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<chapter id="drd-manual" xreflabel="DRD: a thread error detector">
<title>DRD: a thread error detector</title>
<para>To use this tool, you must specify
<option>--tool=drd</option>
on the Valgrind command line.</para>
<sect1 id="drd-manual.overview" xreflabel="Overview">
<title>Overview</title>
<para>
DRD is a Valgrind tool for detecting errors in multithreaded C and C++
programs. The tool works for any program that uses the POSIX threading
primitives or that uses threading concepts built on top of the POSIX threading
primitives.
</para>
<sect2 id="drd-manual.mt-progr-models" xreflabel="MT-progr-models">
<title>Multithreaded Programming Paradigms</title>
<para>
There are two possible reasons for using multithreading in a program:
<itemizedlist>
<listitem>
<para>
To model concurrent activities. Assigning one thread to each activity
can be a great simplification compared to multiplexing the states of
multiple activities in a single thread. This is why most server software
and embedded software is multithreaded.
</para>
</listitem>
<listitem>
<para>
To use multiple CPU cores simultaneously for speeding up
computations. This is why many High Performance Computing (HPC)
applications are multithreaded.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Multithreaded programs can use one or more of the following programming
paradigms. Which paradigm is appropriate depends e.g. on the application type.
Some examples of multithreaded programming paradigms are:
<itemizedlist>
<listitem>
<para>
Locking. Data that is shared over threads is protected from concurrent
accesses via locking. E.g. the POSIX threads library, the Qt library
and the Boost.Thread library support this paradigm directly.
</para>
</listitem>
<listitem>
<para>
Message passing. No data is shared between threads, but threads exchange
data by passing messages to each other. Examples of implementations of
the message passing paradigm are MPI and CORBA.
</para>
</listitem>
<listitem>
<para>
Automatic parallelization. A compiler converts a sequential program into
a multithreaded program. The original program may or may not contain
parallelization hints. One example of such parallelization hints is the
OpenMP standard. In this standard a set of directives are defined which
tell a compiler how to parallelize a C, C++ or Fortran program. OpenMP
is well suited for computational intensive applications. As an example,
an open source image processing software package is using OpenMP to
maximize performance on systems with multiple CPU
cores. GCC supports the
OpenMP standard from version 4.2.0 on.
</para>
</listitem>
<listitem>
<para>
Software Transactional Memory (STM). Any data that is shared between
threads is updated via transactions. After each transaction it is
verified whether there were any conflicting transactions. If there were
conflicts, the transaction is aborted, otherwise it is committed. This
is a so-called optimistic approach. There is a prototype of the Intel C++
Compiler available that supports STM. Research about the addition of
STM support to GCC is ongoing.
</para>
</listitem>
</itemizedlist>
</para>
<para>
DRD supports any combination of multithreaded programming paradigms as
long as the implementation of these paradigms is based on the POSIX
threads primitives. DRD however does not support programs that use
e.g. Linux' futexes directly. Attempts to analyze such programs with
DRD will cause DRD to report many false positives.
</para>
</sect2>
<sect2 id="drd-manual.pthreads-model" xreflabel="Pthreads-model">
<title>POSIX Threads Programming Model</title>
<para>
POSIX threads, also known as Pthreads, is the most widely available
threading library on Unix systems.
</para>
<para>
The POSIX threads programming model is based on the following abstractions:
<itemizedlist>
<listitem>
<para>
A shared address space. All threads running within the same
process share the same address space. All data, whether shared or
not, is identified by its address.
</para>
</listitem>
<listitem>
<para>
Regular load and store operations, which allow to read values
from or to write values to the memory shared by all threads
running in the same process.
</para>
</listitem>
<listitem>
<para>
Atomic store and load-modify-store operations. While these are
not mentioned in the POSIX threads standard, most
microprocessors support atomic memory operations.
</para>
</listitem>
<listitem>
<para>
Threads. Each thread represents a concurrent activity.
</para>
</listitem>
<listitem>
<para>
Synchronization objects and operations on these synchronization
objects. The following types of synchronization objects have been
defined in the POSIX threads standard: mutexes, condition variables,
semaphores, reader-writer synchronization objects, barriers and
spinlocks.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Which source code statements generate which memory accesses depends on
the <emphasis>memory model</emphasis> of the programming language being
used. There is not yet a definitive memory model for the C and C++
languages. For a draft memory model, see also the document
<ulink url="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2338.html">
WG21/N2338: Concurrency memory model compiler consequences</ulink>.
</para>
<para>
For more information about POSIX threads, see also the Single UNIX
Specification version 3, also known as
<ulink url="http://www.opengroup.org/onlinepubs/000095399/idx/threads.html">
IEEE Std 1003.1</ulink>.
</para>
</sect2>
<sect2 id="drd-manual.mt-problems" xreflabel="MT-Problems">
<title>Multithreaded Programming Problems</title>
<para>
Depending on which multithreading paradigm is being used in a program,
one or more of the following problems can occur:
<itemizedlist>
<listitem>
<para>
Data races. One or more threads access the same memory location without
sufficient locking. Most but not all data races are programming errors
and are the cause of subtle and hard-to-find bugs.
</para>
</listitem>
<listitem>
<para>
Lock contention. One thread blocks the progress of one or more other
threads by holding a lock too long.
</para>
</listitem>
<listitem>
<para>
Improper use of the POSIX threads API. Most implementations of the POSIX
threads API have been optimized for runtime speed. Such implementations
will not complain on certain errors, e.g. when a mutex is being unlocked
by another thread than the thread that obtained a lock on the mutex.
</para>
</listitem>
<listitem>
<para>
Deadlock. A deadlock occurs when two or more threads wait for
each other indefinitely.
</para>
</listitem>
<listitem>
<para>
False sharing. If threads that run on different processor cores
access different variables located in the same cache line
frequently, this will slow down the involved threads a lot due
to frequent exchange of cache lines.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Although the likelihood of the occurrence of data races can be reduced
through a disciplined programming style, a tool for automatic
detection of data races is a necessity when developing multithreaded
software. DRD can detect these, as well as lock contention and
improper use of the POSIX threads API.
