NNAAMMEE db_mpool - shared memory buffer pool SSYYNNOOPPSSIISS ##iinncclluuddee <> iinntt mmeemmpp__ooppeenn((cchhaarr **ddiirr,, uu__iinntt3322__tt ffllaaggss,, iinntt mmooddee,, DDBB__EENNVV **ddbbeennvv,, DDBB__MMPPOOOOLL ****rreeggiioonnpp));; iinntt mmeemmpp__cclloossee((DDBB__MMPPOOOOLL **mmpp));; iinntt mmeemmpp__ffooppeenn((DDBB__MMPPOOOOLL **mmpp,, cchhaarr **ffiillee,, uu__iinntt3322__tt ffllaaggss,, iinntt mmooddee,, ssiizzee__tt ppaaggeessiizzee,, DDBB__MMPPOOOOLL__FFIINNFFOO **ffiinnffoopp,, DDBB__MMPPOOOOLLFFIILLEE ****mmppff));; iinntt mmeemmpp__ffcclloossee((DDBB__MMPPOOOOLLFFIILLEE **mmppff));; iinntt mmeemmpp__ffggeett((DDBB__MMPPOOOOLLFFIILLEE **mmppff,, ddbb__ppggnnoo__tt **ppggnnooaaddddrr,, uu__iinntt3322__tt ffllaaggss,, vvooiidd ****ppaaggeepp));; iinntt mmeemmpp__ffppuutt((DDBB__MMPPOOOOLLFFIILLEE **mmppff,, vvooiidd **ppggaaddddrr,, uu__iinntt3322__tt ffllaaggss));; iinntt mmeemmpp__ffsseett((DDBB__MMPPOOOOLLFFIILLEE **mmppff,, vvooiidd **ppggaaddddrr,, uu__iinntt3322__tt ffllaaggss));; iinntt mmeemmpp__ffssyynncc((DDBB__MMPPOOOOLLFFIILLEE **mmppff));; iinntt mmeemmpp__uunnlliinnkk((ccoonnsstt cchhaarr **ddiirr,, iinntt ffoorrccee,, DDBB__EENNVV **));; iinntt mmeemmpp__rreeggiisstteerr((DDBB__MMPPOOOOLL **mmpp,, iinntt ffttyyppee,, iinntt ((**ppggiinn))((ddbb__ppggnnoo__tt ppggnnoo,, vvooiidd **ppggaaddddrr,, DDBBTT **ppggccooookkiiee)),, iinntt ((**ppggoouutt))((ddbb__ppggnnoo__tt ppggnnoo,, vvooiidd **ppggaaddddrr,, DDBBTT **ppggccooookkiiee))));; iinntt mmeemmpp__ttrriicckkllee((DDBB__MMPPOOOOLL **mmpp,, iinntt ppcctt,, iinntt **nnwwrrootteepp));; iinntt mmeemmpp__ssyynncc((DDBB__MMPPOOOOLL **mmpp,, LLSSNN **llssnn));; iinntt mmeemmpp__ssttaatt((DDBB__MMPPOOOOLL **mmpp,, DDBB__MMPPOOOOLL__SSTTAATT ****ggsspp,, DDBB__MMPPOOOOLL__FFSSTTAATT **((**ffsspp))[[]],, vvooiidd **((**ddbb__mmaalllloocc))((ssiizzee__tt))));; DDEESSCCRRIIPPTTIIOONN The DB library is a family of groups of functions that provides a modular programming interface to transactions and record-oriented file access. The library includes support for transactions, locking, logging and file page caching, as well as various indexed access methods. Many of the functional groups (e.g., the file page caching functions) are useful independent of the other DB func- tions, although some functional groups are explicitly based on other functional groups (e.g., transactions and logging). For a general description of the DB package, see _d_b___i_n_t_r_o(3). This manual page describes the specific details of the memory pool interface. The _d_b___m_p_o_o_l functions are the library interface intended to provide general-purpose, page-oriented buffer manage- ment of one or more files. While designed to work with the other DB functions, these functions are also useful for more general purposes. The memory pools (DB_MPOOL's) are referred to in this document as simply ``pools''. Pools may be shared between processes. Pools are usually filled by pages from one or more files (DB_MPOOLFILE's). Pages in the pool are replaced in LRU (least-recently- used) order, with each new page replacing the page that has been unused the longest. Pages retrieved from the pool using _m_e_m_p___f_g_e_t are ``pinned'' in the pool, by default, until they are returned to the pool's control using the _m_e_m_p___f_p_u_t function. _m_e_m_p___o_p_e_n The _m_e_m_p___o_p_e_n function copies a pointer, to the memory pool identified by the ddiirreeccttoorryy _d_i_r, into the memory location referenced by _r_e_g_i_o_n_p. If the _d_b_e_n_v argument to _m_e_m_p___o_p_e_n was initialized using _d_b___a_p_p_i_n_i_t, _d_i_r is interpreted as described by _d_b___a_p_p_i_n_i_t(3). Otherwise, if _d_i_r is not NULL, it is interpreted relative to the current working directory of the process. If _d_i_r is NULL, the following environment variables are checked in order: ``TMPDIR'', ``TEMP'', and ``TMP''. If one of them is set, memory pool files are created relative to the directory it specifies. If none of them are set, the first possible one of the following directories is used: _/_v_a_r_/_t_m_p, _/_u_s_r_/_t_m_p, _/_t_e_m_p, _/_t_m_p, _C_:_/_t_e_m_p and _C_:_/_t_m_p. All files associated with the memory pool are created in this directory. This directory must already exist when memory pool_open is called. If the memory pool already exists, the process must have permission to read and