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<! "@(#)open.so 10.12 (Sleepycat) 10/2/98">
<!Copyright 1997, 1998 by Sleepycat Software, Inc. All rights reserved.>
<html>
<body bgcolor=white>
<head>
<title>Berkeley DB: memp_open</title>
<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btr
ee,hash,hashing,transaction,transactions,locking,logging,access method,access me
thods,java,C,C++">
</head>
<h1>memp_open</h1>
<hr size=1 noshade>
<tt>
<h3>
<pre>
#include <db.h>
<p>
int memp_open(char *dir, u_int32_t flags,
int mode, DB_ENV *dbenv, DB_MPOOL **regionp);
</pre>
</h3>
<h1>Description</h1>
<p>
The memp_open
function copies a pointer, to the "memory pool" identified by the <i>directory</i>
<b>dir</b>, into the memory location referenced by <b>regionp</b>.
<p>
If the <b>dir</b> pathname argument is NULL or the DB_MPOOL_PRIVATE flag
is set, any necessary temporary files are created as described for the
<a href="../../ref/env/naming.html#DB_TMP_DIR">DB_TMP_DIR</a> value in <a href="../../ref/env/naming.html">Berkeley DB File Naming</a>. Otherwise, the
<b>dir</b> pathname argument is interpreted as described in
<a href="../../ref/env/naming.html">Berkeley DB File Naming</a>.
<p>
The <b>flags</b> and <b>mode</b> arguments specify how files will be opened
and/or created if they do not already exist.
The flags value is specified by logically <b>OR</b>'ing together one or more of the
following values:
<dl compact>
<a name="DB_CREATE">
<p><dt>DB_CREATE<dd>Create any underlying files, as necessary. If the files do not already
exist and the DB_CREATE flag is not specified, the call will fail.
<a name="DB_MPOOL_PRIVATE">
<p><dt>DB_MPOOL_PRIVATE<dd>Create a private MPOOL that is not shared with any other process (although
it may be shared with other threads).
<a name="DB_NOMMAP">
<p><dt>DB_NOMMAP<dd>Always copy files in this memory pool into the local cache instead of
mapping them into process memory (see the description of the
<b>mp_mmapsize</b> field of the DB_ENV structure for further
information).
<a name="DB_THREAD">
<p><dt>DB_THREAD<dd>Cause the m4_reg(DB_MPOOL) handle returned by memp_open to be useable
by multiple threads within a single address space, i.e., to be
<i>free-threaded</i>.
</dl>
<p>
All files created by the memory pool subsystem (other than files created by the
<a href="../../api_c/DbMpoolFile/open.html">memp_fopen</a> function, which are separately specified) are created with mode <b>mode</b> (as described
in <b>chmod</b>(2)) and modified by the process' umask value at the time
of creation (see <b>umask</b>(2)))).
The group ownership of created files is based on the system and directory
defaults, and is not further specified by Berkeley DB.
<p>
The memory pool subsystem is configured
based on the <b>dbenv</b> argument to memp_open which is a pointer to
a structure of type DB_ENV. Applications normally use the same DB_ENV
structure (initialized by <a href="../../api_c/DbEnv/appinit.html">db_appinit</a>) as an argument to all of
the subsystems in the Berkeley DB package.
<p>
References to the DB_ENV structure are maintained by Berkeley DB, so it may not
be discarded until the last close function, corresponding to an open
function for which it was an argument, has returned. To ensure
compatibility with future releases of Berkeley DB, all fields of the DB_ENV
structure that are not explicitly set should be initialized to 0 before
the first time the structure is used. Do this by declaring the structure
external or static, or by calling one of the C library routines
<b>bzero</b>(3) or <b>memset</b>(3).
<p>
The fields of the DB_ENV structure used by memp_open are described below.
If <b>dbenv</b> is NULL or any of its fields are set to 0, defaults
appropriate for the system are used where possible.
<p>
The following fields in the DB_ENV structure may be initialized before
calling memp_open:
<dl compact>
<p><dt>void *(*db_errcall)(char *db_errpfx, char *buffer);<dd>
<dt>FILE *db_errfile;<dd>
<dt>const char *db_errpfx;<dd>
<dt>int db_verbose;<dd>The error fields of the DB_ENV behave as described for <a href="../../api_c/DbEnv/appinit.html">db_appinit</a>.
<a name="mp_mmapsize">
<p><dt>size_t mp_mmapsize;<dd>Files that are opened read-only in the pool (and that satisfy a few other
criteria) are, by default, mapped into the process address space instead
of being copied into the local cache. This can result in better-than-usual
performance, as available virtual memory is normally much larger than the
local cache, and page faults are faster than page copying on many systems.
