213 threads API have been optimized for runtime speed. Such implementations 214 will not complain on certain errors, e.g. when a mutex is being unlocked 215 by another thread than the thread that obtained a lock on the mutex. 216 </p></li> 217 <li class="listitem"><p> 218 Deadlock. A deadlock occurs when two or more threads wait for 219 each other indefinitely. 220 </p></li> 221 <li class="listitem"><p> 222 False sharing. If threads that run on different processor cores 223 access different variables located in the same cache line 224 frequently, this will slow down the involved threads a lot due 225 to frequent exchange of cache lines. 226 </p></li> 227 </ul></div> 228 <p> 229 </p> 230 <p> 231 Although the likelihood of the occurrence of data races can be reduced 232 through a disciplined programming style, a tool for automatic 233 detection of data races is a necessity when developing multithreaded 234 software. DRD can detect these, as well as lock contention and 235 improper use of the POSIX threads API. 236 </p> 237 </div> 238 <div class="sect2"> 239 <div class="titlepage"><div><div><h3 class="title"> 240 <a name="drd-manual.data-race-detection"></a>8.1.4.Data Race Detection</h3></div></div></div> 241 <p> 242 The result of load and store operations performed by a multithreaded program 243 depends on the order in which memory operations are performed. This order is 244 determined by: 245 </p> 246 <div class="orderedlist"><ol class="orderedlist" type="1"> 247 <li class="listitem"><p> 248 All memory operations performed by the same thread are performed in 249 <span class="emphasis"><em>program order</em></span>, that is, the order determined by the 250 program source code and the results of previous load operations. 251 </p></li> 252 <li class="listitem"><p> 253 Synchronization operations determine certain ordering constraints on 254 memory operations performed by different threads. These ordering 255 constraints are called the <span class="emphasis"><em>synchronization order</em></span>. 256 </p></li> 257 </ol></div> 258 <p> 259 The combination of program order and synchronization order is called the 260 <span class="emphasis"><em>happens-before relationship</em></span>. This concept was first 261 defined by S. Adve et al in the paper <span class="emphasis"><em>Detecting data races on weak 262 memory systems</em></span>, ACM SIGARCH Computer Architecture News, v.19 n.3, 263 p.234-243, May 1991. 264 </p> 265 <p> 266 Two memory operations <span class="emphasis"><em>conflict</em></span> if both operations are 267 performed by different threads, refer to the same memory location and at least 268 one of them is a store operation. 269 </p> 270 <p> 271 A multithreaded program is <span class="emphasis"><em>data-race free</em></span> if all 272 conflicting memory accesses are ordered by synchronization 273 operations. 274 </p> 275 <p> 276 A well known way to ensure that a multithreaded program is data-race 277 free is to ensure that a locking discipline is followed. It is e.g. 278 possible to associate a mutex with each shared data item, and to hold 279 a lock on the associated mutex while the shared data is accessed. 280 </p> 281 <p> 282 All programs that follow a locking discipline are data-race free, but not all 283 data-race free programs follow a locking discipline. There exist multithreaded 284 programs where access to shared data is arbitrated via condition variables, 285 semaphores or barriers. As an example, a certain class of HPC applications 286 consists of a sequence of computation steps separated in time by barriers, and 287 where these barriers are the only means of synchronization. Although there are 288 many conflicting memory accesses in such applications and although such 289 applications do not make use mutexes, most of these applications do not 290 contain data races. 291 </p> 292 <p> 293 There exist two different approaches for verifying the correctness of 294 multithreaded programs at runtime. The approach of the so-called Eraser 295 algorithm is to verify whether all shared memory accesses follow a consistent 296 locking strategy. And the happens-before data race detectors verify directly 297 whether all interthread memory accesses are ordered by synchronization 298 operations. While the last approach is more complex to implement, and while it 299 is more sensitive to OS scheduling, it is a general approach that works for 300 all classes of multithreaded programs. An important advantage of 301 happens-before data race detectors is that these do not report any false 302 positives. 303 </p> 304 <p> 305 DRD is based on the happens-before algorithm. 306 </p> 307 </div> 308 </div> 309 <div class="sect1"> 310 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> 311 <a name="drd-manual.using-drd"></a>8.2.Using DRD</h2></div></div></div> 312 <div class="sect2"> 313 <div class="titlepage"><div><div><h3 class="title"> 314 <a name="drd-manual.options"></a>8.2.1.DRD Command-line Options</h3></div></div></div> 315 <p>The following command-line options are available for controlling the 316 behavior of the DRD tool itself:</p> 317 <div class="variablelist"> 318 <a name="drd.opts.list"></a><dl class="variablelist"> 319 <dt><span class="term"> 320 <code class="option">--check-stack-var=<yes|no> [default: no]</code> 321 </span></dt> 322 <dd><p> 323 Controls whether DRD detects data races on stack 324 variables. Verifying stack variables is disabled by default because 325 most programs do not share stack variables over threads. 326 </p></dd> 327 <dt><span class="term"> 328 <code class="option">--exclusive-threshold=<n> [default: off]</code> 329 </span></dt> 330 <dd><p> 331 Print an error message if any mutex or writer lock has been 332 held longer than the time specified in milliseconds. This 333 option enables the detection of lock contention. 334 </p></dd> 335 <dt><span class="term"> 336 <code class="option">--join-list-vol=<n> [default: 10]</code> 337 </span></dt> 338 <dd><p> 339 Data races that occur between a statement at the end of one thread 340 and another thread can be missed if memory access information is 341 discarded immediately after a thread has been joined. This option 342 allows to specify for how many joined threads memory access information 343 should be retained. 344 </p></dd> 345 <dt><span class="term"> 346 <code class="option"> 347 --first-race-only=<yes|no> [default: no] 348 </code> 349 </span></dt> 350 <dd><p> 351 Whether to report only the first data race that has been detected on a 352 memory location or all data races that have been detected on a memory 353 location. 354 </p></dd> 355 <dt><span class="term"> 356 <code class="option"> 357 --free-is-write=<yes|no> [default: no] 358 </code> 359 </span></dt> 360 <dd> 361 <p> 362 Whether to report races between accessing memory and freeing 363 memory. Enabling this option may cause DRD to run slightly 364 slower. Notes:</p> 365 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 366 <li class="listitem"><p> 367 Don't enable this option when using custom memory allocators 368 that use 369 the <code class="computeroutput">VG_USERREQ__MALLOCLIKE_BLOCK</code> 370 and <code class="computeroutput">VG_USERREQ__FREELIKE_BLOCK</code> 371 because that would result in false positives. 372 </p></li> 373 <li class="listitem"><p>Don't enable this option when using reference-counted 374 objects because that will result in false positives, even when 375 that code has been annotated properly with 376 <code class="computeroutput">ANNOTATE_HAPPENS_BEFORE</code> 377 and <code class="computeroutput">ANNOTATE_HAPPENS_AFTER</code>. See 378 e.g. the output of the following command for an example: 379 <code class="computeroutput">valgrind --tool=drd --free-is-write=yes 380 drd/tests/annotate_smart_pointer</code>. 