write the existing files. If the memory pool does not already exist, it is optionally created and initialized. The _f_l_a_g_s and _m_o_d_e arguments specify how files will be opened and/or created when they don't already exist. The flags value is specified by oorr'ing together one or more of the following values: DB_CREATE 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. DB_MPOOL_PRIVATE Create a private MPOOL that is not shared with any other process (although it may be shared with other threads). DB_NOMMAP Always copy files in this memory pool into the local cache instead of mapping them into process memory (see the description of the _m_p___m_m_a_p_s_i_z_e field of the DB_ENV structure for further information). DB_THREAD Cause the DB_MPOOL handle returned by the _m_e_m_p___o_p_e_n function to be useable by multiple threads within a single address space, i.e., to be ``free-threaded''. All files created by the memory pool subsystem (other than files created by the _m_e_m_p___f_o_p_e_n function, which are sepa- rately specified) are created with mode _m_o_d_e (as described in _c_h_m_o_d(2)) and modified by the process' umask value at the time of creation (see _u_m_a_s_k(2)). The group ownership of created files is based on the system and directory defaults, and is not further specified by DB. The memory pool subsystem is configured based on the _d_b_e_n_v argument to _m_e_m_p___o_p_e_n, which is a pointer to a structure of type DB_ENV (typedef'd in ). Applications will normally use the same DB_ENV structure (initialized by _d_b___a_p_p_i_n_i_t(3)), as an argument to all of the subsystems in the DB package. References to the DB_ENV structure are maintained by 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. In order to ensure compatibility with future releases of DB, all fields of the DB_ENV structure that are not explicitly set should be initial- ized to 0 before the first time the structure is used. Do this by declaring the structure external or static, or by calling the C library routine _b_z_e_r_o(3) or _m_e_m_s_e_t(3). The fields of the DB_ENV structure used by _m_e_m_p___o_p_e_n are described below. If _d_b_e_n_v is NULL or any of its fields are set to 0, defaults appropriate for the system are used where possible. The following fields in the DB_ENV structure may be ini- tialized before calling _m_e_m_p___o_p_e_n: void *(*db_errcall)(char *db_errpfx, char *buffer); FILE *db_errfile; const char *db_errpfx; int db_verbose; The error fields of the DB_ENV behave as described for _d_b___a_p_p_i_n_i_t(3). size_t mp_mmapsize; 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 bet- ter-than-usual performance, as available virtual mem- ory is normally much larger than the local cache, and page faults are faster than page copying on many sys- tems. 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 _m_p___m_m_a_p_s_i_z_e is non-zero, it speci- fies the maximum file size, in bytes, for a file to be mapped into the process address space. By default, it is set to 10Mb. size_t mp_size; The suggested size of the pool, 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 (16 8K byte pages), and may not be less than 20K bytes. The _m_e_m_p___o_p_e_n function returns the value of _e_r_r_n_o on fail- ure and 0 on success. _m_e_m_p___c_l_o_s_e The _m_e_m_p___c_l_o_s_e function closes the pool indicated by the DB_MPOOL pointer _m_p, as returned by _m_e_m_p___o_p_e_n. This func- tion does not imply a call to _m_e_m_p___f_s_y_n_c, but does imply a call to _m_e_m_p___f_c_l_o_s_e for any remaining open DB_MPOOLFILE pointers returned to this process by calls to _m_e_m_p___f_o_p_e_n. In addition, if the _d_i_r argument to _m_e_m_p___o_p_e_n was NULL and _d_b_e_n_v was not initialized using _d_b___a_p_p_i_n_i_t, or the DB_MPOOL_PRIVATE flag was set, all files created for this shared region will be removed, as if _m_e_m_p___u_n_l_i_n_k were called. When multiple threads are using the DB_MPOOL handle con- currently, only a single thread may call the _m_e_m_p___c_l_o_s_e function. The _m_e_m_p___c_l_o_s_e function returns the value of _e_r_r_n_o on failure and 0 on success. _m_e_m_p___f_o_p_e_n The _m_e_m_p___f_o_p_e_n function opens a file in the pool specified by the DB_MPOOL argument, copying the DB_MPOOLFILE pointer representing it into the memory location referenced by _m_p_f. The _f_i_l_e argument is the name of