db_internal(3db)

NAME

db_jump_set, db_value_set - replace underlying DB functionality

SYNOPSIS

#include <db2/db.h>
int
db_jump_set(void *func, int which);
int
db_value_set(int value, int which);

DESCRIPTION

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 functions, 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 db_intro(3).

This manual page describes interfaces to tune or replace underlying system functionality used by the DB library. Few applications should have any need for these interfaces.

db_jump_set The db_jump_set function enables applications to replace underlying DB library functionality by replacing entries in a function call jump table. The which argument specifies the entry to be replaced by the argument func.

The following values of which are supported:

DB_FUNC_CLOSE: Replace all DB calls to the IEEE Std 1003.1b-1993 (``POSIX'') close function with func, which must conform to the standard interface.

DB_FUNC_DIRFREE

The DB library requires the ability to return any memory allocated as part of the routine which reads through a directory and creates a list of files that that the directory contains (see DB_FUNC_DIRLIST). The func argument must conform to the following interface:

int dirfree(char **namesp, int cnt);

The namesp and cnt arguments are the same values as were returned by the DB_FUNC_DIRLIST function.

The dirfree function returns the value of errno on failure and 0 on success.

DB_FUNC_DIRLIST

The DB library requires the ability to read through a directory and create a list of files that that the directory contains. The func argument must conform to the following interface:

int dirlist(const char *dir,
char ***namesp, int *cntp);

The dir argument is the name of the directory to be searched. The function must return a pointer to an array of nul-terminated file names in the memory location referenced by the argument namesp, and a count of the number of elements in the array in the memory location referenced by cntp.

The dirlist function returns the value of errno on failure and 0 on success.

DB_FUNC_EXISTS

The DB library requires the ability to determine if a file exists, and optionally, if it is a file of type directory. The func argument must conform to the following interface:

int exists(const char *path, int *isdirp);

The path argument is the pathname of the file to be checked.

If the isdirp argument is non-NULL, it must be set to non-0 if path is a directory, and 0 if path is not a directory.

The exists function returns the value of errno on failure and 0 on success.

DB_FUNC_FREE: Replace all DB calls to the ANSI C X3.159-1989 (``ANSI C'') standard free function with func, which must conform to the standard interface.

DB_FUNC_FSYNC: Replace all DB calls to the IEEE Std 1003.1b-1993 (``POSIX'') fsync function with func, which must conform to the standard interface.

DB_FUNC_IOINFO

The DB library requires the ability to determine the size and I/O characteristics of a file. The func argument must conform to the following interface:

int ioinfo(const char *path, int fd,
u_int32_t *mbytesp, u_int32_t *bytesp,
u_int32_t *iosizep);

The path argument is the pathname of the file to be checked, and the fd argument is an open file descriptor on the file.

If the mbytesp and bytesp arguments are non-NULL, the ioinfo function must return in them the size of the file: the number of megabytes in the file into the memory location referenced by the mbytesp argument, and the number of bytes over and above that number of megabytes into the memory location referenced by the bytesp argument.

In addition, if the iosizep argument is non-NULL, the ioinfo function must return the optimum granularity for I/O operations to the file in the memory location referenced by it.

The ioinfo function returns the value of errno on failure and 0 on success.

DB_FUNC_MALLOC: Replace all DB calls to the ANSI C X3.159-1989 (``ANSI C'') standard malloc function with func, which must conform to the standard interface.

DB_FUNC_MAP

The DB library requires the ability to map a file into memory and to create shared memory regions (which may or may not be backed by files). The func argument must conform to the following interface:

int map(char *path, int fd, size_t len,
int is_region, int is_anonymous, int is_rdonly,
void **addr);

The path argument is the name of a file. The fd argument is an open file descriptor on that file.

The is_region argument will be zero if the intention is to map a file into shared memory. In this case, the map function must map the first len bytes of the file into memory and return a pointer to the mapped location in the memory location referenced by the argument addr. In this case, the is_anonymous argument will always be zero. The is_rdonly argument will be non-zero if the file is considered read-only by the caller.

The is_region argument will be non-zero if the memory is intended to be used as a shared memory region for synchronization between DB threads/processes. In this case, the returned memory may be of any kind (e.g., anonymous), but must be able to support semaphores. If the application has previously specified that regions are to be instantiated in anonymous memory (see DB_REGION_ANON below), or the region is being joined and is believed to have been allocated in anonymous shared memory, the is_anonymous argument will be non-zero. Additionally, the path and fd arguments may be ignored (although future map calls using the same path must return the same memory), and the is_rdonly argument will always be zero.

By default, on UNIX systems, the DB library will use the IEEE Std 1003.1b-1993 (``POSIX'') mmap(2) interface to both map regular files into shared memory and create shared memory regions. If the application specifies that shared memory regions be instantiated in anonymous memory (see DB_REGION_ANON below), the shmget(2) shared memory segment interface will be used, where available, and the MAP_ANON or MAP_ANONYMOUS options to mmap(2) when shmget(2) is not available.

When using shmget(2), shared memory regions are named, and so multiple processes may share them. When using the mmap MAP_ANON or MAP_ANONYMOUS options, shared memory regions are not named, and so may only be accessed by a single process and its threads.

HP/UX note:: The shmget(2) interfaces are not used on HP/UX, even though they exist, as anonymous memory allocated using shmget(2) cannot be used to store semaphores.

The map function returns the value of errno on failure and 0 on success.

DB_FUNC_OPEN: Replace all DB calls to the IEEE Std 1003.1b-1993 (``POSIX'') open function with func, which must conform to the standard interface.

DB_FUNC_READ: Replace all DB calls to the IEEE Std 1003.1b-1993 (``POSIX'') read function with func, which must conform to the standard interface.