381 </p></li> 382 </ul></div> 383 </dd> 384 <dt><span class="term"> 385 <code class="option"> 386 --report-signal-unlocked=<yes|no> [default: yes] 387 </code> 388 </span></dt> 389 <dd><p> 390 Whether to report calls to 391 <code class="function">pthread_cond_signal</code> and 392 <code class="function">pthread_cond_broadcast</code> where the mutex 393 associated with the signal through 394 <code class="function">pthread_cond_wait</code> or 395 <code class="function">pthread_cond_timed_wait</code>is not locked at 396 the time the signal is sent. Sending a signal without holding 397 a lock on the associated mutex is a common programming error 398 which can cause subtle race conditions and unpredictable 399 behavior. There exist some uncommon synchronization patterns 400 however where it is safe to send a signal without holding a 401 lock on the associated mutex. 402 </p></dd> 403 <dt><span class="term"> 404 <code class="option">--segment-merging=<yes|no> [default: yes]</code> 405 </span></dt> 406 <dd><p> 407 Controls segment merging. Segment merging is an algorithm to 408 limit memory usage of the data race detection 409 algorithm. Disabling segment merging may improve the accuracy 410 of the so-called 'other segments' displayed in race reports 411 but can also trigger an out of memory error. 412 </p></dd> 413 <dt><span class="term"> 414 <code class="option">--segment-merging-interval=<n> [default: 10]</code> 415 </span></dt> 416 <dd><p> 417 Perform segment merging only after the specified number of new 418 segments have been created. This is an advanced configuration option 419 that allows to choose whether to minimize DRD's memory usage by 420 choosing a low value or to let DRD run faster by choosing a slightly 421 higher value. The optimal value for this parameter depends on the 422 program being analyzed. The default value works well for most programs. 423 </p></dd> 424 <dt><span class="term"> 425 <code class="option">--shared-threshold=<n> [default: off]</code> 426 </span></dt> 427 <dd><p> 428 Print an error message if a reader lock has been held longer 429 than the specified time (in milliseconds). This option enables 430 the detection of lock contention. 431 </p></dd> 432 <dt><span class="term"> 433 <code class="option">--show-confl-seg=<yes|no> [default: yes]</code> 434 </span></dt> 435 <dd><p> 436 Show conflicting segments in race reports. Since this 437 information can help to find the cause of a data race, this 438 option is enabled by default. Disabling this option makes the 439 output of DRD more compact. 440 </p></dd> 441 <dt><span class="term"> 442 <code class="option">--show-stack-usage=<yes|no> [default: no]</code> 443 </span></dt> 444 <dd><p> 445 Print stack usage at thread exit time. When a program creates a large 446 number of threads it becomes important to limit the amount of virtual 447 memory allocated for thread stacks. This option makes it possible to 448 observe how much stack memory has been used by each thread of the 449 client program. Note: the DRD tool itself allocates some temporary 450 data on the client thread stack. The space necessary for this 451 temporary data must be allocated by the client program when it 452 allocates stack memory, but is not included in stack usage reported by 453 DRD. 454 </p></dd> 455 <dt><span class="term"> 456 <code class="option">--ignore-thread-creation=<yes|no> [default: no]</code> 457 </span></dt> 458 <dd> 459 <p> 460 Controls whether all activities during thread creation should be 461 ignored. By default enabled only on Solaris. 462 Solaris provides higher throughput, parallelism and scalability than 463 other operating systems, at the cost of more fine-grained locking 464 activity. This means for example that when a thread is created under 465 glibc, just one big lock is used for all thread setup. Solaris libc 466 uses several fine-grained locks and the creator thread resumes its 467 activities as soon as possible, leaving for example stack and TLS setup 468 sequence to the created thread. 469 This situation confuses DRD as it assumes there is some false ordering 470 in place between creator and created thread; and therefore many types 471 of race conditions in the application would not be reported. To prevent 472 such false ordering, this command line option is set to 473 <code class="computeroutput">yes</code> by default on Solaris. 474 All activity (loads, stores, client requests) is therefore ignored 475 during:</p> 476 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 477 <li class="listitem"><p> 478 pthread_create() call in the creator thread 479 </p></li> 480 <li class="listitem"><p> 481 thread creation phase (stack and TLS setup) in the created thread 482 </p></li> 483 </ul></div> 484 </dd> 485 </dl> 486 </div> 487 <p> 488 The following options are available for monitoring the behavior of the 489 client program: 490 </p> 491 <div class="variablelist"> 492 <a name="drd.debugopts.list"></a><dl class="variablelist"> 493 <dt><span class="term"> 494 <code class="option">--trace-addr=<address> [default: none]</code> 495 </span></dt> 496 <dd><p> 497 Trace all load and store activity for the specified 498 address. This option may be specified more than once. 499 </p></dd> 500 <dt><span class="term"> 501 <code class="option">--ptrace-addr=<address> [default: none]</code> 502 </span></dt> 503 <dd><p> 504 Trace all load and store activity for the specified address and keep 505 doing that even after the memory at that address has been freed and 506 reallocated. 507 </p></dd> 508 <dt><span class="term"> 509 <code class="option">--trace-alloc=<yes|no> [default: no]</code> 510 </span></dt> 511 <dd><p> 512 Trace all memory allocations and deallocations. May produce a huge 513 amount of output. 514 </p></dd> 515 <dt><span class="term"> 516 <code class="option">--trace-barrier=<yes|no> [default: no]</code> 517 </span></dt> 518 <dd><p> 519 Trace all barrier activity. 520 </p></dd> 521 <dt><span class="term"> 522 <code class="option">--trace-cond=<yes|no> [default: no]</code> 523 </span></dt> 524 <dd><p> 525 Trace all condition variable activity. 526 </p></dd> 527 <dt><span class="term"> 528 <code class="option">--trace-fork-join=<yes|no> [default: no]</code> 529 </span></dt> 530 <dd><p> 531 Trace all thread creation and all thread termination events. 532 </p></dd> 533 <dt><span class="term"> 534 <code class="option">--trace-hb=<yes|no> [default: no]</code> 535 </span></dt> 536 <dd><p> 537 Trace execution of the <code class="literal">ANNOTATE_HAPPENS_BEFORE()</code>, 538 <code class="literal">ANNOTATE_HAPPENS_AFTER()</code> and 539 <code class="literal">ANNOTATE_HAPPENS_DONE()</code> client requests. 540 </p></dd> 541 <dt><span class="term"> 542 <code class="option">--trace-mutex=<yes|no> [default: no]</code> 543 </span></dt> 544 <dd><p> 545 Trace all mutex activity. 546 </p></dd> 547 <dt><span class="term"> 548 <code class="option">--trace-rwlock=<yes|no> [default: no]</code> 549 </span></dt> 550 <dd><p> 551 Trace all reader-writer lock activity. 552 </p></dd> 553 <dt><span class="term"> 554 <code class="option">--trace-semaphore=<yes|no> [default: no]</code> 555 </span></dt> 556 <dd><p> 557 Trace all semaphore activity. 558 </p></dd> 559 </dl> 560 </div> 561 </div> 562 <div class="sect2"> 563 <div class="titlepage"><div><div><h3 class="title"> 564 <a name="drd-manual.data-races"></a>8.2.2.Detected Errors: Data Races</h3></div></div></div> 565 <p> 566 DRD prints a message every time it detects a data race. Please keep 567 the following in mind when interpreting DRD's output: 568 </p> 569 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 570 <li class="listitem"><p> 571 Every thread is assigned a <span class="emphasis"><em>thread ID</em></span> by the DRD 572 tool. A thread ID is a number. Thread ID's start at one and are never 573 recycled. 