the file to be opened. If _f_i_l_e is NULL, a private file is created that cannot be shared with any other process (although it may be shared with other threads). The _f_l_a_g_s and _m_o_d_e arguments specify how files will be opened and/or created when they don't already exist. The flags value is specified by oorr'ing together one or more of the following values: DB_CREATE 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. DB_NOMMAP Always copy this file into the local cache instead of mapping it into process memory (see the description of the _m_p___m_m_a_p_s_i_z_e field of the DB_ENV structure for further information). DB_RDONLY Open any underlying files for reading only. Any attempt to write the file using the pool functions will fail, regardless of the actual permissions of the file. All files created by the function _m_e_m_p___f_o_p_e_n are created with mode _m_o_d_e (as described in _c_h_m_o_d(2)) and modified by the process' umask value at the time of creation (see _u_m_a_s_k(2)). The group ownership of created files is based on the system and directory defaults, and is not further specified by DB. The _p_a_g_e_s_i_z_e argument is the size, in bytes, of the unit of transfer between the application and the pool, although it is not necessarily the unit of transfer between the pool and the source file. Files opened in the pool may be further configured based on the _f_i_n_f_o_p argument to _m_e_m_p___f_o_p_e_n, which is a pointer to a structure of type DB_MPOOL_FINFO (typedef'd in ). No references to the _f_i_n_f_o_p structure are main- tained by DB, so it may be discarded when the _m_e_m_p___f_o_p_e_n function returns. In order to ensure compatibility with future releases of DB, all fields of the DB_MPOOL_FINFO structure that are not explicitly set should be initial- ized to 0 before the first time the structure is used. Do this by declaring the structure external or static, or by calling the C library routine _b_z_e_r_o(3) or _m_e_m_s_e_t(3). The fields of the DB_MPOOL_FINFO structure used by _m_e_m_p___f_o_p_e_n are described below. If _f_i_n_f_o_p is NULL or any of its fields are set to their default value, defaults appropriate for the system are used. int ftype; The _f_t_y_p_e field should be the same as a _f_t_y_p_e argu- ment previously specified to the _m_e_m_p___r_e_g_i_s_t_e_r func- tion, unless no input or output processing of the file's pages are necessary, in which case it should be 0. (See the description of the _m_e_m_p___r_e_g_i_s_t_e_r function for more information.) DBT *pgcookie; The _p_g_c_o_o_k_i_e field contains the byte string that is passed to the _p_g_i_n and _p_g_o_u_t functions for this file, if any. If no _p_g_i_n or _p_g_o_u_t functions are specified, the _p_g_c_o_o_k_i_e field should be NULL. (See the descrip- tion of the _m_e_m_p___r_e_g_i_s_t_e_r function for more informa- tion.) u_int8_t *fileid; The _f_i_l_e_i_d field is a unique identifier for the file. The mpool functions must be able to uniquely identify files in order that multiple processes sharing a file will correctly share its underlying pages. Normally, the _f_i_l_e_i_d field should be NULL and the mpool func- tions will use the file's device and inode numbers (see _s_t_a_t(2)) for this purpose. On some filesystems, (e.g., FAT or NFS) file device and inode numbers are not necessarily unique across system reboots. AAppppllii-- ccaattiioonnss wwaannttiinngg ttoo mmaaiinnttaaiinn aa sshhaarreedd mmeemmoorryy bbuuffffeerr ppooooll aaccrroossss ssyysstteemm rreebboooottss,, wwhheerree tthhee ppooooll ccoonnttaaiinnss ppaaggeess ffrroomm ffiilleess ssttoorreedd oonn ssuucchh ffiilleessyysstteemmss,, mmuusstt ssppeecciiffyy aa uunniiqquuee ffiillee iiddeennttiiffiieerr ttoo tthhee _m_e_m_p___f_o_p_e_n call and each process opening or registering the file must provide the same unique identifier. If the _f_i_l_e_i_d field is non-NULL, it must reference a DB_FILE_ID_LEN (as defined in ) length array of bytes that will be used to uniquely identify the file. This should not be necessary for most applica- tions. Specifically, it is not necessary if the mem- ory pool is re-instantiated after each system reboot, the application is using the DB access methods instead of calling the pool functions explicitly, or the files in the memory pool are stored on filesys- tems where the file device and inode numbers do not change across