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<b>DbEnv::set_thread_id</b>
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<b><pre>
#include <db_cxx.h>
<p>
int
DbEnv::set_thread_id(void (*thread_id)(DbEnv *dbenv, pid_t *pid, db_threadid_t *tid));
</pre></b>
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<b>Description: DbEnv::set_thread_id</b>
<p>Declare a function that returns a unique identifier pair for the current
thread of control. The DbEnv::set_thread_id method supports the
<a href="../api_cxx/env_failchk.html">DbEnv::failchk</a> method. For more information, see
<a href="../ref/cam/app.html">Architecting Data Store and Concurrent
Data Store applications</a>, and <a href="../ref/transapp/app.html">Architecting Transactional Data Store applications</a>.</p>
<p>The DbEnv::set_thread_id method configures operations performed using the specified
<a href="../api_cxx/env_class.html">DbEnv</a> handle, not all operations performed on the underlying
database environment.</p>
<p>The DbEnv::set_thread_id method may be called at any time during the life of the
application.</p>
<p>The DbEnv::set_thread_id method
either returns a non-zero error value
or throws an exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
</p>
<b>Parameters</b> <br>
<b>thread_id</b><ul compact><li>The <b>thread_id</b> parameter is a function which returns a unique
identifier pair for a thread of control in a Berkeley DB application. The
function takes three arguments:</ul>
<br>
<b><b>dbenv</b></b><ul compact><li>The <b>dbenv</b> parameter is the enclosing database environment
handle, allowing application access to the application-private fields
of that object.</ul>
<b><b>pid</b></b><ul compact><li>The <b>pid</b> points to a memory location of type pid_t. The
process ID of the current thread of control may be returned in
this memory location.</ul>
<b><b>tid</b></b><ul compact><li>The <b>tid</b> points to a memory location of type db_threadid_t. The
thread ID of the current thread of control may be returned in this
memory location.</ul>
<br>
<br>
<p>The standard system library calls to return process and thread IDs are
often sufficient for this purpose (for example, getpid() and
pthread_self() on POSIX systems or GetCurrentThreadID on Windows
systems). However, if the Berkeley DB application dynamically creates
processes or threads, some care may be necessary in assigning unique
IDs. In most threading systems, process and thread IDs are available
for re-use as soon as the process or thread exits. If a new process or
thread is created between the time of process or thread exit, and the
<a href="../api_cxx/env_failchk.html">DbEnv::failchk</a> method is run, it may be possible for
<a href="../api_cxx/env_failchk.html">DbEnv::failchk</a> to not detect that a thread of control exited
without properly releasing all Berkeley DB resources.</p>
<p>It may be possible to handle this problem by inhibiting process or
thread creation between thread of control exit and calling
<a href="../api_cxx/env_failchk.html">DbEnv::failchk</a>. Alternatively, the <b>thread_id</b> function
must be constructed to not re-use <b>pid</b>/<b>tid</b> pairs. For
example, in a single process application, the returned process ID might
be used as an incremental counter, with the returned thread ID set to
the actual thread ID. Obviously, the <b>is_alive</b> function
specified to the <a href="../api_cxx/env_set_isalive.html">DbEnv::set_isalive</a> method must be compatible with any
<b>thread_id</b> function specified to DbEnv::set_thread_id.</p>
<p>The db_threadid_t type is configured to be the same type as a standard
thread identifier, in Berkeley DB configurations where this type is known (for
example, systems supporting pthread_t or thread_t, or DWORD on Windows).
If the Berkeley DB configuration process is unable to determine the type of a
standard thread identifier, the db_thread_t type is set to uintmax_t (or
the largest available unsigned integral type, on systems lacking the
uintmax_t type). Applications running on systems lacking a detectable
standard thread type, and which are also using thread APIs where a
thread identifier is not an integral value and so will not fit into the
configured db_threadid_t type, must either translate between the
db_threadid_t type and the thread identifier (mapping the thread
identifier to a unique identifier of the appropriate size), or modify
the Berkeley DB sources to use an appropriate db_threadid_t type. Note: we
do not currently know of any systems where this is necessary. If your
application has to solve this problem, please contact our support group
and let us know.</p>
<p>If no <b>thread_id</b> function is specified by the application, the
Berkeley DB library will identify threads of control by using the taskIdSelf()
call on VxWorks, the getpid() and GetCurrentThreadID() calls on Windows,
the getpid() and pthread_self() calls when the Berkeley DB library has been
configured for POSIX pthreads or Solaris LWP threads, the getpid() and
thr_self() calls when the Berkeley DB library has been configured for UI
threads, and otherwise getpid().</p>
<br><b>Errors</b>
<p>The DbEnv::set_thread_id method
may fail and throw
<a href="../api_cxx/except_class.html">DbException</a>,
encapsulating one of the following non-zero errors, or return one of
the following non-zero errors:</p>
<br>
<b>EINVAL</b><ul compact><li>An
invalid flag value or parameter was specified.</ul>
<br>
<hr size=1 noshade>
<br><b>Class</b>
<a href="../api_cxx/env_class.html">DbEnv</a>
<br><b>See Also</b>
<a href="../api_cxx/env_list.html">Database Environments and Related Methods</a>
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