574 </p></li> 575 <li class="listitem"><p> 576 The term <span class="emphasis"><em>segment</em></span> refers to a consecutive 577 sequence of load, store and synchronization operations, all 578 issued by the same thread. A segment always starts and ends at a 579 synchronization operation. Data race analysis is performed 580 between segments instead of between individual load and store 581 operations because of performance reasons. 582 </p></li> 583 <li class="listitem"><p> 584 There are always at least two memory accesses involved in a data 585 race. Memory accesses involved in a data race are called 586 <span class="emphasis"><em>conflicting memory accesses</em></span>. DRD prints a 587 report for each memory access that conflicts with a past memory 588 access. 589 </p></li> 590 </ul></div> 591 <p> 592 </p> 593 <p> 594 Below you can find an example of a message printed by DRD when it 595 detects a data race: 596 </p> 597 <pre class="programlisting"> 598 $ valgrind --tool=drd --read-var-info=yes drd/tests/rwlock_race 599 ... 600 ==9466== Thread 3: 601 ==9466== Conflicting load by thread 3 at 0x006020b8 size 4 602 ==9466== at 0x400B6C: thread_func (rwlock_race.c:29) 603 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186) 604 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so) 605 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so) 606 ==9466== Location 0x6020b8 is 0 bytes inside local var "s_racy" 607 ==9466== declared at rwlock_race.c:18, in frame #0 of thread 3 608 ==9466== Other segment start (thread 2) 609 ==9466== at 0x4C2847D: pthread_rwlock_rdlock* (drd_pthread_intercepts.c:813) 610 ==9466== by 0x400B6B: thread_func (rwlock_race.c:28) 611 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186) 612 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so) 613 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so) 614 ==9466== Other segment end (thread 2) 615 ==9466== at 0x4C28B54: pthread_rwlock_unlock* (drd_pthread_intercepts.c:912) 616 ==9466== by 0x400B84: thread_func (rwlock_race.c:30) 617 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186) 618 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so) 619 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so) 620 ... 621 </pre> 622 <p> 623 The above report has the following meaning: 624 </p> 625 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 626 <li class="listitem"><p> 627 The number in the column on the left is the process ID of the 628 process being analyzed by DRD. 629 </p></li> 630 <li class="listitem"><p> 631 The first line ("Thread 3") tells you the thread ID for 632 the thread in which context the data race has been detected. 633 </p></li> 634 <li class="listitem"><p> 635 The next line tells which kind of operation was performed (load or 636 store) and by which thread. On the same line the start address and the 637 number of bytes involved in the conflicting access are also displayed. 638 </p></li> 639 <li class="listitem"><p> 640 Next, the call stack of the conflicting access is displayed. If 641 your program has been compiled with debug information 642 (<code class="option">-g</code>), this call stack will include file names and 643 line numbers. The two 644 bottommost frames in this call stack (<code class="function">clone</code> 645 and <code class="function">start_thread</code>) show how the NPTL starts 646 a thread. The third frame 647 (<code class="function">vg_thread_wrapper</code>) is added by DRD. The 648 fourth frame (<code class="function">thread_func</code>) is the first 649 interesting line because it shows the thread entry point, that 650 is the function that has been passed as the third argument to 651 <code class="function">pthread_create</code>. 652 </p></li> 653 <li class="listitem"><p> 654 Next, the allocation context for the conflicting address is 655 displayed. For dynamically allocated data the allocation call 656 stack is shown. For static variables and stack variables the 657 allocation context is only shown when the option 658 <code class="option">--read-var-info=yes</code> has been 659 specified. Otherwise DRD will print <code class="computeroutput">Allocation 660 context: unknown</code>. 661 </p></li> 662 <li class="listitem"> 663 <p> 664 A conflicting access involves at least two memory accesses. For 665 one of these accesses an exact call stack is displayed, and for 666 the other accesses an approximate call stack is displayed, 667 namely the start and the end of the segments of the other 668 accesses. This information can be interpreted as follows: 669 </p> 670 <div class="orderedlist"><ol class="orderedlist" type="1"> 671 <li class="listitem"><p> 672 Start at the bottom of both call stacks, and count the 673 number stack frames with identical function name, file 674 name and line number. In the above example the three 675 bottommost frames are identical 676 (<code class="function">clone</code>, 677 <code class="function">start_thread</code> and 678 <code class="function">vg_thread_wrapper</code>). 679 </p></li> 680 <li class="listitem"><p> 681 The next higher stack frame in both call stacks now tells 682 you between in which source code region the other memory 683 access happened. The above output tells that the other 684 memory access involved in the data race happened between 685 source code lines 28 and 30 in file 686 <code class="computeroutput">rwlock_race.c</code>. 687 </p></li> 688 </ol></div> 689 <p> 690 </p> 691 </li> 692 </ul></div> 693 <p> 694 </p> 695 </div> 696 <div class="sect2"> 697 <div class="titlepage"><div><div><h3 class="title"> 698 <a name="drd-manual.lock-contention"></a>8.2.3.Detected Errors: Lock Contention</h3></div></div></div> 699 <p> 700 Threads must be able to make progress without being blocked for too long by 701 other threads. Sometimes a thread has to wait until a mutex or reader-writer 702 synchronization object is unlocked by another thread. This is called 703 <span class="emphasis"><em>lock contention</em></span>. 704 </p> 705 <p> 706 Lock contention causes delays. Such delays should be as short as 707 possible. The two command line options 708 <code class="literal">--exclusive-threshold=<n></code> and 709 <code class="literal">--shared-threshold=<n></code> make it possible to 710 detect excessive lock contention by making DRD report any lock that 711 has been held longer than the specified threshold. An example: 712 </p> 713 <pre class="programlisting"> 714 $ valgrind --tool=drd --exclusive-threshold=10 drd/tests/hold_lock -i 500 715 ... 716 ==10668== Acquired at: 717 ==10668== at 0x4C267C8: pthread_mutex_lock (drd_pthread_intercepts.c:395) 718 ==10668== by 0x400D92: main (hold_lock.c:51) 719 ==10668== Lock on mutex 0x7fefffd50 was held during 503 ms (threshold: 10 ms). 720 ==10668== at 0x4C26ADA: pthread_mutex_unlock (drd_pthread_intercepts.c:441) 721 ==10668== by 0x400DB5: main (hold_lock.c:55) 722 ... 723 </pre> 724 <p> 725 The <code class="literal">hold_lock</code> test program holds a lock as long as 726 specified by the <code class="literal">-i</code> (interval) argument. The DRD 727 output reports that the lock acquired at line 51 in source file 728 <code class="literal">hold_lock.c</code> and released at line 55 was held during 729 503 ms, while a threshold of 10 ms was specified to DRD. 730 </p> 731 </div> 732 <div class="sect2"> 733 <div class="titlepage"><div><div><h3 class="title"> 734 <a name="drd-manual.api-checks"></a>8.2.4.Detected Errors: Misuse of the POSIX threads API</h3></div></div></div> 735 <p> 736 DRD is able to detect and report the following misuses of the POSIX 737 threads API: 738 </p> 739 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 740 <li class="listitem"><p> 741 Passing the address of one type of synchronization object 742 (e.g. a mutex) to a POSIX API call that expects a pointer to 743 another type of synchronization object (e.g. a condition 744 variable). 