system reboots. int32_t lsn_offset; The _l_s_n___o_f_f_s_e_t field is the zero-based byte offset in the page of the page's log sequence number (LSN), or -1 if no LSN offset is specified. (See the descrip- tion of the _m_e_m_p___s_y_n_c function for more information.) u_int32_t clear_len; The _c_l_e_a_r___l_e_n field is the number of initial bytes in a page that should be set to zero when the page is created as a result of the DB_MPOOL_CREATE or DB_MPOOL_NEW flags being specified to _m_e_m_p___f_g_e_t. If _f_i_n_f_o_p is NULL or _c_l_e_a_r___l_e_n is 0, the entire page is cleared. The _m_e_m_p___f_o_p_e_n function returns the value of _e_r_r_n_o on failure and 0 on success. _m_e_m_p___f_c_l_o_s_e The _m_e_m_p___f_c_l_o_s_e function closes the source file indicated by the DB_MPOOLFILE pointer _m_p_f. This function does not imply a call to _m_e_m_p___f_s_y_n_c, i.e. no pages are written to the source file as as a result of calling _m_e_m_p___f_c_l_o_s_e. In addition, if the _f_i_l_e argument to _m_e_m_p___f_o_p_e_n was NULL, any underlying files created for this DB_MPOOLFILE will be removed. The _m_e_m_p___f_c_l_o_s_e function returns the value of _e_r_r_n_o on failure and 0 on success. _m_e_m_p___f_g_e_t The _m_e_m_p___f_g_e_t function copies a pointer to the page with the page number specified by _p_g_n_o_a_d_d_r, from the source file specified by the DB_MPOOLFILE pointer _m_p_f, into the memory location referenced by _p_a_g_e_p. If the page does not exist or cannot be retrieved, _m_e_m_p___f_g_e_t will fail. The returned page is size_t type aligned. PPaaggee nnuummbbeerrss bbeeggiinn aatt 00,, ee..gg..,, tthhee ffiirrsstt ppaaggee iinn tthhee ffiillee iiss ppaaggee nnuummbbeerr 00,, nnoott ppaaggee nnuummbbeerr 11.. The _f_l_a_g_s argument is specified by oorr'ing together one or more of the following values: DB_MPOOL_CREATE If the specified page does not exist, create it. In this case, the _p_g_i_n function, if specified, is called. DB_MPOOL_LAST Return the last page of the source file and copy its page number to the location referenced by _p_g_n_o_a_d_d_r. DB_MPOOL_NEW Create a new page in the file and copy its page num- ber to the location referenced by _p_g_n_o_a_d_d_r. In this case, the _p_g_i_n function, if specified, is not called. The DB_MPOOL_CREATE, DB_MPOOL_LAST and DB_MPOOL_NEW flags are mutually exclusive. Created pages have all their bytes set to 0, unless other- wise specified when the file was opened. All pages returned by _m_e_m_p___f_g_e_t will be retained (i.e. ``pinned'') in the pool until a subsequent call to _m_e_m_p___f_p_u_t. The _m_e_m_p___f_g_e_t function returns the value of _e_r_r_n_o on fail- ure and 0 on success. _m_e_m_p___f_p_u_t The _m_e_m_p___f_p_u_t function indicates that the page referenced by _p_g_a_d_d_r can be evicted from the pool. _P_g_a_d_d_r must be an address previously returned by _m_e_m_p___f_g_e_t. The _f_l_a_g_s argument is specified by oorr'ing together one or more of the following values: DB_MPOOL_CLEAN Clear any previously set modification information (i.e., don't bother writing the page back to the source file). DB_MPOOL_DIRTY The page has been modified and must be written to the source file before being evicted from the pool. DB_MPOOL_DISCARD The page is unlikely to be useful in the near future, and should be discarded before other pages in the pool. The DB_MPOOL_CLEAN and DB_MPOOL_DIRTY flags are mutually exclusive. The _m_e_m_p___f_p_u_t function returns the value of _e_r_r_n_o on fail- ure and 0 on success. _m_e_m_p___f_s_e_t The _m_e_m_p___f_s_e_t function sets the flags associated with the page referenced by _p_g_a_d_d_r without unpinning it from the pool. _P_g_a_d_d_r must be an address previously returned by _m_e_m_p___f_g_e_t. The _f_l_a_g_s argument to _m_e_m_p___f_s_e_t is specified by oorr'ing together one or more of the values specified as flags for the _m_e_m_p___f_p_u_t call. The _m_e_m_p___f_s_e_t function returns the value of _e_r_r_n_o on fail- ure and 0 on success. _m_e_m_p___f_s_y_n_c The _m_e_m_p___f_s_y_n_c function writes all pages associated with the DB_MPOOLFILE pointer _m_p_f, that were marked as modified using _m_e_m_p___f_p_u_t or _m_e_m_p___f_s_e_t, back to the source file. If any of the modified pages are also pinned (i.e., currently referenced by this or another process) _m_e_m_p___f_s_y_n_c will ignore them. The _m_e_m_p___f_s_y_n_c