DB_FUNC_REALLOC: Replace all DB calls to the ANSI C X3.159-1989 (``ANSI C'') standard realloc function with func, which must conform to the standard interface.

DB_FUNC_RUNLINK

The DB library requires the ability to remove shared memory regions from the system, whether or not they are backed by regular files. The func argument must conform to the following interface:

int runlink(char *path);

The path argument is the path argument specified to the DB_FUNC_MAP function when the region was mapped into memory.

The runlink function returns the value of errno on failure and 0 on success.

DB_FUNC_SEEK

The DB library requires the ability to specify that a subsequent read from or write to a file will occur at a specific location in that file. The func argument must conform to the following interface:

int seek(int fd, size_t pgsize, db_pgno_t pageno,
u_int32_t relative, int rewind, int whence);

The fd argument is an open file descriptor on the file. The seek function must cause a subsequent read from or write to the file to occur at a byte offset specified by the calculation:

(pgsize * pageno) + relative

If rewind is non-zero, the byte offset is treated as a backwards seek, not a forwards one.

The whence argument specifies where in the file the byte offset is relative to, as described by the IEEE Std 1003.1b-1993 (``POSIX'') lseek system call.

The seek function returns the value of errno on failure and 0 on success.

DB_FUNC_SLEEP

The DB library requires the ability to cause a process to suspend itself for a period of time, relinquishing control of the processor to any other waiting thread of control. The func argument must conform to the following interface:

int sleep(u_long seconds, u_long microseconds);

The seconds and microseconds arguments specify the amount of time to wait until the suspending thread of control should run again.

The seconds and microseconds arguments may not be normalized when the sleep function is called, i.e., the microseconds argument may be greater than 1000000.

The sleep function returns the value of errno on failure and 0 on success.

DB_FUNC_UNLINK: Replace all DB calls to the IEEE Std 1003.1b-1993 (``POSIX'') unlink function with func, which must conform to the standard interface.

DB_FUNC_UNMAP

The DB library requires the ability to unmap a file or shared memory region from memory. The func argument must conform to the following interface:

int unmap(void *addr, size_t len);

The addr argument is the argument returned by the DB_FUNC_MAP function when the file or region was mapped into memory, and the len argument is the same as the len argument specified to the DB_FUNC_MAP function when the file or region was mapped into memory.

The unmap function returns the value of errno on failure and 0 on success.

DB_FUNC_WRITE: Replace all DB calls to the IEEE Std 1003.1b-1993 (``POSIX'') write function with func, which must conform to the standard interface.

DB_FUNC_YIELD

The DB library requires the ability to yield the processor from the current thread of control to any other waiting threads of control. The func argument must conform to the following interface:

int yield(void);

The yield function must be able to cause the rescheduling all participants in the current DB environment, whether threaded or not. It may be incorrect to supply a thread yield function if more than a single process is operating in the DB environment. This is because many thread-yield functions will not allow other processes to run, and the contested lock may be held by another process, not by another thread.

If no yield function is specified, or if the yield function returns an error, the function specified by the DB_FUNC_SLEEP entry will be used instead or subsequently, i.e., if no yield function is specified, or it is possible for the yield function to fail, the sleep function must cause the processor to reschedule any waiting threads of control for execution.

The yield function returns the value of errno on failure and 0 on success.

The db_jump_set function returns the value of errno on failure and 0 on success.

db_value_set The db_value_set function enables applications to specify underlying DB library functionality. The which argument specifies the information being set by the argument value.

The following values of which are supported:

DB_MUTEXLOCKS: Grant all requested mutual exclusion mutexes without testing for their availability. This flag should never be used for any other purpose than debugging.

DB_REGION_ANON: Setting value to a non-zero value specifies that shared memory regions are to be created in anonymous memory, and not backed by a regular file. DB_REGION_NAME differs from DB_REGION_ANON in that the former will fail if the shared memory regions cannot be named, that is, if multiple processes cannot use them. See DB_FUNC_MAP for more information.

DB_REGION_INIT: In some applications, the expense of page-faulting the shared memory regions can affect performance, e.g., when the page-fault occurs while holding a lock, other lock requests can convoy and overall throughput may decrease. Setting value to a non-zero value specifies that one byte be read from each 4K page of the shared memory region when the region is initialized.

DB_REGION_NAME: Setting value to a non-zero value specifies that shared memory regions are to be created in anonymous memory, and not backed by a regular file. DB_REGION_NAME differs from DB_REGION_ANON in that the former will fail if the shared memory regions cannot be named, that is, if multiple processes cannot use them. See DB_FUNC_MAP for more information.

DB_TSL_SPINS

Specify the number of times mutexes should spin without blocking.

This value defaults to 1 on uniprocessor systems and to 50 times the number of processors on multiprocessor systems.

The db_value_set function returns the value of errno on failure and 0 on success.

ERRORS

The db_jump_set function may fail and return errno for the following conditions:

[EINVAL]: An invalid flag value or parameter was specified.

BUGS

No type checking is done of the func argument, and specifying an invalid replacement routine will cause unpredictable results.

Applications should be careful to replace related functions as a group and at the same time. Replacing DB_FUNC_MALLOC without replacing DB_FUNC_REALLOC is likely to result in unpredictable results.

On Windows/95, files that are opened by multiple processes do not share data correctly. To tell Berkeley DB to use a named region of the paging file to share memory instead, use:

db_value_set(1, DB_REGION_NAME);

You do not need to do this if your application can guarantee that only one process will be accessing DB files.

On Windows/NT, sharing of data between processes through the paging file does not work correctly, so you should not call db_value_set. That will allow DB to use the file itself for sharing, which works correctly.