745 </p></li> 746 <li class="listitem"><p> 747 Attempts to unlock a mutex that has not been locked. 748 </p></li> 749 <li class="listitem"><p> 750 Attempts to unlock a mutex that was locked by another thread. 751 </p></li> 752 <li class="listitem"><p> 753 Attempts to lock a mutex of type 754 <code class="literal">PTHREAD_MUTEX_NORMAL</code> or a spinlock 755 recursively. 756 </p></li> 757 <li class="listitem"><p> 758 Destruction or deallocation of a locked mutex. 759 </p></li> 760 <li class="listitem"><p> 761 Sending a signal to a condition variable while no lock is held 762 on the mutex associated with the condition variable. 763 </p></li> 764 <li class="listitem"><p> 765 Calling <code class="function">pthread_cond_wait</code> on a mutex 766 that is not locked, that is locked by another thread or that 767 has been locked recursively. 768 </p></li> 769 <li class="listitem"><p> 770 Associating two different mutexes with a condition variable 771 through <code class="function">pthread_cond_wait</code>. 772 </p></li> 773 <li class="listitem"><p> 774 Destruction or deallocation of a condition variable that is 775 being waited upon. 776 </p></li> 777 <li class="listitem"><p> 778 Destruction or deallocation of a locked reader-writer synchronization 779 object. 780 </p></li> 781 <li class="listitem"><p> 782 Attempts to unlock a reader-writer synchronization object that was not 783 locked by the calling thread. 784 </p></li> 785 <li class="listitem"><p> 786 Attempts to recursively lock a reader-writer synchronization object 787 exclusively. 788 </p></li> 789 <li class="listitem"><p> 790 Attempts to pass the address of a user-defined reader-writer 791 synchronization object to a POSIX threads function. 792 </p></li> 793 <li class="listitem"><p> 794 Attempts to pass the address of a POSIX reader-writer synchronization 795 object to one of the annotations for user-defined reader-writer 796 synchronization objects. 797 </p></li> 798 <li class="listitem"><p> 799 Reinitialization of a mutex, condition variable, reader-writer 800 lock, semaphore or barrier. 801 </p></li> 802 <li class="listitem"><p> 803 Destruction or deallocation of a semaphore or barrier that is 804 being waited upon. 805 </p></li> 806 <li class="listitem"><p> 807 Missing synchronization between barrier wait and barrier destruction. 808 </p></li> 809 <li class="listitem"><p> 810 Exiting a thread without first unlocking the spinlocks, mutexes or 811 reader-writer synchronization objects that were locked by that thread. 812 </p></li> 813 <li class="listitem"><p> 814 Passing an invalid thread ID to <code class="function">pthread_join</code> 815 or <code class="function">pthread_cancel</code>. 816 </p></li> 817 </ul></div> 818 <p> 819 </p> 820 </div> 821 <div class="sect2"> 822 <div class="titlepage"><div><div><h3 class="title"> 823 <a name="drd-manual.clientreqs"></a>8.2.5.Client Requests</h3></div></div></div> 824 <p> 825 Just as for other Valgrind tools it is possible to let a client program 826 interact with the DRD tool through client requests. In addition to the 827 client requests several macros have been defined that allow to use the 828 client requests in a convenient way. 829 </p> 830 <p> 831 The interface between client programs and the DRD tool is defined in 832 the header file <code class="literal"><valgrind/drd.h></code>. The 833 available macros and client requests are: 834 </p> 835 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 836 <li class="listitem"><p> 837 The macro <code class="literal">DRD_GET_VALGRIND_THREADID</code> and the 838 corresponding client 839 request <code class="varname">VG_USERREQ__DRD_GET_VALGRIND_THREAD_ID</code>. 840 Query the thread ID that has been assigned by the Valgrind core to the 841 thread executing this client request. Valgrind's thread ID's start at 842 one and are recycled in case a thread stops. 843 </p></li> 844 <li class="listitem"><p> 845 The macro <code class="literal">DRD_GET_DRD_THREADID</code> and the corresponding 846 client request <code class="varname">VG_USERREQ__DRD_GET_DRD_THREAD_ID</code>. 847 Query the thread ID that has been assigned by DRD to the thread 848 executing this client request. These are the thread ID's reported by DRD 849 in data race reports and in trace messages. DRD's thread ID's start at 850 one and are never recycled. 851 </p></li> 852 <li class="listitem"><p> 853 The macros <code class="literal">DRD_IGNORE_VAR(x)</code>, 854 <code class="literal">ANNOTATE_TRACE_MEMORY(&x)</code> and the corresponding 855 client request <code class="varname">VG_USERREQ__DRD_START_SUPPRESSION</code>. Some 856 applications contain intentional races. There exist e.g. applications 857 where the same value is assigned to a shared variable from two different 858 threads. It may be more convenient to suppress such races than to solve 859 these. This client request allows to suppress such races. 860 </p></li> 861 <li class="listitem"><p> 862 The macro <code class="literal">DRD_STOP_IGNORING_VAR(x)</code> and the 863 corresponding client request 864 <code class="varname">VG_USERREQ__DRD_FINISH_SUPPRESSION</code>. Tell DRD 865 to no longer ignore data races for the address range that was suppressed 866 either via the macro <code class="literal">DRD_IGNORE_VAR(x)</code> or via the 867 client request <code class="varname">VG_USERREQ__DRD_START_SUPPRESSION</code>. 868 </p></li> 869 <li class="listitem"><p> 870 The macro <code class="literal">DRD_TRACE_VAR(x)</code>. Trace all load and store 871 activity for the address range starting at <code class="literal">&x</code> and 872 occupying <code class="literal">sizeof(x)</code> bytes. When DRD reports a data 873 race on a specified variable, and it's not immediately clear which 874 source code statements triggered the conflicting accesses, it can be 875 very helpful to trace all activity on the offending memory location. 876 </p></li> 877 <li class="listitem"><p> 878 The macro <code class="literal">DRD_STOP_TRACING_VAR(x)</code>. Stop tracing load 879 and store activity for the address range starting 880 at <code class="literal">&x</code> and occupying <code class="literal">sizeof(x)</code> 881 bytes. 882 </p></li> 883 <li class="listitem"><p> 884 The macro <code class="literal">ANNOTATE_TRACE_MEMORY(&x)</code>. Trace all 885 load and store activity that touches at least the single byte at the 886 address <code class="literal">&x</code>. 887 </p></li> 888 <li class="listitem"><p> 889 The client request <code class="varname">VG_USERREQ__DRD_START_TRACE_ADDR</code>, 890 which allows to trace all load and store activity for the specified 891 address range. 892 </p></li> 893 <li class="listitem"><p> 894 The client 895 request <code class="varname">VG_USERREQ__DRD_STOP_TRACE_ADDR</code>. Do no longer 896 trace load and store activity for the specified address range. 897 </p></li> 898 <li class="listitem"><p> 899 The macro <code class="literal">ANNOTATE_HAPPENS_BEFORE(addr)</code> tells DRD to 900 insert a mark. Insert this macro just after an access to the variable at 901 the specified address has been performed. 902 </p></li> 903 <li class="listitem"><p> 904 The macro <code class="literal">ANNOTATE_HAPPENS_AFTER(addr)</code> tells DRD that 905 the next access to the variable at the specified address should be 906 considered to have happened after the access just before the latest 907 <code class="literal">ANNOTATE_HAPPENS_BEFORE(addr)</code> annotation that 908 references the same variable. The purpose of these two macros is to tell 909 DRD about the order of inter-thread memory accesses implemented via 910 atomic memory operations. See 911 also <code class="literal">drd/tests/annotate_smart_pointer.cpp</code> for an 912 example. 