function returns the value of _e_r_r_n_o on failure, 0 on success, and DB_INCOMPLETE if there were pages which were modified but which _m_e_m_p___f_s_y_n_c was unable to write. _m_e_m_p___u_n_l_i_n_k The _m_e_m_p___u_n_l_i_n_k function destroys the memory pool identi- fied by the directory _d_i_r, removing all files used to implement the memory pool. (The directory _d_i_r is not removed.) If there are processes that have called _m_e_m_p___o_p_e_n without calling _m_e_m_p___c_l_o_s_e (i.e., there are pro- cesses currently using the memory pool), _m_e_m_p___u_n_l_i_n_k will fail without further action, unless the force flag is set, in which case _m_e_m_p___u_n_l_i_n_k will attempt to remove the mem- ory pool files regardless of any processes still using the memory pool. The result of attempting to forcibly destroy the region when a process has the region open is unspecified. Pro- cesses 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 memory pool will either fail or attempt to create a new region. On other systems, e.g., WNT, where the _u_n_l_i_n_k(2) system call will fail if any process has an open file descriptor for the file, the region removal will fail. In the case of catastrophic or system failure, database recovery must be performed (see _d_b___r_e_c_o_v_e_r(1) or the DB_RECOVER and DB_RECOVER_FATAL flags to _d_b___a_p_p_i_n_i_t(3)). Alternatively, if recovery is not required because no database state is maintained across failures, it is possi- ble to clean up a memory pool by removing all of the files in the directory specified to the _m_e_m_p___o_p_e_n function, as memory pool files are never created in any directory other than the one specified to _m_e_m_p___o_p_e_n. Note, however, that this has the potential to remove files created by the other DB subsystems in this database environment. The _m_e_m_p___u_n_l_i_n_k function returns the value of _e_r_r_n_o on failure and 0 on success. _m_e_m_p___r_e_g_i_s_t_e_r The _m_e_m_p___r_e_g_i_s_t_e_r function registers page-in and page-out functions for files of type _f_t_y_p_e in the specified pool. If the _p_g_i_n function is non-NULL, it is called each time a page is read into the memory pool from a file of type _f_t_y_p_e, or a page is created for a file of type _f_t_y_p_e (see the DB_MPOOL_CREATE flag for the _m_e_m_p___f_g_e_t function). If the _p_g_o_u_t function is non-NULL, it is called each time a page is written to a file of type _f_t_y_p_e. Both the _p_g_i_n and _p_g_o_u_t functions are called with the page number, a pointer to the page being read or written, and any argument _p_g_c_o_o_k_i_e that was specified to the _m_e_m_p___f_o_p_e_n function when the file was opened. The _p_g_i_n and _p_g_o_u_t functions should return 0 on success, and an applicable non-zero _e_r_r_n_o value on failure, in which case the _d_b___m_p_o_o_l function calling it will also fail, returning that _e_r_r_n_o value. The purpose of the _m_e_m_p___r_e_g_i_s_t_e_r function is to support processing when pages are entered into, or flushed from, the pool. A file type must be specified to make it possi- ble for unrelated threads or processes, that are sharing a pool, to evict each other's pages from the pool. Applica- tions should call _m_e_m_p___r_e_g_i_s_t_e_r, during initialization, for each type of file requiring input or output processing that will be sharing the underlying pool. (No registry is necessary for the standard access method types, btree, hash and recno, as _d_b___o_p_e_n(3) registers them separately.) If a thread or process does not call _m_e_m_p___r_e_g_i_s_t_e_r for a file type, it is impossible for it to evict pages for any file requiring input or output processing from the pool. For this reason, _m_e_m_p___r_e_g_i_s_t_e_r should always be called by each application sharing a pool for each type of file included in the pool, regardless of whether or not the application itself uses files of that type. There are no standard values for _f_t_y_p_e, _p_g_i_n, _p_g_o_u_t and _p_g_c_o_o_k_i_e, except that the _f_t_y_p_e value for a file must be a non-zero positive number, as negative numbers are reserved for internal use by the DB library. For this reason, applications sharing a pool must coordinate their values amongst themselves. The _m_e_m_p___r_e_g_i_s_t_e_r function returns the value of _e_r_r_n_o on failure and 0 on success. _m_e_m_p___t_r_i_c_k_l_e