</para>
</sect2>
<sect2 id="drd-manual.data-race-detection" xreflabel="data-race-detection">
<title>Data Race Detection</title>
<para>
The result of load and store operations performed by a multithreaded program
depends on the order in which memory operations are performed. This order is
determined by:
<orderedlist>
<listitem>
<para>
All memory operations performed by the same thread are performed in
<emphasis>program order</emphasis>, that is, the order determined by the
program source code and the results of previous load operations.
</para>
</listitem>
<listitem>
<para>
Synchronization operations determine certain ordering constraints on
memory operations performed by different threads. These ordering
constraints are called the <emphasis>synchronization order</emphasis>.
</para>
</listitem>
</orderedlist>
The combination of program order and synchronization order is called the
<emphasis>happens-before relationship</emphasis>. This concept was first
defined by S. Adve et al in the paper <emphasis>Detecting data races on weak
memory systems</emphasis>, ACM SIGARCH Computer Architecture News, v.19 n.3,
p.234-243, May 1991.
</para>
<para>
Two memory operations <emphasis>conflict</emphasis> if both operations are
performed by different threads, refer to the same memory location and at least
one of them is a store operation.
</para>
<para>
A multithreaded program is <emphasis>data-race free</emphasis> if all
conflicting memory accesses are ordered by synchronization
operations.
</para>
<para>
A well known way to ensure that a multithreaded program is data-race
free is to ensure that a locking discipline is followed. It is e.g.
possible to associate a mutex with each shared data item, and to hold
a lock on the associated mutex while the shared data is accessed.
</para>
<para>
All programs that follow a locking discipline are data-race free, but not all
data-race free programs follow a locking discipline. There exist multithreaded
programs where access to shared data is arbitrated via condition variables,
semaphores or barriers. As an example, a certain class of HPC applications
consists of a sequence of computation steps separated in time by barriers, and
where these barriers are the only means of synchronization. Although there are
many conflicting memory accesses in such applications and although such
applications do not make use mutexes, most of these applications do not
contain data races.
</para>
<para>
There exist two different approaches for verifying the correctness of
multithreaded programs at runtime. The approach of the so-called Eraser
algorithm is to verify whether all shared memory accesses follow a consistent
locking strategy. And the happens-before data race detectors verify directly
whether all interthread memory accesses are ordered by synchronization
operations. While the last approach is more complex to implement, and while it
is more sensitive to OS scheduling, it is a general approach that works for
all classes of multithreaded programs. An important advantage of
happens-before data race detectors is that these do not report any false
positives.
</para>
<para>
DRD is based on the happens-before algorithm.
</para>
</sect2>
</sect1>
<sect1 id="drd-manual.using-drd" xreflabel="Using DRD">
<title>Using DRD</title>
<sect2 id="drd-manual.options" xreflabel="DRD Command-line Options">
<title>DRD Command-line Options</title>
<para>The following command-line options are available for controlling the
behavior of the DRD tool itself:</para>
<!-- start of xi:include in the manpage -->
<variablelist id="drd.opts.list">
<varlistentry>
<term>
<option><![CDATA[--check-stack-var=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Controls whether DRD detects data races on stack
variables. Verifying stack variables is disabled by default because
most programs do not share stack variables over threads.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--exclusive-threshold=<n> [default: off]]]></option>
</term>
<listitem>
<para>
Print an error message if any mutex or writer lock has been
held longer than the time specified in milliseconds. This
option enables the detection of lock contention.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--join-list-vol=<n> [default: 10]]]></option>
</term>
<listitem>
<para>
Data races that occur between a statement at the end of one thread
and another thread can be missed if memory access information is
discarded immediately after a thread has been joined. This option
allows to specify for how many joined threads memory access information
should be retained.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>
<![CDATA[--first-race-only=<yes|no> [default: no]]]>
</option>
</term>
<listitem>
<para>
Whether to report only the first data race that has been detected on a
memory location or all data races that have been detected on a memory
location.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>
<![CDATA[--free-is-write=<yes|no> [default: no]]]>
</option>
</term>
<listitem>
<para>
Whether to report races between accessing memory and freeing
memory. Enabling this option may cause DRD to run slightly
slower. Notes:</para>
<itemizedlist>
<listitem>
<para>
Don't enable this option when using custom memory allocators
that use
the <computeroutput>VG_USERREQ__MALLOCLIKE_BLOCK</computeroutput>
and <computeroutput>VG_USERREQ__FREELIKE_BLOCK</computeroutput>
because that would result in false positives.
</para>
</listitem>
<listitem>
<para>Don't enable this option when using reference-counted
objects because that will result in false positives, even when
that code has been annotated properly with
<computeroutput>ANNOTATE_HAPPENS_BEFORE</computeroutput>
and <computeroutput>ANNOTATE_HAPPENS_AFTER</computeroutput>. See
e.g. the output of the following command for an example:
<computeroutput>valgrind --tool=drd --free-is-write=yes
drd/tests/annotate_smart_pointer</computeroutput>.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>
<![CDATA[--report-signal-unlocked=<yes|no> [default: yes]]]>
</option>
</term>
<listitem>
<para>
Whether to report calls to
<function>pthread_cond_signal</function> and
<function>pthread_cond_broadcast</function> where the mutex
associated with the signal through
<function>pthread_cond_wait</function> or
<function>pthread_cond_timed_wait</function>is not locked at
the time the signal is sent. Sending a signal without holding
a lock on the associated mutex is a common programming error
which can cause subtle race conditions and unpredictable
behavior. There exist some uncommon synchronization patterns
however where it is safe to send a signal without holding a
lock on the associated mutex.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--segment-merging=<yes|no> [default: yes]]]></option>
</term>
<listitem>
<para>
Controls segment merging. Segment merging is an algorithm to
limit memory usage of the data race detection
algorithm. Disabling segment merging may improve the accuracy
of the so-called 'other segments' displayed in race reports
but can also trigger an out of memory error.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--segment-merging-interval=<n> [default: 10]]]></option>
</term>
<listitem>
<para>
Perform segment merging only after the specified number of new
segments have been created. This is an advanced configuration option
that allows to choose whether to minimize DRD's memory usage by
choosing a low value or to let DRD run faster by choosing a slightly
higher value. The optimal value for this parameter depends on the
program being analyzed. The default value works well for most programs.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--shared-threshold=<n> [default: off]]]></option>
</term>
<listitem>
<para>
Print an error message if a reader lock has been held longer
than the specified time (in milliseconds). This option enables
the detection of lock contention.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--show-confl-seg=<yes|no> [default: yes]]]></option>
</term>
<listitem>
<para>
Show conflicting segments in race reports. Since this
information can help to find the cause of a data race, this
option is enabled by default. Disabling this option makes the
output of DRD more compact.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--show-stack-usage=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Print stack usage at thread exit time. When a program creates a large
number of threads it becomes important to limit the amount of virtual
memory allocated for thread stacks. This option makes it possible to
observe how much stack memory has been used by each thread of the
client program. Note: the DRD tool itself allocates some temporary
data on the client thread stack. The space necessary for this
temporary data must be allocated by the client program when it
allocates stack memory, but is not included in stack usage reported by
DRD.