913 </p></li> 914 <li class="listitem"><p> 915 The macro <code class="literal">ANNOTATE_RWLOCK_CREATE(rwlock)</code> tells DRD 916 that the object at address <code class="literal">rwlock</code> is a 917 reader-writer synchronization object that is not a 918 <code class="literal">pthread_rwlock_t</code> synchronization object. See 919 also <code class="literal">drd/tests/annotate_rwlock.c</code> for an example. 920 </p></li> 921 <li class="listitem"><p> 922 The macro <code class="literal">ANNOTATE_RWLOCK_DESTROY(rwlock)</code> tells DRD 923 that the reader-writer synchronization object at 924 address <code class="literal">rwlock</code> has been destroyed. 925 </p></li> 926 <li class="listitem"><p> 927 The macro <code class="literal">ANNOTATE_WRITERLOCK_ACQUIRED(rwlock)</code> tells 928 DRD that a writer lock has been acquired on the reader-writer 929 synchronization object at address <code class="literal">rwlock</code>. 930 </p></li> 931 <li class="listitem"><p> 932 The macro <code class="literal">ANNOTATE_READERLOCK_ACQUIRED(rwlock)</code> tells 933 DRD that a reader lock has been acquired on the reader-writer 934 synchronization object at address <code class="literal">rwlock</code>. 935 </p></li> 936 <li class="listitem"><p> 937 The macro <code class="literal">ANNOTATE_RWLOCK_ACQUIRED(rwlock, is_w)</code> 938 tells DRD that a writer lock (when <code class="literal">is_w != 0</code>) or that 939 a reader lock (when <code class="literal">is_w == 0</code>) has been acquired on 940 the reader-writer synchronization object at 941 address <code class="literal">rwlock</code>. 942 </p></li> 943 <li class="listitem"><p> 944 The macro <code class="literal">ANNOTATE_WRITERLOCK_RELEASED(rwlock)</code> tells 945 DRD that a writer lock has been released on the reader-writer 946 synchronization object at address <code class="literal">rwlock</code>. 947 </p></li> 948 <li class="listitem"><p> 949 The macro <code class="literal">ANNOTATE_READERLOCK_RELEASED(rwlock)</code> tells 950 DRD that a reader lock has been released on the reader-writer 951 synchronization object at address <code class="literal">rwlock</code>. 952 </p></li> 953 <li class="listitem"><p> 954 The macro <code class="literal">ANNOTATE_RWLOCK_RELEASED(rwlock, is_w)</code> 955 tells DRD that a writer lock (when <code class="literal">is_w != 0</code>) or that 956 a reader lock (when <code class="literal">is_w == 0</code>) has been released on 957 the reader-writer synchronization object at 958 address <code class="literal">rwlock</code>. 959 </p></li> 960 <li class="listitem"><p> 961 The macro <code class="literal">ANNOTATE_BARRIER_INIT(barrier, count, 962 reinitialization_allowed)</code> tells DRD that a new barrier object 963 at the address <code class="literal">barrier</code> has been initialized, 964 that <code class="literal">count</code> threads participate in each barrier and 965 also whether or not barrier reinitialization without intervening 966 destruction should be reported as an error. See 967 also <code class="literal">drd/tests/annotate_barrier.c</code> for an example. 968 </p></li> 969 <li class="listitem"><p> 970 The macro <code class="literal">ANNOTATE_BARRIER_DESTROY(barrier)</code> 971 tells DRD that a barrier object is about to be destroyed. 972 </p></li> 973 <li class="listitem"><p> 974 The macro <code class="literal">ANNOTATE_BARRIER_WAIT_BEFORE(barrier)</code> 975 tells DRD that waiting for a barrier will start. 976 </p></li> 977 <li class="listitem"><p> 978 The macro <code class="literal">ANNOTATE_BARRIER_WAIT_AFTER(barrier)</code> 979 tells DRD that waiting for a barrier has finished. 980 </p></li> 981 <li class="listitem"><p> 982 The macro <code class="literal">ANNOTATE_BENIGN_RACE_SIZED(addr, size, 983 descr)</code> tells DRD that any races detected on the specified 984 address are benign and hence should not be 985 reported. The <code class="literal">descr</code> argument is ignored but can be 986 used to document why data races on <code class="literal">addr</code> are benign. 987 </p></li> 988 <li class="listitem"><p> 989 The macro <code class="literal">ANNOTATE_BENIGN_RACE_STATIC(var, descr)</code> 990 tells DRD that any races detected on the specified static variable are 991 benign and hence should not be reported. The <code class="literal">descr</code> 992 argument is ignored but can be used to document why data races 993 on <code class="literal">var</code> are benign. Note: this macro can only be 994 used in C++ programs and not in C programs. 995 </p></li> 996 <li class="listitem"><p> 997 The macro <code class="literal">ANNOTATE_IGNORE_READS_BEGIN</code> tells 998 DRD to ignore all memory loads performed by the current thread. 999 </p></li> 1000 <li class="listitem"><p> 1001 The macro <code class="literal">ANNOTATE_IGNORE_READS_END</code> tells 1002 DRD to stop ignoring the memory loads performed by the current thread. 1003 </p></li> 1004 <li class="listitem"><p> 1005 The macro <code class="literal">ANNOTATE_IGNORE_WRITES_BEGIN</code> tells 1006 DRD to ignore all memory stores performed by the current thread. 1007 </p></li> 1008 <li class="listitem"><p> 1009 The macro <code class="literal">ANNOTATE_IGNORE_WRITES_END</code> tells 1010 DRD to stop ignoring the memory stores performed by the current thread. 1011 </p></li> 1012 <li class="listitem"><p> 1013 The macro <code class="literal">ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN</code> tells 1014 DRD to ignore all memory accesses performed by the current thread. 1015 </p></li> 1016 <li class="listitem"><p> 1017 The macro <code class="literal">ANNOTATE_IGNORE_READS_AND_WRITES_END</code> tells 1018 DRD to stop ignoring the memory accesses performed by the current thread. 1019 </p></li> 1020 <li class="listitem"><p> 1021 The macro <code class="literal">ANNOTATE_NEW_MEMORY(addr, size)</code> tells 1022 DRD that the specified memory range has been allocated by a custom 1023 memory allocator in the client program and that the client program 1024 will start using this memory range. 1025 </p></li> 1026 <li class="listitem"><p> 1027 The macro <code class="literal">ANNOTATE_THREAD_NAME(name)</code> tells DRD to 1028 associate the specified name with the current thread and to include this 1029 name in the error messages printed by DRD. 1030 </p></li> 1031 <li class="listitem"><p> 1032 The macros <code class="literal">VALGRIND_MALLOCLIKE_BLOCK</code> and 1033 <code class="literal">VALGRIND_FREELIKE_BLOCK</code> from the Valgrind core are 1034 implemented; they are described in 1035 <a class="xref" href="manual-core-adv.html#manual-core-adv.clientreq" title="3.1.The Client Request mechanism">The Client Request mechanism</a>. 1036 </p></li> 1037 </ul></div> 1038 <p> 1039 </p> 1040 <p> 1041 Note: if you compiled Valgrind yourself, the header file 1042 <code class="literal"><valgrind/drd.h></code> will have been installed in 1043 the directory <code class="literal">/usr/include</code> by the command 1044 <code class="literal">make install</code>. If you obtained Valgrind by 1045 installing it as a package however, you will probably have to install 1046 another package with a name like <code class="literal">valgrind-devel</code> 1047 before Valgrind's header files are available. 