The _m_e_m_p___t_r_i_c_k_l_e function ensures that at least _p_c_t per- cent of the pages in the shared memory pool are clean by writing dirty pages to their backing files. If the _n_w_r_o_t_e_p argument is non-NULL, the number of pages that were written to reach the correct percentage is returned in the memory location it references. The purpose of the _m_e_m_p___t_r_i_c_k_l_e function is to enable a memory pool manager to ensure that a page is always avail- able for reading in new information without having to wait for a write. The _m_e_m_p___t_r_i_c_k_l_e function returns the value of _e_r_r_n_o on failure and 0 on success. _m_e_m_p___s_y_n_c The _m_e_m_p___s_y_n_c function ensures that all the modified pages in the pool with log sequence numbers (LSNs) less than the _l_s_n argument are written to disk. The _m_e_m_p___s_y_n_c function returns the value of _e_r_r_n_o on fail- ure, 0 on success, and DB_INCOMPLETE if there were pages which need to be written but which _m_e_m_p___s_y_n_c was unable to write immediately. In addition, if _m_e_m_p___s_y_n_c returns suc- cess, the value of _l_s_n will be overwritten with the largest LSN from any page which was written by _m_e_m_p___s_y_n_c to satisfy this request. The purpose of the _m_e_m_p___s_y_n_c function is to enable a transaction manager to ensure, as part of a checkpoint, that all pages modified by a certain time have been writ- ten to disk. Pages in the pool which cannot be written back to disk immediately (e.g., are currently pinned) are written to disk as soon as it is possible to do so. The expected behavior of the transaction manager is to call the _m_e_m_p___s_y_n_c function and then, if the return indicates that some pages could not be written immediately, to wait briefly and retry again with the same LSN until the _m_e_m_p___s_y_n_c function returns that all pages have been writ- ten. To support the _m_e_m_p___s_y_n_c functionality, it is necessary that the pool functions know the location of the LSN on the page for each file type. This location should be specified when the file is opened using the _m_e_m_p___f_o_p_e_n function. (Note, it is not required that the LSN be aligned on the page in any way.) _m_e_m_p___s_t_a_t The _m_e_m_p___s_t_a_t function creates statistical structures and copies pointers to them into user-specified memory loca- tions. The statistics include the number of files partic- ipating in the pool, the active pages in the pool, and information as to how effective the cache has been. Statistical structures are created in allocated memory. If _d_b___m_a_l_l_o_c is non-NULL, it is called to allocate the memory, otherwise, the library function _m_a_l_l_o_c(3) is used. The function _d_b___m_a_l_l_o_c must match the calling conventions of the _m_a_l_l_o_c(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. If _g_s_p is non-NULL, the global statistics for the memory pool _m_p are copied into the memory location it references. The global statistics are stored in a structure of type DB_MPOOL_STAT (typedef'd in ). The following DB_MPOOL_STAT fields will be filled in: u_int32_t st_refcnt; The number of references to the region. u_int32_t st_regsize; The size of the region. size_t st_cachesize; Cache size in bytes. u_int32_t st_cache_hit; Requested pages found in the cache. u_int32_t st_cache_miss; Requested pages not found in the cache. u_int32_t st_map; Requested pages mapped into the process' address space (there is no available information as to whether or not this request caused disk I/O, although examining the application page fault rate may be helpful). u_int32_t st_page_create; Pages created in the cache. u_int32_t st_page_in; Pages read into the cache. u_int32_t st_page_out; Pages written from the cache to the backing file. u_int32_t st_ro_evict; Clean pages forced from the cache. u_int32_t st_rw_evict; Dirty pages forced from the cache. u_int32_t st_hash_buckets; Number of hash buckets in buffer hash table. u_int32_t st_hash_searches; Total number of buffer hash table lookups. u_int32_t st_hash_longest; The longest chain ever encountered in buffer hash table lookups. u_int32_t st_hash_examined; Total number of hash elements traversed during hash table lookups. u_int32_t st_page_clean; Clean pages currently in the cache. u_int32_t st_page_dirty; Dirty pages currently in the cache. u_int32_t st_page_trickle; Dirty pages written