File: db_thread.3

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.ds TYPE C
.\"
.\" See the file LICENSE for redistribution information.
.\"
.\" Copyright (c) 1996, 1997
.\"	Sleepycat Software.  All rights reserved.
.\"
.\"	@(#)db_thread.so	10.3 (Sleepycat) 1/18/98
.\"
.\"
.\" See the file LICENSE for redistribution information.
.\"
.\" Copyright (c) 1997
.\"	Sleepycat Software.  All rights reserved.
.\"
.\"	@(#)macros.so	10.38 (Sleepycat) 1/18/98
.\"
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.\" The open flags.
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The
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and
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arguments specify how files will be opened and/or created when they
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together one or more of the following values:
.Sj DB_CREATE
Create any underlying files, as necessary.
If the files do not already exist and the DB_CREATE flag is not specified,
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..
.\" DB_THREAD open flag macro.
.\" This macro takes two arguments:
.\"	+ the open function name
.\"	+ the object it returns.
.de Ft
.TP 5
.Sj DB_THREAD
Cause the \\$2 handle returned by the
.I \\$1
.Al function method
to be useable by multiple threads within a single address space,
i.e., to be ``free-threaded''.
.if '\*[TYPE]'JAVA'\{\
Threading is assumed in the Java API,
so no special flags are required,
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..
.\" The mode macro.
.\" This macro takes one argument:
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All files created by the \\$1 are created with mode
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and modified by the process' umask value at the time of creation (see
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.\" The application exits macro.
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The
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.\" The application -h section.
.\" This macro takes one argument:
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.de Dh
DB_HOME
If the
.B \-h
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.\" The function DB_HOME ENVIRONMENT VARIABLES section.
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If the
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.\" The function TMPDIR ENVIRONMENT VARIABLES section.
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was NULL or not initialized using
.IR db_appinit ,
the environment variable TMPDIR may be used as the directory in which to
create the \\$1,
as described in the
.I \\*(Vo
section above.
.rm Vo
..
.\" The unused flags macro.
.de Fl
The
.I flags
parameter is currently unused, and must be set to 0.
..
.\" The no-space TP macro.
.de Nt
.br
.ns
.TP 5
..
.\" The return values of the functions macros.
.\" Rc is the standard two-value return with a suffix for more values.
.\" Ro is the standard two-value return but there were previous values.
.\" Rt is the standard two-value return, returning errno, 0, or < 0.
.\" These macros take one argument:
.\"	+ the routine name
.de Rc
The
.I \\$1
.ie '\*[TYPE]'C'\{function returns the value of
.I errno
on failure,
0 on success,
\}
.el\{method throws a
.IR DbException (3)
.ie '\*[TYPE]'CXX'\{or returns the value of
.I errno
on failure,
0 on success,
\}
.el\{that encapsulates an
.I errno
on failure,
\}
\}
..
.de Ro
Otherwise, the
.I \\$1
.ie '\*[TYPE]'C'\{function returns the value of
.I errno
on failure and 0 on success.
\}
.el\{method throws a
.IR DbException (3)
.ie '\*[TYPE]'CXX'\{or returns the value of
.I errno
on failure and 0 on success.
\}
.el\{that encapsulates an
.I errno
on failure,
\}
\}
..
.de Rt
The
.I \\$1
.ie '\*[TYPE]'C'\{function returns the value of
.I errno
on failure and 0 on success.
\}
.el\{method throws a
.IR DbException (3)
.ie '\*[TYPE]'CXX'\{or returns the value of
.I errno
on failure and 0 on success.
\}
.el\{that encapsulates an
.I errno
on failure.
\}
\}
..
.\" The TXN id macro.
.de Tx
.IP
If the file is being accessed under transaction protection,
the
.I txnid
parameter is a transaction ID returned from
.IR txn_begin ,
otherwise, NULL.
..
.\" The XXX_unlink function text macro.
.\" This macro takes two arguments:
.\"	+ the interface, e.g., "transaction region"
.\"	+ the prefix (for C++, this is the class name)
.de Un
.ie '\*[TYPE]'C'\{\
.ds Va db_appinit
.ds Vc \\$2_close
.ds Vo \\$2_open
.ds Vu \\$2_unlink
\}
.el\{\
.ds Va DbEnv::appinit
.ds Vc \\$2::close
.ds Vo \\$2::open
.ds Vu \\$2::unlink
\}
.Fn \\*(Vu
The
.I \\*(Vu
.Al function method
destroys the \\$1 identified by the directory
.IR dir ,
removing all files used to implement the \\$1.