1048 </p> 1049 </div> 1050 <div class="sect2"> 1051 <div class="titlepage"><div><div><h3 class="title"> 1052 <a name="drd-manual.C++11"></a>8.2.6.Debugging C++11 Programs</h3></div></div></div> 1053 <p>If you want to use the C++11 class std::thread you will need to do the 1054 following to annotate the std::shared_ptr<> objects used in the 1055 implementation of that class: 1056 </p> 1057 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1058 <li class="listitem"> 1059 <p>Add the following code at the start of a common header or at the 1060 start of each source file, before any C++ header files are included:</p> 1061 <pre class="programlisting"> 1062 #include <valgrind/drd.h> 1063 #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(addr) ANNOTATE_HAPPENS_BEFORE(addr) 1064 #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(addr) ANNOTATE_HAPPENS_AFTER(addr) 1065 </pre> 1066 </li> 1067 <li class="listitem"><p>Download the gcc source code and from source file 1068 libstdc++-v3/src/c++11/thread.cc copy the implementation of the 1069 <code class="computeroutput">execute_native_thread_routine()</code> 1070 and <code class="computeroutput">std::thread::_M_start_thread()</code> 1071 functions into a source file that is linked with your application. Make 1072 sure that also in this source file the 1073 _GLIBCXX_SYNCHRONIZATION_HAPPENS_*() macros are defined properly.</p></li> 1074 </ul></div> 1075 <p> 1076 </p> 1077 <p>For more information, see also <span class="emphasis"><em>The 1078 GNU C++ Library Manual, Debugging Support</em></span> 1079 (<a class="ulink" href="http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html" target="_top">http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html</a>).</p> 1080 </div> 1081 <div class="sect2"> 1082 <div class="titlepage"><div><div><h3 class="title"> 1083 <a name="drd-manual.gnome"></a>8.2.7.Debugging GNOME Programs</h3></div></div></div> 1084 <p> 1085 GNOME applications use the threading primitives provided by the 1086 <code class="computeroutput">glib</code> and 1087 <code class="computeroutput">gthread</code> libraries. These libraries 1088 are built on top of POSIX threads, and hence are directly supported by 1089 DRD. Please keep in mind that you have to call 1090 <code class="function">g_thread_init</code> before creating any threads, or 1091 DRD will report several data races on glib functions. See also the 1092 <a class="ulink" href="http://library.gnome.org/devel/glib/stable/glib-Threads.html" target="_top">GLib 1093 Reference Manual</a> for more information about 1094 <code class="function">g_thread_init</code>. 1095 </p> 1096 <p> 1097 One of the many facilities provided by the <code class="literal">glib</code> 1098 library is a block allocator, called <code class="literal">g_slice</code>. You 1099 have to disable this block allocator when using DRD by adding the 1100 following to the shell environment variables: 1101 <code class="literal">G_SLICE=always-malloc</code>. See also the <a class="ulink" href="http://library.gnome.org/devel/glib/stable/glib-Memory-Slices.html" target="_top">GLib 1102 Reference Manual</a> for more information. 1103 </p> 1104 </div> 1105 <div class="sect2"> 1106 <div class="titlepage"><div><div><h3 class="title"> 1107 <a name="drd-manual.boost.thread"></a>8.2.8.Debugging Boost.Thread Programs</h3></div></div></div> 1108 <p> 1109 The Boost.Thread library is the threading library included with the 1110 cross-platform Boost Libraries. This threading library is an early 1111 implementation of the upcoming C++0x threading library. 1112 </p> 1113 <p> 1114 Applications that use the Boost.Thread library should run fine under DRD. 1115 </p> 1116 <p> 1117 More information about Boost.Thread can be found here: 1118 </p> 1119 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1120 <li class="listitem"><p> 1121 Anthony Williams, <a class="ulink" href="http://www.boost.org/doc/libs/1_37_0/doc/html/thread.html" target="_top">Boost.Thread</a> 1122 Library Documentation, Boost website, 2007. 1123 </p></li> 1124 <li class="listitem"><p> 1125 Anthony Williams, <a class="ulink" href="http://www.ddj.com/cpp/211600441" target="_top">What's New in Boost 1126 Threads?</a>, Recent changes to the Boost Thread library, 1127 Dr. Dobbs Magazine, October 2008. 1128 </p></li> 1129 </ul></div> 1130 <p> 1131 </p> 1132 </div> 1133 <div class="sect2"> 1134 <div class="titlepage"><div><div><h3 class="title"> 1135 <a name="drd-manual.openmp"></a>8.2.9.Debugging OpenMP Programs</h3></div></div></div> 1136 <p> 1137 OpenMP stands for <span class="emphasis"><em>Open Multi-Processing</em></span>. The OpenMP 1138 standard consists of a set of compiler directives for C, C++ and Fortran 1139 programs that allows a compiler to transform a sequential program into a 1140 parallel program. OpenMP is well suited for HPC applications and allows to 1141 work at a higher level compared to direct use of the POSIX threads API. While 1142 OpenMP ensures that the POSIX API is used correctly, OpenMP programs can still 1143 contain data races. So it definitely makes sense to verify OpenMP programs 1144 with a thread checking tool. 1145 </p> 1146 <p> 1147 DRD supports OpenMP shared-memory programs generated by GCC. GCC 1148 supports OpenMP since version 4.2.0. GCC's runtime support 1149 for OpenMP programs is provided by a library called 1150 <code class="literal">libgomp</code>. The synchronization primitives implemented 1151 in this library use Linux' futex system call directly, unless the 1152 library has been configured with the 1153 <code class="literal">--disable-linux-futex</code> option. DRD only supports 1154 libgomp libraries that have been configured with this option and in 1155 which symbol information is present. For most Linux distributions this 1156 means that you will have to recompile GCC. See also the script 1157 <code class="literal">drd/scripts/download-and-build-gcc</code> in the 1158 Valgrind source tree for an example of how to compile GCC. You will 1159 also have to make sure that the newly compiled 1160 <code class="literal">libgomp.so</code> library is loaded when OpenMP programs 1161 are started. This is possible by adding a line similar to the 1162 following to your shell startup script: 1163 </p> 1164 <pre class="programlisting"> 1165 export LD_LIBRARY_PATH=~/gcc-4.4.0/lib64:~/gcc-4.4.0/lib: 1166 </pre> 1167 <p> 1168 As an example, the test OpenMP test program 1169 <code class="literal">drd/tests/omp_matinv</code> triggers a data race 1170 when the option -r has been specified on the command line. The data 1171 race is triggered by the following code: 1172 </p> 1173 <pre class="programlisting"> 1174 #pragma omp parallel for private(j) 1175 for (j = 0; j < rows; j++) 1176 { 1177 if (i != j) 1178 { 1179 const elem_t factor = a[j * cols + i]; 1180 for (k = 0; k < cols; k++) 1181 { 1182 a[j * cols + k] -= a[i * cols + k] * factor; 1183 } 1184 } 1185 } 1186 </pre> 1187 <p> 1188 The above code is racy because the variable <code class="literal">k</code> has 1189 not been declared private. DRD will print the following error message 1190 for the above code: 1191 </p> 1192 <pre class="programlisting"> 1193 $ valgrind --tool=drd --check-stack-var=yes --read-var-info=yes drd/tests/omp_matinv 3 -t 2 -r 1194 ... 1195 Conflicting store by thread 1/1 at 0x7fefffbc4 size 4 1196 at 0x4014A0: gj.omp_fn.0 (omp_matinv.c:203) 1197 by 0x401211: gj (omp_matinv.c:159) 1198 by 0x40166A: invert_matrix (omp_matinv.c:238) 1199 by 0x4019B4: main (omp_matinv.c:316) 1200 Location 0x7fefffbc4 is 0 bytes inside local var "k" 1201 declared at omp_matinv.c:160, in frame #0 of thread 1 1202 ... 1203 </pre> 1204 <p> 1205 In the above output the function name <code class="function">gj.omp_fn.0</code> 1206 has been generated by GCC from the function name 1207 <code class="function">gj</code>. The allocation context information shows that the 1208 data race has been caused by modifying the variable <code class="literal">k</code>. 1209 </p> 1210 <p> 1211 Note: for GCC versions before 4.4.0, no allocation context information is 1212 shown. With these GCC versions the most usable information in the above output 1213 is the source file name and the line number where the data race has been 1214 detected (<code class="literal">omp_matinv.c:203</code>). 