However, in the presence of limited virtual memory it can cause resource
starvation, and in the presence of large databases, it can result in immense
process sizes. If <b>mp_mmapsize</b> is non-zero, it specifies the maximum
file size, in bytes, for a file to be mapped into the process address space.
By default, it is set to 10Mb.
<a name="mp_size">
<p><dt>size_t mp_size;<dd>The suggested size of the shared memory buffer pool, i.e., the cache, in
bytes. This should be the size of the normal working data set of the
application, with some small amount of additional memory for unusual
situations. (Note, the working set is not the same as the number of
simultaneously referenced pages, and should be quite a bit larger!) The
default cache size is 128K bytes, and may not be specified as less than
20K bytes.
<p>
For information on tuning the Berkeley DB cache size, see
<a href="../../ref/am/cachesize.html">Selecting a cache size</a>.
</dl>
<p>
The memp_open
function returns the value of <b>errno</b> on failure, and 0 on success.
<p>
<h1>Environment Variables</h1>
<dl compact>
<p><dt>DB_HOME<dd>If the <b>dbenv</b> argument to memp_open was initialized using
<a href="../../api_c/DbEnv/appinit.html">db_appinit</a> the environment variable <b>DB_HOME</b> may
be used as the path of the database home for the interpretation of
the <b>dir</b> argument.
</dl>
<p>
<dl compact>
<p><dt>TMPDIR<dd>If the <b>dbenv</b> argument to memp_open was NULL or not initialized
using <a href="../../api_c/DbEnv/appinit.html">db_appinit</a> the environment variable <b>TMPDIR</b> may
be used as the directory in which to create the memory pool, as described in
memp_open.
</dl>
<p>
<h1>Errors</h1>
If a fatal error occurs in Berkeley DB, the memp_open function may fail and return
DB_RUNRECOVERY, at which point all subsequent database calls will also
return DB_RUNRECOVERY.
<p>
The memp_open
function may fail and return <b>errno</b>
for any of the errors specified for the following Berkeley DB and C library
functions:
DBmemp->pgin(3),
DBmemp->pgout(3),
abort(3),
close(3),
<a href="../../api_c/DbEnv/version.html">db_version</a>,
dbenv->db_paniccall(3),
fcntl(3),
fflush(3),
fprintf(3),
free(3),
fstat(3),
fsync(3),
getenv(3),
getpid(3),
getuid(3),
isdigit(3),
<a href="../../api_c/DbLog/compare.html">log_compare</a>,
<a href="../../api_c/DbLog/flush.html">log_flush</a>,
lseek(3),
malloc(3),
memcmp(3),
memcpy(3),
<a href="../../api_c/DbMpool/close.html">memp_close</a>,
<a href="../../api_c/DbMpool/unlink.html">memp_unlink</a>,
memset(3),
mmap(3),
munmap(3),
open(3),
pread(3),
pstat_getdynamic(3),
pwrite(3),
read(3),
shmat(3),
shmctl(3),
shmdt(3),
sigfillset(3),
sigprocmask(3),
stat(3),
strerror(3),
strlen(3),
sysconf(3),
time(3),
unlink(3),
vfprintf(3),
vsnprintf(3),
and
write(3).
<p>
In addition, the memp_open
function may fail and return <b>errno</b>
for the following conditions:
<dl compact>
<p><dt>EAGAIN<dd>The shared memory region was locked and (repeatedly) unavailable.
</dl>
<dl compact>
<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
<p>
The DB_THREAD flag was specified and spinlocks are not implemented for
this architecture.
<p>
A NULL pathname was specified without the DB_MPOOL_PRIVATE flag.
<p>
The specified cache size was impossibly small.
</dl>
<p>
<h1>See Also</h1>
<a href="../../api_c/DbMpool/close.html">memp_close</a>,
<a href="../../api_c/DbMpoolFile/close.html">memp_fclose</a>,
<a href="../../api_c/DbMpoolFile/get.html">memp_fget</a>,
<a href="../../api_c/DbMpoolFile/open.html">memp_fopen</a>,
<a href="../../api_c/DbMpoolFile/put.html">memp_fput</a>,
<a href="../../api_c/DbMpoolFile/set.html">memp_fset</a>,
<a href="../../api_c/DbMpoolFile/sync.html">memp_fsync</a>,
memp_open,
<a href="../../api_c/DbMpool/db_register.html">memp_register</a>,
<a href="../../api_c/DbMpool/stat.html">memp_stat</a>,
<a href="../../api_c/DbMpool/stat.html">memp_stat</a>,
<a href="../../api_c/DbMpool/sync.html">memp_sync</a>,
<a href="../../api_c/DbMpool/trickle.html">memp_trickle</a>
and
<a href="../../api_c/DbMpool/unlink.html">memp_unlink</a>.
</tt>
</body>
</html>
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