.ie '\\$2'log' \{(The log files themselves and the directory
.I dir
are not removed.)\}
.el \{(The directory
.I dir
is not removed.)\}
If there are processes that have called
.I \\*(Vo
without calling
.I \\*(Vc
(i.e., there are processes currently using the \\$1),
.I \\*(Vu
will fail without further action,
unless the force flag is set,
in which case
.I \\*(Vu
will attempt to remove the \\$1 files regardless of any processes
still using the \\$1.
.PP
The result of attempting to forcibly destroy the region when a process
has the region open is unspecified.
Processes using a shared memory region maintain an open file descriptor
for it.
On UNIX systems, the region removal should succeed
and processes that have already joined the region should continue to
run in the region without change,
however processes attempting to join the \\$1 will either fail or
attempt to create a new region.
On other systems, e.g., WNT, where the
.IR unlink (2)
system call will fail if any process has an open file descriptor
for the file,
the region removal will fail.
.PP
In the case of catastrophic or system failure,
database recovery must be performed (see
.IR db_recover (1)
or the DB_RECOVER flags to
.IR \\*(Va (3)).
Alternatively, if recovery is not required because no database state is
maintained across failures,
it is possible to clean up a \\$1 by removing all of the
files in the directory specified to the
.I \\*(Vo
.Al function, method,
as \\$1 files are never created in any directory other than the one
specified to
.IR \\*(Vo .
Note, however,
that this has the potential to remove files created by the other DB
subsystems in this database environment.
.PP
.Rt \\*(Vu
.rm Va
.rm Vo
.rm Vu
.rm Vc
..
.\" Signal paragraph for standard utilities.
.\" This macro takes one argument:
.\"	+ the utility name.
.de Si
The
.I \\$1
utility attaches to DB shared memory regions.
In order to avoid region corruption,
it should always be given the chance to detach and exit gracefully.
To cause
.I \\$1
to clean up after itself and exit,
send it an interrupt signal (SIGINT).
..
.\" Logging paragraph for standard utilities.
.\" This macro takes one argument:
.\"	+ the utility name.
.de Pi
.B \-L
Log the execution of the \\$1 utility to the specified file in the
following format, where ``###'' is the process ID, and the date is
the time the utility starting running.
.sp
\\$1: ### Wed Jun 15 01:23:45 EDT 1995
.sp
This file will be removed if the \\$1 utility exits gracefully.
..
.\" Malloc paragraph.
.\" This macro takes one argument:
.\"	+ the allocated object
.de Ma
.if !'\*[TYPE]'JAVA'\{\
\\$1 are created in allocated memory.
If
.I db_malloc
is non-NULL,
it is called to allocate the memory,
otherwise,
the library function
.IR malloc (3)
is used.
The function
.I db_malloc
must match the calling conventions of the
.IR malloc (3)
library routine.
Regardless,
the caller is responsible for deallocating the returned memory.
To deallocate the returned memory,
free each returned memory pointer;
pointers inside the memory do not need to be individually freed.
\}
..
.\" Underlying function paragraph.
.\" This macro takes two arguments:
.\"	+ the function name
.\"	+ the utility name
.de Uf
The
.I \\$1
.Al function method
is the underlying function used by the
.IR \\$2 (1)
utility.
See the source code for the
.I \\$2
utility for an example of using
.I \\$1
in a UNIX environment.
..
.\" Underlying function paragraph, for C++.
.\" This macro takes three arguments:
.\"	+ the C++ method name
.\"	+ the function name for C
.\"	+ the utility name
.de Ux
The
.I \\$1
method is based on the C
.I \\$2
function, which
is the underlying function used by the
.IR \\$3 (1)
utility.
See the source code for the
.I \\$3
utility for an example of using
.I \\$2
in a UNIX environment.
..
.TH DB_THREAD 3 "January 18, 1998"
.UC 7
.SH "NAME
db_thread \- using threads in the DB library.
.SH "DESCRIPTION
.Gn
.PP
This manual pages describes the specific details of using DB from
within threaded programs.
.PP
The DB library is not itself multi-threaded.