</para>
</listitem>
</varlistentry>
</variablelist>
<!-- end of xi:include in the manpage -->
<!-- start of xi:include in the manpage -->
<para>
The following options are available for monitoring the behavior of the
client program:
</para>
<variablelist id="drd.debugopts.list">
<varlistentry>
<term>
<option><![CDATA[--trace-addr=<address> [default: none]]]></option>
</term>
<listitem>
<para>
Trace all load and store activity for the specified
address. This option may be specified more than once.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--ptrace-addr=<address> [default: none]]]></option>
</term>
<listitem>
<para>
Trace all load and store activity for the specified address and keep
doing that even after the memory at that address has been freed and
reallocated.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--trace-alloc=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Trace all memory allocations and deallocations. May produce a huge
amount of output.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--trace-barrier=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Trace all barrier activity.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--trace-cond=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Trace all condition variable activity.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--trace-fork-join=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Trace all thread creation and all thread termination events.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--trace-hb=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Trace execution of the <literal>ANNOTATE_HAPPENS_BEFORE()</literal>,
<literal>ANNOTATE_HAPPENS_AFTER()</literal> and
<literal>ANNOTATE_HAPPENS_DONE()</literal> client requests.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--trace-mutex=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Trace all mutex activity.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--trace-rwlock=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Trace all reader-writer lock activity.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option><![CDATA[--trace-semaphore=<yes|no> [default: no]]]></option>
</term>
<listitem>
<para>
Trace all semaphore activity.
</para>
</listitem>
</varlistentry>
</variablelist>
<!-- end of xi:include in the manpage -->
</sect2>
<sect2 id="drd-manual.data-races" xreflabel="Data Races">
<title>Detected Errors: Data Races</title>
<para>
DRD prints a message every time it detects a data race. Please keep
the following in mind when interpreting DRD's output:
<itemizedlist>
<listitem>
<para>
Every thread is assigned a <emphasis>thread ID</emphasis> by the DRD
tool. A thread ID is a number. Thread ID's start at one and are never
recycled.
</para>
</listitem>
<listitem>
<para>
The term <emphasis>segment</emphasis> refers to a consecutive
sequence of load, store and synchronization operations, all
issued by the same thread. A segment always starts and ends at a
synchronization operation. Data race analysis is performed
between segments instead of between individual load and store
operations because of performance reasons.
</para>
</listitem>
<listitem>
<para>
There are always at least two memory accesses involved in a data
race. Memory accesses involved in a data race are called
<emphasis>conflicting memory accesses</emphasis>. DRD prints a
report for each memory access that conflicts with a past memory
access.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Below you can find an example of a message printed by DRD when it
detects a data race:
</para>
<programlisting><![CDATA[
$ valgrind --tool=drd --read-var-info=yes drd/tests/rwlock_race
...
==9466== Thread 3:
==9466== Conflicting load by thread 3 at 0x006020b8 size 4
==9466== at 0x400B6C: thread_func (rwlock_race.c:29)
==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
==9466== Location 0x6020b8 is 0 bytes inside local var "s_racy"
==9466== declared at rwlock_race.c:18, in frame #0 of thread 3
==9466== Other segment start (thread 2)
==9466== at 0x4C2847D: pthread_rwlock_rdlock* (drd_pthread_intercepts.c:813)
==9466== by 0x400B6B: thread_func (rwlock_race.c:28)
==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
==9466== Other segment end (thread 2)
==9466== at 0x4C28B54: pthread_rwlock_unlock* (drd_pthread_intercepts.c:912)
==9466== by 0x400B84: thread_func (rwlock_race.c:30)
==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
...
]]></programlisting>
<para>
The above report has the following meaning:
<itemizedlist>
<listitem>
<para>
The number in the column on the left is the process ID of the
process being analyzed by DRD.
</para>
</listitem>
<listitem>
<para>
The first line ("Thread 3") tells you the thread ID for
the thread in which context the data race has been detected.
</para>
</listitem>
<listitem>
<para>
The next line tells which kind of operation was performed (load or
store) and by which thread. On the same line the start address and the
number of bytes involved in the conflicting access are also displayed.
</para>
</listitem>
<listitem>
<para>
Next, the call stack of the conflicting access is displayed. If
your program has been compiled with debug information
(<option>-g</option>), this call stack will include file names and
line numbers. The two
bottommost frames in this call stack (<function>clone</function>
and <function>start_thread</function>) show how the NPTL starts
a thread. The third frame
(<function>vg_thread_wrapper</function>) is added by DRD. The
fourth frame (<function>thread_func</function>) is the first
interesting line because it shows the thread entry point, that
is the function that has been passed as the third argument to
<function>pthread_create</function>.
</para>
</listitem>
<listitem>
<para>
Next, the allocation context for the conflicting address is
displayed. For dynamically allocated data the allocation call
stack is shown. For static variables and stack variables the
allocation context is only shown when the option
<option>--read-var-info=yes</option> has been
specified. Otherwise DRD will print <computeroutput>Allocation
context: unknown</computeroutput>.
</para>
</listitem>
<listitem>
<para>
A conflicting access involves at least two memory accesses. For
one of these accesses an exact call stack is displayed, and for
the other accesses an approximate call stack is displayed,
namely the start and the end of the segments of the other
accesses. This information can be interpreted as follows:
<orderedlist>
<listitem>
<para>
Start at the bottom of both call stacks, and count the
number stack frames with identical function name, file
name and line number. In the above example the three
bottommost frames are identical
(<function>clone</function>,
<function>start_thread</function> and
<function>vg_thread_wrapper</function>).
