#include <db.h>
int
DB_ENV->set_thread_id(DB_ENV *dbenv,
void (*thread_id)(DB_ENV *dbenv, pid_t *pid, db_threadid_t *tid));
Declare a function that returns a unique identifier pair for the
current thread of control. The DB_ENV->set_thread_id() method
supports the DB_ENV->failchk() method.
For more information, see
Architecting Data Store and
Concurrent Data Store applications, and
Architecting Transactional Data Store
applications, both in the
Berkeley DB Programmer's Reference Guide.
The DB_ENV->set_thread_id() method configures operations performed
using the specified DB_ENV handle, not all
operations performed on the underlying database environment.
The DB_ENV->set_thread_id() method may be called at any time during
the life of the application.
The DB_ENV->set_thread_id()
method returns a non-zero error value on failure and 0 on success.
The thread_id 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:
dbenv
The dbenv parameter is the enclosing database environment handle, allowing application access to the application-private fields of that object.
pid
The pid points to a memory location of
type pid_t, or NULL. The process ID of the current
thread of control may be returned in this memory location, if it is not
NULL.
tid
The tid points to a memory location of
type db_threadid_t, or NULL. The thread ID of the
current thread of control may be returned in this memory location, if it
is not NULL.
The DB_ENV->set_thread_id()
method may fail and return one of the following non-zero errors:
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
DB_ENV->failchk() method is run, it may be possible for
DB_ENV->failchk()
to not detect that a thread of control exited without properly
releasing all Berkeley DB resources.
It may be possible to handle this problem by inhibiting process or thread
creation between thread of control exit and calling the
DB_ENV->failchk()
method. Alternatively, the thread_id function
must be constructed to not re-use pid/tid
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 is_alive
function specified to the
DB_ENV->set_isalive()
method must be compatible with any
thread_id function specified to
DB_ENV->set_thread_id().
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.
If no thread_id 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().