The library was deliberately architected to not use threads internally
because of the portability problems that using threads within the library
would introduce.
Object handles returned from DB library functions are free-threaded,
i.e., threads may use handles concurrently,
by specifying the DB_THREAD flag to
.IR db_appinit (3)
(\c
or, in the C++ API,
.IR DbEnv::appinit (3))
and the other subsystem open functions.
Threading is assumed in the Java API,
so no special flags are required,
and DB functions will always behave as if the DB_THREAD flag was specified.
.PP
DB supports multi-threaded applications with the caveat that it loads
and calls functions that are commonly available in C language environments
and which may not themselves be thread-safe.
Other than this usage,
DB has no static data and maintains no local context between calls to
DB functions.
To ensure that applications can safely use threads in the context of DB,
porters to new operating systems and/or C libraries must confirm that the
system and C library functions used by the DB library are thread-safe.
.PP
There are some additional caveats about using threads to access
the DB library:
.TP 2
\(bu
The DB_THREAD flag must be specified for all subsystems either explicitly
or via the
.I db_appinit
(\c
.IR DbEnv::appinit )
function in the C and C++ APIs.
Threading is assumed in the Java API,
so no special flags are required,
and DB functions will always behave as if the DB_THREAD flag was specified.
.sp
Setting the DB_THREAD flag inconsistently may result in database corruption.
.TP 2
\(bu
Only a single thread may call the close function for a returned database
or subsystem handle.
See
.IR db_open (3)
(\c
.IR Db::open (3))
and the appropriate subsystem manual pages for more information.
.TP 2
\(bu
Either the DB_DBT_MALLOC or DB_DBT_USERMEM flags must be set in a DBT used
for key or data retrieval.
See
.IR db_dbt (3)
(\c
.IR Dbt (3))
for more information.
.TP 2
\(bu
The DB_CURRENT, DB_NEXT and DB_PREV flags to the
.I log_get
(\c
.IR DbLog::get )
function may not be used by a free-threaded handle.  If such calls are
necessary, a thread should explicitly create a unique DB_LOG handle by
calling
.IR log_open (3)
(\c
.IR DbLog::open ).
See
.IR db_log (3)
(\c
.IR DbLog (3))
for more information.
.TP 2
\(bu
Each database operation (i.e., any call to a function underlying the
handles returned by
.IR db_open (3)
and
.IR db_cursor (3))
(\c
.IR Db (3)
and
.IR Dbc (3))
is normally performed on behalf of a unique locker.
If, within a single thread of control,
multiple calls on behalf of the same locker are desired,
then transactions must be used.  For example, consider the case where
a cursor scan locates a record, and then based on that record, accesses
some other item in the database.  If these are done using the default
lockers for the handle, there is no guarantee that these two operations
will not conflict.  If the application wishes to guarantee that the
operations do not conflict, locks must be obtained on behalf of a transaction,
instead of the default locker id, and a transaction must be specified to
the cursor creation and the subsequent db call.
.TP 2
\(bu
Transactions may not span threads, i.e.,
each transaction must begin and end in the same thread,
and each transaction may only be used by a single thread.
.TP 2
\(bu
Spinlocks must have been implemented for the compiler/architecture
combination.
Attempting to specify the DB_THREAD flag will fail if spinlocks are not
available.
.TP 2
\(bu
The DB library makes a system call to pause for some number of microseconds
when it is necessary to wait on a lock.
This may not be optimal,
especially in a thread-only environment where it will be more efficient to
explicitly yield the processor to another thread.
It is possible to specify a yield function on an per-application basis,
see
.IR db_jump_set (3)
for more information.
.SH "COMPILING THREADED APPLICATIONS
Special compile-time flags are required when compiling threaded applications
with the UNIX include files on some architectures.
.PP
On IRIX, if you are compiling a threaded application,
you must compile with the -D_SGI_MP_SOURCE flag:
.PP
.ti +5
cc -D_SGI_MP_SOURCE ...
.PP
On OSF/1, if you are compiling a threaded application,
you must compile with the -D_REENTRANT flag:
.PP
.ti +5
cc -D_REENTRANT ...
.PP
On Solaris, if you are compiling a threaded application,
you must compile with the -D_REENTRANT flag and link with the -lthread
library:
.PP
.ti +5
cc -D_REENTRANT ... -lthread
.SH "SEE ALSO"
.Sa