</para>
</listitem>
<listitem>
<para>
The next higher stack frame in both call stacks now tells
you between in which source code region the other memory
access happened. The above output tells that the other
memory access involved in the data race happened between
source code lines 28 and 30 in file
<computeroutput>rwlock_race.c</computeroutput>.
</para>
</listitem>
</orderedlist>
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
<sect2 id="drd-manual.lock-contention" xreflabel="Lock Contention">
<title>Detected Errors: Lock Contention</title>
<para>
Threads must be able to make progress without being blocked for too long by
other threads. Sometimes a thread has to wait until a mutex or reader-writer
synchronization object is unlocked by another thread. This is called
<emphasis>lock contention</emphasis>.
</para>
<para>
Lock contention causes delays. Such delays should be as short as
possible. The two command line options
<literal>--exclusive-threshold=<n></literal> and
<literal>--shared-threshold=<n></literal> make it possible to
detect excessive lock contention by making DRD report any lock that
has been held longer than the specified threshold. An example:
</para>
<programlisting><![CDATA[
$ valgrind --tool=drd --exclusive-threshold=10 drd/tests/hold_lock -i 500
...
==10668== Acquired at:
==10668== at 0x4C267C8: pthread_mutex_lock (drd_pthread_intercepts.c:395)
==10668== by 0x400D92: main (hold_lock.c:51)
==10668== Lock on mutex 0x7fefffd50 was held during 503 ms (threshold: 10 ms).
==10668== at 0x4C26ADA: pthread_mutex_unlock (drd_pthread_intercepts.c:441)
==10668== by 0x400DB5: main (hold_lock.c:55)
...
]]></programlisting>
<para>
The <literal>hold_lock</literal> test program holds a lock as long as
specified by the <literal>-i</literal> (interval) argument. The DRD
output reports that the lock acquired at line 51 in source file
<literal>hold_lock.c</literal> and released at line 55 was held during
503 ms, while a threshold of 10 ms was specified to DRD.
</para>
</sect2>
<sect2 id="drd-manual.api-checks" xreflabel="API Checks">
<title>Detected Errors: Misuse of the POSIX threads API</title>
<para>
DRD is able to detect and report the following misuses of the POSIX
threads API:
<itemizedlist>
<listitem>
<para>
Passing the address of one type of synchronization object
(e.g. a mutex) to a POSIX API call that expects a pointer to
another type of synchronization object (e.g. a condition
variable).
</para>
</listitem>
<listitem>
<para>
Attempts to unlock a mutex that has not been locked.
</para>
</listitem>
<listitem>
<para>
Attempts to unlock a mutex that was locked by another thread.
</para>
</listitem>
<listitem>
<para>
Attempts to lock a mutex of type
<literal>PTHREAD_MUTEX_NORMAL</literal> or a spinlock
recursively.
</para>
</listitem>
<listitem>
<para>
Destruction or deallocation of a locked mutex.
</para>
</listitem>
<listitem>
<para>
Sending a signal to a condition variable while no lock is held
on the mutex associated with the condition variable.
</para>
</listitem>
<listitem>
<para>
Calling <function>pthread_cond_wait</function> on a mutex
that is not locked, that is locked by another thread or that
has been locked recursively.
</para>
</listitem>
<listitem>
<para>
Associating two different mutexes with a condition variable
through <function>pthread_cond_wait</function>.
</para>
</listitem>
<listitem>
<para>
Destruction or deallocation of a condition variable that is
being waited upon.
</para>
</listitem>
<listitem>
<para>
Destruction or deallocation of a locked reader-writer synchronization
object.
</para>
</listitem>
<listitem>
<para>
Attempts to unlock a reader-writer synchronization object that was not
locked by the calling thread.
</para>
</listitem>
<listitem>
<para>
Attempts to recursively lock a reader-writer synchronization object
exclusively.
</para>
</listitem>
<listitem>
<para>
Attempts to pass the address of a user-defined reader-writer
synchronization object to a POSIX threads function.
</para>
</listitem>
<listitem>
<para>
Attempts to pass the address of a POSIX reader-writer synchronization
object to one of the annotations for user-defined reader-writer
synchronization objects.
</para>
</listitem>
<listitem>
<para>
Reinitialization of a mutex, condition variable, reader-writer
lock, semaphore or barrier.
</para>
</listitem>
<listitem>
<para>
Destruction or deallocation of a semaphore or barrier that is
being waited upon.
</para>
</listitem>
<listitem>
<para>
Missing synchronization between barrier wait and barrier destruction.
</para>
</listitem>
<listitem>
<para>
Exiting a thread without first unlocking the spinlocks, mutexes or
reader-writer synchronization objects that were locked by that thread.
</para>
</listitem>
<listitem>
<para>
Passing an invalid thread ID to <function>pthread_join</function>
or <function>pthread_cancel</function>.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
<sect2 id="drd-manual.clientreqs" xreflabel="Client requests">
<title>Client Requests</title>
<para>
Just as for other Valgrind tools it is possible to let a client program
interact with the DRD tool through client requests. In addition to the
client requests several macros have been defined that allow to use the
client requests in a convenient way.
</para>
<para>
The interface between client programs and the DRD tool is defined in
the header file <literal><valgrind/drd.h></literal>. The
available macros and client requests are:
<itemizedlist>
<listitem>
<para>
The macro <literal>DRD_GET_VALGRIND_THREADID</literal> and the
corresponding client
request <varname>VG_USERREQ__DRD_GET_VALGRIND_THREAD_ID</varname>.
Query the thread ID that has been assigned by the Valgrind core to the
thread executing this client request. Valgrind's thread ID's start at
one and are recycled in case a thread stops.
</para>
</listitem>
<listitem>
<para>
The macro <literal>DRD_GET_DRD_THREADID</literal> and the corresponding
client request <varname>VG_USERREQ__DRD_GET_DRD_THREAD_ID</varname>.
Query the thread ID that has been assigned by DRD to the thread
executing this client request. These are the thread ID's reported by DRD
in data race reports and in trace messages. DRD's thread ID's start at
one and are never recycled.