1215 </p> 1216 <p> 1217 For more information about OpenMP, see also 1218 <a class="ulink" href="http://openmp.org/" target="_top">openmp.org</a>. 1219 </p> 1220 </div> 1221 <div class="sect2"> 1222 <div class="titlepage"><div><div><h3 class="title"> 1223 <a name="drd-manual.cust-mem-alloc"></a>8.2.10.DRD and Custom Memory Allocators</h3></div></div></div> 1224 <p> 1225 DRD tracks all memory allocation events that happen via the 1226 standard memory allocation and deallocation functions 1227 (<code class="function">malloc</code>, <code class="function">free</code>, 1228 <code class="function">new</code> and <code class="function">delete</code>), via entry 1229 and exit of stack frames or that have been annotated with Valgrind's 1230 memory pool client requests. DRD uses memory allocation and deallocation 1231 information for two purposes: 1232 </p> 1233 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1234 <li class="listitem"><p> 1235 To know where the scope ends of POSIX objects that have not been 1236 destroyed explicitly. It is e.g. not required by the POSIX 1237 threads standard to call 1238 <code class="function">pthread_mutex_destroy</code> before freeing the 1239 memory in which a mutex object resides. 1240 </p></li> 1241 <li class="listitem"><p> 1242 To know where the scope of variables ends. If e.g. heap memory 1243 has been used by one thread, that thread frees that memory, and 1244 another thread allocates and starts using that memory, no data 1245 races must be reported for that memory. 1246 </p></li> 1247 </ul></div> 1248 <p> 1249 </p> 1250 <p> 1251 It is essential for correct operation of DRD that the tool knows about 1252 memory allocation and deallocation events. When analyzing a client program 1253 with DRD that uses a custom memory allocator, either instrument the custom 1254 memory allocator with the <code class="literal">VALGRIND_MALLOCLIKE_BLOCK</code> 1255 and <code class="literal">VALGRIND_FREELIKE_BLOCK</code> macros or disable the 1256 custom memory allocator. 1257 </p> 1258 <p> 1259 As an example, the GNU libstdc++ library can be configured 1260 to use standard memory allocation functions instead of memory pools by 1261 setting the environment variable 1262 <code class="literal">GLIBCXX_FORCE_NEW</code>. For more information, see also 1263 the <a class="ulink" href="http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt04ch11.html" target="_top">libstdc++ 1264 manual</a>. 1265 </p> 1266 </div> 1267 <div class="sect2"> 1268 <div class="titlepage"><div><div><h3 class="title"> 1269 <a name="drd-manual.drd-versus-memcheck"></a>8.2.11.DRD Versus Memcheck</h3></div></div></div> 1270 <p> 1271 It is essential for correct operation of DRD that there are no memory 1272 errors such as dangling pointers in the client program. Which means that 1273 it is a good idea to make sure that your program is Memcheck-clean 1274 before you analyze it with DRD. It is possible however that some of 1275 the Memcheck reports are caused by data races. In this case it makes 1276 sense to run DRD before Memcheck. 1277 </p> 1278 <p> 1279 So which tool should be run first? In case both DRD and Memcheck 1280 complain about a program, a possible approach is to run both tools 1281 alternatingly and to fix as many errors as possible after each run of 1282 each tool until none of the two tools prints any more error messages. 1283 </p> 1284 </div> 1285 <div class="sect2"> 1286 <div class="titlepage"><div><div><h3 class="title"> 1287 <a name="drd-manual.resource-requirements"></a>8.2.12.Resource Requirements</h3></div></div></div> 1288 <p> 1289 The requirements of DRD with regard to heap and stack memory and the 1290 effect on the execution time of client programs are as follows: 1291 </p> 1292 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1293 <li class="listitem"><p> 1294 When running a program under DRD with default DRD options, 1295 between 1.1 and 3.6 times more memory will be needed compared to 1296 a native run of the client program. More memory will be needed 1297 if loading debug information has been enabled 1298 (<code class="literal">--read-var-info=yes</code>). 1299 </p></li> 1300 <li class="listitem"><p> 1301 DRD allocates some of its temporary data structures on the stack 1302 of the client program threads. This amount of data is limited to 1303 1 - 2 KB. Make sure that thread stacks are sufficiently large. 1304 </p></li> 1305 <li class="listitem"><p> 1306 Most applications will run between 20 and 50 times slower under 1307 DRD than a native single-threaded run. The slowdown will be most 1308 noticeable for applications which perform frequent mutex lock / 1309 unlock operations. 1310 </p></li> 1311 </ul></div> 1312 <p> 1313 </p> 1314 </div> 1315 <div class="sect2"> 1316 <div class="titlepage"><div><div><h3 class="title"> 1317 <a name="drd-manual.effective-use"></a>8.2.13.Hints and Tips for Effective Use of DRD</h3></div></div></div> 1318 <p> 1319 The following information may be helpful when using DRD: 1320 </p> 1321 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1322 <li class="listitem"><p> 1323 Make sure that debug information is present in the executable 1324 being analyzed, such that DRD can print function name and line 1325 number information in stack traces. Most compilers can be told 1326 to include debug information via compiler option 1327 <code class="option">-g</code>. 1328 </p></li> 1329 <li class="listitem"><p> 1330 Compile with option <code class="option">-O1</code> instead of 1331 <code class="option">-O0</code>. This will reduce the amount of generated 1332 code, may reduce the amount of debug info and will speed up 1333 DRD's processing of the client program. For more information, 1334 see also <a class="xref" href="manual-core.html#manual-core.started" title="2.2.Getting started">Getting started</a>. 1335 </p></li> 1336 <li class="listitem"><p> 1337 If DRD reports any errors on libraries that are part of your 1338 Linux distribution like e.g. <code class="literal">libc.so</code> or 1339 <code class="literal">libstdc++.so</code>, installing the debug packages 1340 for these libraries will make the output of DRD a lot more 1341 detailed. 1342 </p></li> 1343 <li class="listitem"> 1344 <p> 1345 When using C++, do not send output from more than one thread to 1346 <code class="literal">std::cout</code>. Doing so would not only 1347 generate multiple data race reports, it could also result in 1348 output from several threads getting mixed up. Either use 1349 <code class="function">printf</code> or do the following: 1350 </p> 1351 <div class="orderedlist"><ol class="orderedlist" type="1"> 1352 <li class="listitem"><p>Derive a class from <code class="literal">std::ostreambuf</code> 1353 and let that class send output line by line to 1354 <code class="literal">stdout</code>. This will avoid that individual 1355 lines of text produced by different threads get mixed 1356 up.</p></li> 1357 <li class="listitem"><p>Create one instance of <code class="literal">std::ostream</code> 1358 for each thread. This makes stream formatting settings 1359 thread-local. Pass a per-thread instance of the class 1360 derived from <code class="literal">std::ostreambuf</code> to the 1361 constructor of each instance. </p></li> 1362 <li class="listitem"><p>Let each thread send its output to its own instance of 1363 <code class="literal">std::ostream</code> instead of 1364 <code class="literal">std::cout</code>.