using the _m_e_m_p___t_r_i_c_k_l_e interface. u_int32_t st_region_wait; The number of times that a thread of control was forced to wait before obtaining the region lock. u_int32_t st_region_nowait; The number of times that a thread of control was able to obtain the region lock without waiting. If _f_s_p is non-NULL, a pointer to a NULL-terminated vari- able length array of statistics for individual files, in the memory pool _m_p, is copied into the memory location it references. If no individual files currently exist in the memory pool, _f_s_p will be set to NULL. The per-file statistics are stored in structures of type DB_MPOOL_FSTAT (typedef'd in ). The following DB_MPOOL_FSTAT fields will be filled in for each file in the pool, i.e., each element of the array: char *file_name; The name of the file. size_t st_pagesize; Page size in bytes. u_int32_t st_cache_hit; Requested pages found in the cache. u_int32_t st_cache_miss; Requested pages not found in the cache. u_int32_t st_map; Requested pages mapped into the process' address space. u_int32_t st_page_create; Pages created in the cache. u_int32_t st_page_in; Pages read into the cache. u_int32_t st_page_out; Pages written from the cache to the backing file. The _m_e_m_p___s_t_a_t function returns the value of _e_r_r_n_o on fail- ure and 0 on success. EENNVVIIRROONNMMEENNTT VVAARRIIAABBLLEESS The following environment variables affect the execution of _d_b___m_p_o_o_l: DB_HOME If the _d_b_e_n_v argument to _m_e_m_p___o_p_e_n was initialized using _d_b___a_p_p_i_n_i_t, the environment variable DB_HOME may be used as the path of the database home for the interpretation of the _d_i_r argument to _m_e_m_p___o_p_e_n, as described in _d_b___a_p_p_i_n_i_t(3). TMPDIR If the _d_b_e_n_v argument to _m_e_m_p___o_p_e_n was NULL or not initialized using _d_b___a_p_p_i_n_i_t, the environment vari- able TMPDIR may be used as the directory in which to create the memory pool, as described in the _m_e_m_p___o_p_e_n section above. EERRRROORRSS The _m_e_m_p___o_p_e_n function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: DBmemp->pgin(3), DBmemp->pgout(3), close(2), db_version(3), fcntl(2), fflush(3), fsync(2), log_com- pare(3), log_flush(3), lseek(2), malloc(3), memcmp(3), memcpy(3), memp_close(3), memp_unlink(3), memset(3), mmap(2), munmap(2), open(2), sigfillset(3), sigproc- mask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), time(3), unlink(2), and write(2). In addition, the _m_e_m_p___o_p_e_n function may fail and return _e_r_r_n_o for the following conditions: [EAGAIN] The shared memory region was locked and (repeatedly) unavailable. [EINVAL] An invalid flag value or parameter was specified. The DB_THREAD flag was specified and spinlocks are not implemented for this architecture. A NULL pathname was specified without the DB_MPOOL_PRIVATE flag. The specified cache size was impossibly small. The _m_e_m_p___c_l_o_s_e function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: close(2), fcntl(2), fflush(3), memp_fclose(3), munmap(2), and strerror(3). The _m_e_m_p___f_o_p_e_n function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: DBmemp->pgin(3), DBmemp->pgout(3), close(2), fcntl(2), fflush(3), fsync(2), log_compare(3), log_flush(3), lseek(2), malloc(3), memcmp(3), memcpy(3), memset(3), mmap(2), open(2), sigfillset(3), sigproc- mask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), time(3), unlink(2), and write(2). In addition, the _m_e_m_p___f_o_p_e_n function may fail and return _e_r_r_n_o for the following conditions: [EINVAL] An invalid flag value or parameter was specified. The file has already been entered into the pool, and the _p_a_g_e_s_i_z_e value is not the same as when the file was entered into the pool, or the length of the file is not zero or a multiple of the _p_a_g_e_s_i_z_e. The DB_RDONLY flag was specified for an in-memory pool. The _m_e_m_p___f_c_l_o_s_e function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: close(2), fcntl(2), fflush(3), munmap(2), and strerror(3). The _m_e_m_p___f_g_e_t function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: DBmemp->pgin(3), DBmemp->pgout(3), close(2), fcntl(2), fflush(3), fsync(2), log_compare(3), log_flush(3), lseek(2), malloc(3), memcmp(3), memcpy(3), memset(3), mmap(2), open(2), read(2), sigfillset(3), sig- procmask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), time(3), unlink(2), and