</para>
</listitem>
<listitem>
<para>
The macros <literal>DRD_IGNORE_VAR(x)</literal>,
<literal>ANNOTATE_TRACE_MEMORY(&x)</literal> and the corresponding
client request <varname>VG_USERREQ__DRD_START_SUPPRESSION</varname>. Some
applications contain intentional races. There exist e.g. applications
where the same value is assigned to a shared variable from two different
threads. It may be more convenient to suppress such races than to solve
these. This client request allows to suppress such races.
</para>
</listitem>
<listitem>
<para>
The macro <literal>DRD_STOP_IGNORING_VAR(x)</literal> and the
corresponding client request
<varname>VG_USERREQ__DRD_FINISH_SUPPRESSION</varname>. Tell DRD
to no longer ignore data races for the address range that was suppressed
either via the macro <literal>DRD_IGNORE_VAR(x)</literal> or via the
client request <varname>VG_USERREQ__DRD_START_SUPPRESSION</varname>.
</para>
</listitem>
<listitem>
<para>
The macro <literal>DRD_TRACE_VAR(x)</literal>. Trace all load and store
activity for the address range starting at <literal>&x</literal> and
occupying <literal>sizeof(x)</literal> bytes. When DRD reports a data
race on a specified variable, and it's not immediately clear which
source code statements triggered the conflicting accesses, it can be
very helpful to trace all activity on the offending memory location.
</para>
</listitem>
<listitem>
<para>
The macro <literal>DRD_STOP_TRACING_VAR(x)</literal>. Stop tracing load
and store activity for the address range starting
at <literal>&x</literal> and occupying <literal>sizeof(x)</literal>
bytes.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_TRACE_MEMORY(&x)</literal>. Trace all
load and store activity that touches at least the single byte at the
address <literal>&x</literal>.
</para>
</listitem>
<listitem>
<para>
The client request <varname>VG_USERREQ__DRD_START_TRACE_ADDR</varname>,
which allows to trace all load and store activity for the specified
address range.
</para>
</listitem>
<listitem>
<para>
The client
request <varname>VG_USERREQ__DRD_STOP_TRACE_ADDR</varname>. Do no longer
trace load and store activity for the specified address range.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_HAPPENS_BEFORE(addr)</literal> tells DRD to
insert a mark. Insert this macro just after an access to the variable at
the specified address has been performed.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_HAPPENS_AFTER(addr)</literal> tells DRD that
the next access to the variable at the specified address should be
considered to have happened after the access just before the latest
<literal>ANNOTATE_HAPPENS_BEFORE(addr)</literal> annotation that
references the same variable. The purpose of these two macros is to tell
DRD about the order of inter-thread memory accesses implemented via
atomic memory operations. See
also <literal>drd/tests/annotate_smart_pointer.cpp</literal> for an
example.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_RWLOCK_CREATE(rwlock)</literal> tells DRD
that the object at address <literal>rwlock</literal> is a
reader-writer synchronization object that is not a
<literal>pthread_rwlock_t</literal> synchronization object. See
also <literal>drd/tests/annotate_rwlock.c</literal> for an example.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_RWLOCK_DESTROY(rwlock)</literal> tells DRD
that the reader-writer synchronization object at
address <literal>rwlock</literal> has been destroyed.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_WRITERLOCK_ACQUIRED(rwlock)</literal> tells
DRD that a writer lock has been acquired on the reader-writer
synchronization object at address <literal>rwlock</literal>.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_READERLOCK_ACQUIRED(rwlock)</literal> tells
DRD that a reader lock has been acquired on the reader-writer
synchronization object at address <literal>rwlock</literal>.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_RWLOCK_ACQUIRED(rwlock, is_w)</literal>
tells DRD that a writer lock (when <literal>is_w != 0</literal>) or that
a reader lock (when <literal>is_w == 0</literal>) has been acquired on
the reader-writer synchronization object at
address <literal>rwlock</literal>.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_WRITERLOCK_RELEASED(rwlock)</literal> tells
DRD that a writer lock has been released on the reader-writer
synchronization object at address <literal>rwlock</literal>.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_READERLOCK_RELEASED(rwlock)</literal> tells
DRD that a reader lock has been released on the reader-writer
synchronization object at address <literal>rwlock</literal>.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_RWLOCK_RELEASED(rwlock, is_w)</literal>
tells DRD that a writer lock (when <literal>is_w != 0</literal>) or that
a reader lock (when <literal>is_w == 0</literal>) has been released on
the reader-writer synchronization object at
address <literal>rwlock</literal>.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_BARRIER_INIT(barrier, count,
reinitialization_allowed)</literal> tells DRD that a new barrier object
at the address <literal>barrier</literal> has been initialized,
that <literal>count</literal> threads participate in each barrier and
also whether or not barrier reinitialization without intervening
destruction should be reported as an error. See
also <literal>drd/tests/annotate_barrier.c</literal> for an example.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_BARRIER_DESTROY(barrier)</literal>
tells DRD that a barrier object is about to be destroyed.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_BARRIER_WAIT_BEFORE(barrier)</literal>
tells DRD that waiting for a barrier will start.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_BARRIER_WAIT_AFTER(barrier)</literal>
tells DRD that waiting for a barrier has finished.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_BENIGN_RACE_SIZED(addr, size,
descr)</literal> tells DRD that any races detected on the specified
address are benign and hence should not be
reported. The <literal>descr</literal> argument is ignored but can be
used to document why data races on <literal>addr</literal> are benign.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_BENIGN_RACE_STATIC(var, descr)</literal>
tells DRD that any races detected on the specified static variable are
benign and hence should not be reported. The <literal>descr</literal>
argument is ignored but can be used to document why data races
on <literal>var</literal> are benign. Note: this macro can only be
used in C++ programs and not in C programs.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_IGNORE_READS_BEGIN</literal> tells
DRD to ignore all memory loads performed by the current thread.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_IGNORE_READS_END</literal> tells
DRD to stop ignoring the memory loads performed by the current thread.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_IGNORE_WRITES_BEGIN</literal> tells
DRD to ignore all memory stores performed by the current thread.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_IGNORE_WRITES_END</literal> tells
DRD to stop ignoring the memory stores performed by the current thread.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN</literal> tells
DRD to ignore all memory accesses performed by the current thread.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_IGNORE_READS_AND_WRITES_END</literal> tells
DRD to stop ignoring the memory accesses performed by the current thread.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_NEW_MEMORY(addr, size)</literal> tells
DRD that the specified memory range has been allocated by a custom
memory allocator in the client program and that the client program
will start using this memory range.