</p></li> 1365 </ol></div> 1366 <p> 1367 </p> 1368 </li> 1369 </ul></div> 1370 <p> 1371 </p> 1372 </div> 1373 </div> 1374 <div class="sect1"> 1375 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> 1376 <a name="drd-manual.Pthreads"></a>8.3.Using the POSIX Threads API Effectively</h2></div></div></div> 1377 <div class="sect2"> 1378 <div class="titlepage"><div><div><h3 class="title"> 1379 <a name="drd-manual.mutex-types"></a>8.3.1.Mutex types</h3></div></div></div> 1380 <p> 1381 The Single UNIX Specification version two defines the following four 1382 mutex types (see also the documentation of <a class="ulink" href="http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_mutexattr_settype.html" target="_top"><code class="function">pthread_mutexattr_settype</code></a>): 1383 </p> 1384 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1385 <li class="listitem"><p> 1386 <span class="emphasis"><em>normal</em></span>, which means that no error checking 1387 is performed, and that the mutex is non-recursive. 1388 </p></li> 1389 <li class="listitem"><p> 1390 <span class="emphasis"><em>error checking</em></span>, which means that the mutex 1391 is non-recursive and that error checking is performed. 1392 </p></li> 1393 <li class="listitem"><p> 1394 <span class="emphasis"><em>recursive</em></span>, which means that a mutex may be 1395 locked recursively. 1396 </p></li> 1397 <li class="listitem"><p> 1398 <span class="emphasis"><em>default</em></span>, which means that error checking 1399 behavior is undefined, and that the behavior for recursive 1400 locking is also undefined. Or: portable code must neither 1401 trigger error conditions through the Pthreads API nor attempt to 1402 lock a mutex of default type recursively. 1403 </p></li> 1404 </ul></div> 1405 <p> 1406 </p> 1407 <p> 1408 In complex applications it is not always clear from beforehand which 1409 mutex will be locked recursively and which mutex will not be locked 1410 recursively. Attempts lock a non-recursive mutex recursively will 1411 result in race conditions that are very hard to find without a thread 1412 checking tool. So either use the error checking mutex type and 1413 consistently check the return value of Pthread API mutex calls, or use 1414 the recursive mutex type. 1415 </p> 1416 </div> 1417 <div class="sect2"> 1418 <div class="titlepage"><div><div><h3 class="title"> 1419 <a name="drd-manual.condvar"></a>8.3.2.Condition variables</h3></div></div></div> 1420 <p> 1421 A condition variable allows one thread to wake up one or more other 1422 threads. Condition variables are often used to notify one or more 1423 threads about state changes of shared data. Unfortunately it is very 1424 easy to introduce race conditions by using condition variables as the 1425 only means of state information propagation. A better approach is to 1426 let threads poll for changes of a state variable that is protected by 1427 a mutex, and to use condition variables only as a thread wakeup 1428 mechanism. See also the source file 1429 <code class="computeroutput">drd/tests/monitor_example.cpp</code> for an 1430 example of how to implement this concept in C++. The monitor concept 1431 used in this example is a well known and very useful concept -- see 1432 also Wikipedia for more information about the <a class="ulink" href="http://en.wikipedia.org/wiki/Monitor_(synchronization)" target="_top">monitor</a> 1433 concept. 1434 </p> 1435 </div> 1436 <div class="sect2"> 1437 <div class="titlepage"><div><div><h3 class="title"> 1438 <a name="drd-manual.pctw"></a>8.3.3.pthread_cond_timedwait and timeouts</h3></div></div></div> 1439 <p> 1440 Historically the function 1441 <code class="function">pthread_cond_timedwait</code> only allowed the 1442 specification of an absolute timeout, that is a timeout independent of 1443 the time when this function was called. However, almost every call to 1444 this function expresses a relative timeout. This typically happens by 1445 passing the sum of 1446 <code class="computeroutput">clock_gettime(CLOCK_REALTIME)</code> and a 1447 relative timeout as the third argument. This approach is incorrect 1448 since forward or backward clock adjustments by e.g. ntpd will affect 1449 the timeout. A more reliable approach is as follows: 1450 </p> 1451 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1452 <li class="listitem"><p> 1453 When initializing a condition variable through 1454 <code class="function">pthread_cond_init</code>, specify that the timeout of 1455 <code class="function">pthread_cond_timedwait</code> will use the clock 1456 <code class="literal">CLOCK_MONOTONIC</code> instead of 1457 <code class="literal">CLOCK_REALTIME</code>. You can do this via 1458 <code class="computeroutput">pthread_condattr_setclock(..., 1459 CLOCK_MONOTONIC)</code>. 1460 </p></li> 1461 <li class="listitem"><p> 1462 When calling <code class="function">pthread_cond_timedwait</code>, pass 1463 the sum of 1464 <code class="computeroutput">clock_gettime(CLOCK_MONOTONIC)</code> 1465 and a relative timeout as the third argument. 1466 </p></li> 1467 </ul></div> 1468 <p> 1469 See also 1470 <code class="computeroutput">drd/tests/monitor_example.cpp</code> for an 1471 example. 1472 </p> 1473 </div> 1474 </div> 1475 <div class="sect1"> 1476 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> 1477 <a name="drd-manual.limitations"></a>8.4.Limitations</h2></div></div></div> 1478 <p>DRD currently has the following limitations:</p> 1479 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1480 <li class="listitem"><p> 1481 DRD, just like Memcheck, will refuse to start on Linux 1482 distributions where all symbol information has been removed from 1483 <code class="filename">ld.so</code>. This is e.g. the case for the PPC editions 1484 of openSUSE and Gentoo. You will have to install the glibc debuginfo 1485 package on these platforms before you can use DRD. See also openSUSE 1486 bug <a class="ulink" href="http://bugzilla.novell.com/show_bug.cgi?id=396197" target="_top"> 1487 396197</a> and Gentoo bug <a class="ulink" href="http://bugs.gentoo.org/214065" target="_top">214065</a>. 1488 </p></li> 1489 <li class="listitem"><p> 1490 With gcc 4.4.3 and before, DRD may report data races on the C++ 1491 class <code class="literal">std::string</code> in a multithreaded program. This is 1492 a know <code class="literal">libstdc++</code> issue -- see also GCC bug 1493 <a class="ulink" href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40518" target="_top">40518</a> 1494 for more information. 1495 </p></li> 1496 <li class="listitem"><p> 1497 If you compile the DRD source code yourself, you need GCC 3.0 or 1498 later. GCC 2.95 is not supported. 1499 </p></li> 1500 <li class="listitem"><p> 1501 Of the two POSIX threads implementations for Linux, only the 1502 NPTL (Native POSIX Thread Library) is supported. The older 1503 LinuxThreads library is not supported. 1504 </p></li> 1505 </ul></div> 1506 </div> 1507 <div class="sect1"> 1508 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> 1509 <a name="drd-manual.feedback"></a>8.5.Feedback</h2></div></div></div> 1510 <p> 1511 If you have any comments, suggestions, feedback or bug reports about 1512 DRD, feel free to either post a message on the Valgrind users mailing 1513 list or to file a bug report. See also <a class="ulink" href="http://www.valgrind.org/" target="_top">http://www.valgrind.org/</a> for more information. 1514 </p> 1515 </div> 1516 </div> 1517 <div> 1518 <br><table class="nav" width="100%" cellspacing="3" cellpadding="2" border="0" summary="Navigation footer"> 1519 <tr> 1520 <td rowspan="2" width="40%" align="left"> 1521 <a accesskey="p" href="hg-manual.html"><<7.Helgrind: a thread error detector</a></td> 1522 <td width="20%" align="center"><a accesskey="u" href="manual.html">Up</a></td> 1523 <td rowspan="2" width="40%" align="right"><a accesskey="n" href="ms-manual.html">9.Massif: a heap profiler>></a> 1524 </td> 1525 </tr> 1526 <tr><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td></tr> 1527 </table> 1528 </div> 1529 </body> 1530 </html> 1531