write(2). In addition, the _m_e_m_p___f_g_e_t function may fail and return _e_r_r_n_o for the following conditions: [EAGAIN] The page reference count has overflowed. (This should never happen unless there's a bug in the application.) [EINVAL] An invalid flag value or parameter was specified. The DB_MPOOL_NEW flag was set and the source file was not opened for writing. The requested page does not exist and DB_MPOOL_CREATE was not set. More than one of DB_MPOOL_CREATE, DB_MPOOL_LAST and DB_MPOOL_NEW was set. [ENOMEM] The cache is full and no more pages will fit in the pool. The _m_e_m_p___f_p_u_t function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: DBmemp->pgin(3), DBmemp->pgout(3), close(2), fcntl(2), fflush(3), fsync(2), log_compare(3), log_flush(3), lseek(2), malloc(3), memcmp(3), memcpy(3), memset(3), mmap(2), open(2), sigfillset(3), sigproc- mask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), time(3), unlink(2), and write(2). In addition, the _m_e_m_p___f_p_u_t function may fail and return _e_r_r_n_o for the following conditions: [EACCES] The DB_MPOOL_DIRTY flag was set and the source file was not opened for writing. [EINVAL] An invalid flag value or parameter was specified. The _p_g_a_d_d_r parameter does not reference a page returned by _m_e_m_p___f_g_e_t. More than one of DB_MPOOL_CLEAN and DB_MPOOL_DIRTY was set. The _m_e_m_p___f_s_e_t function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: fcntl(2), and fflush(3). In addition, the _m_e_m_p___f_s_e_t function may fail and return _e_r_r_n_o for the following conditions: [EINVAL] An invalid flag value or parameter was specified. The _m_e_m_p___f_s_y_n_c function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: DBmemp->pgin(3), DBmemp->pgout(3), close(2), fcntl(2), fflush(3), fsync(2), log_compare(3), log_flush(3), lseek(2), malloc(3), memcpy(3), memset(3), open(2), qsort(3), realloc(3), sigfillset(3), sigproc- mask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), unlink(2), and write(2). The _m_e_m_p___u_n_l_i_n_k function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: close(2), fcntl(2), fflush(3), malloc(3), mem- cpy(3), memset(3), mmap(2), munmap(2), open(2), sig- fillset(3), sigprocmask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), and unlink(2). In addition, the _m_e_m_p___u_n_l_i_n_k function may fail and return _e_r_r_n_o for the following conditions: [EBUSY] The shared memory region was in use and the force flag was not set. The _m_e_m_p___r_e_g_i_s_t_e_r function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: fcntl(2), and malloc(3). The _m_e_m_p___t_r_i_c_k_l_e function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: DBmemp->pgin(3), DBmemp->pgout(3), close(2), fcntl(2), fflush(3), fsync(2), log_compare(3), log_flush(3), lseek(2), malloc(3), memcmp(3), memcpy(3), memset(3), mmap(2), open(2), sigfillset(3), sigproc- mask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), time(3), unlink(2), and write(2). In addition, the _m_e_m_p___t_r_i_c_k_l_e function may fail and return _e_r_r_n_o for the following conditions: [EINVAL] An invalid flag value or parameter was specified. The _m_e_m_p___s_y_n_c function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: DBmemp->pgin(3), DBmemp->pgout(3), close(2), fcntl(2), fflush(3), fsync(2), log_compare(3), log_flush(3), lseek(2), malloc(3), memcmp(3), memcpy(3), memset(3), mmap(2), open(2), qsort(3), realloc(3), sig- fillset(3), sigprocmask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), time(3), unlink(2), and write(2). In addition, the _m_e_m_p___s_y_n_c function may fail and return _e_r_r_n_o for the following conditions: [EINVAL] An invalid flag value or parameter was specified. The _m_e_m_p___s_y_n_c function was called without logging having been initialized in the environment. The _m_e_m_p___s_t_a_t function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: fcntl(2), malloc(3), memcpy(3), and strlen(3). SSEEEE AALLSSOO _d_b___a_r_c_h_i_v_e(1), _d_b___c_h_e_c_k_p_o_i_n_t(1), _d_b___d_e_a_d_l_o_c_k(1), _d_b___d_u_m_p(1), _d_b___l_o_a_d(1), _d_b___r_e_c_o_v_e_r(1), _d_b___s_t_a_t(1), _d_b___i_n_t_r_o(3), _d_b___a_p_p_i_n_i_t(3), _d_b___c_u_r_s_o_r(3), _d_b___d_b_m(3), _d_b___i_n_t_e_r_n_a_l(3), _d_b___l_o_c_k(3), _d_b___l_o_g(3), _d_b___m_p_o_o_l(3), _d_b___o_p_e_n(3), _d_b___t_h_r_e_a_d(3), _d_b___t_x_n(3)