</para>
</listitem>
<listitem>
<para>
The macro <literal>ANNOTATE_THREAD_NAME(name)</literal> tells DRD to
associate the specified name with the current thread and to include this
name in the error messages printed by DRD.
</para>
</listitem>
<listitem>
<para>
The macros <literal>VALGRIND_MALLOCLIKE_BLOCK</literal> and
<literal>VALGRIND_FREELIKE_BLOCK</literal> from the Valgrind core are
implemented; they are described in
<xref linkend="manual-core-adv.clientreq"/>.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Note: if you compiled Valgrind yourself, the header file
<literal><valgrind/drd.h></literal> will have been installed in
the directory <literal>/usr/include</literal> by the command
<literal>make install</literal>. If you obtained Valgrind by
installing it as a package however, you will probably have to install
another package with a name like <literal>valgrind-devel</literal>
before Valgrind's header files are available.
</para>
</sect2>
<sect2 id="drd-manual.C++11" xreflabel="C++11">
<title>Debugging C++11 Programs</title>
<para>If you want to use the C++11 class std::thread you will need to do the
following to annotate the std::shared_ptr<> objects used in the
implementation of that class:
<itemizedlist>
<listitem>
<para>Add the following code at the start of a common header or at the
start of each source file, before any C++ header files are included:</para>
<programlisting>
#include <valgrind/drd.h>
#define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(addr) ANNOTATE_HAPPENS_BEFORE(addr)
#define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(addr) ANNOTATE_HAPPENS_AFTER(addr)
</programlisting>
</listitem>
<listitem>
<para>Download the gcc source code and from source file
libstdc++-v3/src/c++11/thread.cc copy the implementation of the
<computeroutput>execute_native_thread_routine()</computeroutput>
and <computeroutput>std::thread::_M_start_thread()</computeroutput>
functions into a source file that is linked with your application. Make
sure that also in this source file the
_GLIBCXX_SYNCHRONIZATION_HAPPENS_*() macros are defined properly.</para>
</listitem>
</itemizedlist>
</para>
<para>For more information, see also <emphasis>The
GNU C++ Library Manual, Debugging Support</emphasis>
(<ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html">http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html</ulink>).</para>
</sect2>
<sect2 id="drd-manual.gnome" xreflabel="GNOME">
<title>Debugging GNOME Programs</title>
<para>
GNOME applications use the threading primitives provided by the
<computeroutput>glib</computeroutput> and
<computeroutput>gthread</computeroutput> libraries. These libraries
are built on top of POSIX threads, and hence are directly supported by
DRD. Please keep in mind that you have to call
<function>g_thread_init</function> before creating any threads, or
DRD will report several data races on glib functions. See also the
<ulink
url="http://library.gnome.org/devel/glib/stable/glib-Threads.html">GLib
Reference Manual</ulink> for more information about
<function>g_thread_init</function>.
</para>
<para>
One of the many facilities provided by the <literal>glib</literal>
library is a block allocator, called <literal>g_slice</literal>. You
have to disable this block allocator when using DRD by adding the
following to the shell environment variables:
<literal>G_SLICE=always-malloc</literal>. See also the <ulink
url="http://library.gnome.org/devel/glib/stable/glib-Memory-Slices.html">GLib
Reference Manual</ulink> for more information.
</para>
</sect2>
<sect2 id="drd-manual.boost.thread" xreflabel="Boost.Thread">
<title>Debugging Boost.Thread Programs</title>
<para>
The Boost.Thread library is the threading library included with the
cross-platform Boost Libraries. This threading library is an early
implementation of the upcoming C++0x threading library.
</para>
<para>
Applications that use the Boost.Thread library should run fine under DRD.
</para>
<para>
More information about Boost.Thread can be found here:
<itemizedlist>
<listitem>
<para>
Anthony Williams, <ulink
url="http://www.boost.org/doc/libs/1_37_0/doc/html/thread.html">Boost.Thread</ulink>
Library Documentation, Boost website, 2007.
</para>
</listitem>
<listitem>
<para>
Anthony Williams, <ulink
url="http://www.ddj.com/cpp/211600441">What's New in Boost
Threads?</ulink>, Recent changes to the Boost Thread library,
Dr. Dobbs Magazine, October 2008.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
<sect2 id="drd-manual.openmp" xreflabel="OpenMP">
<title>Debugging OpenMP Programs</title>
<para>
OpenMP stands for <emphasis>Open Multi-Processing</emphasis>. The OpenMP
standard consists of a set of compiler directives for C, C++ and Fortran
programs that allows a compiler to transform a sequential program into a
parallel program. OpenMP is well suited for HPC applications and allows to
work at a higher level compared to direct use of the POSIX threads API. While
OpenMP ensures that the POSIX API is used correctly, OpenMP programs can still
contain data races. So it definitely makes sense to verify OpenMP programs
with a thread checking tool.
</para>
<para>
DRD supports OpenMP shared-memory programs generated by GCC. GCC
supports OpenMP since version 4.2.0. GCC's runtime support
for OpenMP programs is provided by a library called
<literal>libgomp</literal>. The synchronization primitives implemented
in this library use Linux' futex system call directly, unless the
library has been configured with the
<literal>--disable-linux-futex</literal> option. DRD only supports
libgomp libraries that have been configured with this option and in
which symbol information is present. For most Linux distributions this
means that you will have to recompile GCC. See also the script
<literal>drd/scripts/download-and-build-gcc</literal> in the
Valgrind source tree for an example of how to compile GCC. You will
also have to make sure that the newly compiled
<literal>libgomp.so</literal> library is loaded when OpenMP programs
are started. This is possible by adding a line similar to the
following to your shell startup script:
</para>
<programlisting><![CDATA[
export LD_LIBRARY_PATH=~/gcc-4.4.0/lib64:~/gcc-4.4.0/lib:
]]></programlisting>
<para>
As an example, the test OpenMP test program
<literal>drd/tests/omp_matinv</literal> triggers a data race
when the option -r has been specified on the command line. The data
race is triggered by the following code:
</para>
<programlisting><![CDATA[
#pragma omp parallel for private(j)
for (j = 0; j < rows; j++)
{
if (i != j)
{
const elem_t factor = a[j * cols + i];
for (k = 0; k < cols; k++)
{
a[j * cols + k] -= a[i * cols + k] * factor;
}
}
}
]]></programlisting>
<para>
The above code is racy because the variable <literal>k</literal> has
not been declared private. DRD will print the following error message
for the above code:
</para>
<programlisting><![CDATA[
$ valgrind --tool=drd --check-stack-var=yes --read-var-info=yes drd/tests/omp_matinv 3 -t 2 -r
...
Conflicting store by thread 1/1 at 0x7fefffbc4 size 4
at 0x4014A0: gj.omp_fn.0 (omp_matinv.c:203)
by 0x401211: gj (omp_matinv.c:159)
by 0x40166A: invert_matrix (omp_matinv.c:238)
by 0x4019B4: main (omp_matinv.c:316)
Location 0x7fefffbc4 is 0 bytes inside local var "k"
declared at omp_matinv.c:160, in frame #0 of thread 1
...
]]></programlisting>
<para>
In the above output the function name <function>gj.omp_fn.0</function>
has been generated by GCC from the function name
<function>gj</function>. The allocation context information shows that the
data race has been caused by modifying the variable <literal>k</literal>.
</para>
<para>
Note: for GCC versions before 4.4.0, no allocation context information is
shown. With these GCC versions the most usable information in the above output
is the source file name and the line number where the data race has been
detected (<literal>omp_matinv.c:203</literal>).
</para>
<para>
For more information about OpenMP, see also
<ulink url="http://openmp.org/">openmp.org</ulink>.
</para>
</sect2>
<sect2 id="drd-manual.cust-mem-alloc" xreflabel="Custom Memory Allocators">
<title>DRD and Custom Memory Allocators</title>
<para>
DRD tracks all memory allocation events that happen via the
standard memory allocation and deallocation functions
(<function>malloc</function>, <function>free</function>,
<function>new</function> and <function>delete</function>), via entry
and exit of stack frames or that have been annotated with Valgrind's
memory pool client requests. DRD uses memory allocation and deallocation
information for two purposes:
<itemizedlist>
<listitem>
<para>
To know where the scope ends of POSIX objects that have not been
destroyed explicitly. It is e.g. not required by the POSIX
threads standard to call
<function>pthread_mutex_destroy</function> before freeing the
memory in which a mutex object resides.
</para>
</listitem>
<listitem>
<para>
To know where the scope of variables ends. If e.g. heap memory
has been used by one thread, that thread frees that memory, and
another thread allocates and starts using that memory, no data
races must be reported for that memory.
</para>
</listitem>
</itemizedlist>
</para>
<para>
It is essential for correct operation of DRD that the tool knows about
memory allocation and deallocation events. When analyzing a client program
with DRD that uses a custom memory allocator, either instrument the custom
memory allocator with the <literal>VALGRIND_MALLOCLIKE_BLOCK</literal>
and <literal>VALGRIND_FREELIKE_BLOCK</literal> macros or disable the
custom memory allocator.
</para>
<para>
As an example, the GNU libstdc++ library can be configured
to use standard memory allocation functions instead of memory pools by
setting the environment variable
<literal>GLIBCXX_FORCE_NEW</literal>. For more information, see also
the <ulink
url="http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt04ch11.html">libstdc++
manual</ulink>.
</para>
</sect2>
<sect2 id="drd-manual.drd-versus-memcheck" xreflabel="DRD Versus Memcheck">
<title>DRD Versus Memcheck</title>
<para>
It is essential for correct operation of DRD that there are no memory
errors such as dangling pointers in the client program. Which means that
it is a good idea to make sure that your program is Memcheck-clean
before you analyze it with DRD. It is possible however that some of
the Memcheck reports are caused by data races. In this case it makes
sense to run DRD before Memcheck.
</para>
<para>
So which tool should be run first? In case both DRD and Memcheck
complain about a program, a possible approach is to run both tools
alternatingly and to fix as many errors as possible after each run of
each tool until none of the two tools prints any more error messages.
</para>
</sect2>
<sect2 id="drd-manual.resource-requirements" xreflabel="Resource Requirements">
<title>Resource Requirements</title>
<para>
The requirements of DRD with regard to heap and stack memory and the
effect on the execution time of client programs are as follows:
<itemizedlist>
<listitem>
<para>
When running a program under DRD with default DRD options,
between 1.1 and 3.6 times more memory will be needed compared to
a native run of the client program. More memory will be needed
if loading debug information has been enabled
(<literal>--read-var-info=yes</literal>).
</para>
</listitem>
<listitem>
<para>
DRD allocates some of its temporary data structures on the stack
of the client program threads. This amount of data is limited to
1 - 2 KB. Make sure that thread stacks are sufficiently large.
</para>
</listitem>
<listitem>
<para>
Most applications will run between 20 and 50 times slower under
DRD than a native single-threaded run. The slowdown will be most
noticeable for applications which perform frequent mutex lock /
unlock operations.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
<sect2 id="drd-manual.effective-use" xreflabel="Effective Use">
<title>Hints and Tips for Effective Use of DRD</title>
<para>
The following information may be helpful when using DRD:
<itemizedlist>
<listitem>
<para>
Make sure that debug information is present in the executable
being analyzed, such that DRD can print function name and line
number information in stack traces. Most compilers can be told
to include debug information via compiler option
<option>-g</option>.
</para>
</listitem>
<listitem>
<para>
Compile with option <option>-O1</option> instead of
<option>-O0</option>. This will reduce the amount of generated
code, may reduce the amount of debug info and will speed up
DRD's processing of the client program. For more information,
see also <xref linkend="manual-core.started"/>.
</para>
</listitem>
<listitem>
<para>
If DRD reports any errors on libraries that are part of your
Linux distribution like e.g. <literal>libc.so</literal> or
<literal>libstdc++.so</literal>, installing the debug packages
for these libraries will make the output of DRD a lot more
detailed.
</para>
</listitem>
<listitem>
<para>
When using C++, do not send output from more than one thread to
<literal>std::cout</literal>. Doing so would not only
generate multiple data race reports, it could also result in
output from several threads getting mixed up. Either use
<function>printf</function> or do the following:
<orderedlist>
<listitem>
<para>Derive a class from <literal>std::ostreambuf</literal>
and let that class send output line by line to
<literal>stdout</literal>. This will avoid that individual
lines of text produced by different threads get mixed
up.</para>
</listitem>
<listitem>
<para>Create one instance of <literal>std::ostream</literal>
for each thread. This makes stream formatting settings
thread-local. Pass a per-thread instance of the class
derived from <literal>std::ostreambuf</literal> to the
constructor of each instance. </para>
</listitem>
<listitem>
<para>Let each thread send its output to its own instance of
<literal>std::ostream</literal> instead of
<literal>std::cout</literal>.</para>
</listitem>
</orderedlist>
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1 id="drd-manual.Pthreads" xreflabel="Pthreads">
<title>Using the POSIX Threads API Effectively</title>
<sect2 id="drd-manual.mutex-types" xreflabel="mutex-types">
<title>Mutex types</title>
<para>
The Single UNIX Specification version two defines the following four
mutex types (see also the documentation of <ulink
url="http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_mutexattr_settype.html"><function>pthread_mutexattr_settype</function></ulink>):
<itemizedlist>
<listitem>
<para>
<emphasis>normal</emphasis>, which means that no error checking
is performed, and that the mutex is non-recursive.
</para>
</listitem>
<listitem>
<para>
<emphasis>error checking</emphasis>, which means that the mutex
is non-recursive and that error checking is performed.
</para>
</listitem>
<listitem>
<para>
<emphasis>recursive</emphasis>, which means that a mutex may be
locked recursively.
</para>
</listitem>
<listitem>
<para>
<emphasis>default</emphasis>, which means that error checking
behavior is undefined, and that the behavior for recursive
locking is also undefined. Or: portable code must neither
trigger error conditions through the Pthreads API nor attempt to
lock a mutex of default type recursively.
</para>
</listitem>
</itemizedlist>
</para>
<para>
In complex applications it is not always clear from beforehand which
mutex will be locked recursively and which mutex will not be locked
recursively. Attempts lock a non-recursive mutex recursively will
result in race conditions that are very hard to find without a thread
checking tool. So either use the error checking mutex type and
consistently check the return value of Pthread API mutex calls, or use
the recursive mutex type.
</para>
</sect2>
<sect2 id="drd-manual.condvar" xreflabel="condition-variables">
<title>Condition variables</title>
<para>
A condition variable allows one thread to wake up one or more other
threads. Condition variables are often used to notify one or more
threads about state changes of shared data. Unfortunately it is very
easy to introduce race conditions by using condition variables as the
only means of state information propagation. A better approach is to
let threads poll for changes of a state variable that is protected by
a mutex, and to use condition variables only as a thread wakeup
mechanism. See also the source file
<computeroutput>drd/tests/monitor_example.cpp</computeroutput> for an
example of how to implement this concept in C++. The monitor concept
used in this example is a well known and very useful concept -- see
also Wikipedia for more information about the <ulink
url="http://en.wikipedia.org/wiki/Monitor_(synchronization)">monitor</ulink>
concept.
</para>
</sect2>
<sect2 id="drd-manual.pctw" xreflabel="pthread_cond_timedwait">
<title>pthread_cond_timedwait and timeouts</title>
<para>
Historically the function
<function>pthread_cond_timedwait</function> only allowed the
specification of an absolute timeout, that is a timeout independent of
the time when this function was called. However, almost every call to
this function expresses a relative timeout. This typically happens by
passing the sum of
<computeroutput>clock_gettime(CLOCK_REALTIME)</computeroutput> and a
relative timeout as the third argument. This approach is incorrect
since forward or backward clock adjustments by e.g. ntpd will affect
the timeout. A more reliable approach is as follows:
<itemizedlist>
<listitem>
<para>
When initializing a condition variable through
<function>pthread_cond_init</function>, specify that the timeout of
<function>pthread_cond_timedwait</function> will use the clock
<literal>CLOCK_MONOTONIC</literal> instead of
<literal>CLOCK_REALTIME</literal>. You can do this via
<computeroutput>pthread_condattr_setclock(...,
CLOCK_MONOTONIC)</computeroutput>.
</para>
</listitem>
<listitem>
<para>
When calling <function>pthread_cond_timedwait</function>, pass
the sum of
<computeroutput>clock_gettime(CLOCK_MONOTONIC)</computeroutput>
and a relative timeout as the third argument.
</para>
</listitem>
</itemizedlist>
See also
<computeroutput>drd/tests/monitor_example.cpp</computeroutput> for an
example.
</para>
</sect2>
</sect1>
<sect1 id="drd-manual.limitations" xreflabel="Limitations">
<title>Limitations</title>
<para>DRD currently has the following limitations:</para>
<itemizedlist>
<listitem>
<para>
DRD, just like Memcheck, will refuse to start on Linux
distributions where all symbol information has been removed from
<filename>ld.so</filename>. This is e.g. the case for the PPC editions
of openSUSE and Gentoo. You will have to install the glibc debuginfo
package on these platforms before you can use DRD. See also openSUSE
bug <ulink url="http://bugzilla.novell.com/show_bug.cgi?id=396197">
396197</ulink> and Gentoo bug <ulink
url="http://bugs.gentoo.org/214065">214065</ulink>.
</para>
</listitem>
<listitem>
<para>
With gcc 4.4.3 and before, DRD may report data races on the C++
class <literal>std::string</literal> in a multithreaded program. This is
a know <literal>libstdc++</literal> issue -- see also GCC bug
<ulink url="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40518">40518</ulink>
for more information.
</para>
</listitem>
<listitem>
<para>
If you compile the DRD source code yourself, you need GCC 3.0 or
later. GCC 2.95 is not supported.
</para>
</listitem>
<listitem>
<para>
Of the two POSIX threads implementations for Linux, only the
NPTL (Native POSIX Thread Library) is supported. The older
LinuxThreads library is not supported.
</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="drd-manual.feedback" xreflabel="Feedback">
<title>Feedback</title>
<para>
If you have any comments, suggestions, feedback or bug reports about
DRD, feel free to either post a message on the Valgrind users mailing
list or to file a bug report. See also <ulink
url="&vg-url;">&vg-url;</ulink> for more information.
</para>
</sect1>
</chapter>