Description - Introduction

2.5.2 Description
GNU dbm is a library of routines that manages data files that contain key/data pairs. The access provided is that of storing, retrieval, and deletion by key and a non-sorted traversal of all keys. A process is allowed to use multiple data files at the same time.
A process that opens a gdbm file is designated as a "reader" or a "writer". Only one writer may open a gdbm file and many readers may open the file. Readers and writers can not open the gdbm file at the same time. The procedure for opening a gdbm file is:
GDBM_FILE dbf;
dbf = gdbm_open ( name, block_size, read_write, mode, fatal_func )
Name is the name of the file (the complete name, gdbm does not append any characters to this name). Block_size is the size of a single transfer from disk to memory. This parameter is ignored unless the file is a new file. The minimum size is 512. If it is less than 512, dbm will use the stat block size for the file system. Read_write can have one of the following values:
GDBM_READER reader
GDBM_WRITER writer
GDBM_WRCREAT writer - if database does not exist create new one
GDBM_NEWDB writer - create new database regardless if one exists
For the last three (writers of the database) there is an extra value that that can be added to read_write by bitwise or, GDBM_FAST. This requests that gdbm write the database with no disk file syncronization. This allows faster writes, but may produce an inconsistant database in the event of abnormal termination of the writer.
Mode is the file mode (Read, Write or both) if the file is created. (*Fatal_func) () is a function for dbm to call if it detects a fatal error. The only parameter of this function is a string. If the value of 0 is provided, gdbm will use a default function.
The return value dbf is the pointer needed by all other routines to access that gdbm file. If the return is the NULL pointer, gdbm_open was not successful. The errors can be found in gdbm_errno for gdbm errors and in errno for system errors. (For error codes, refer to gdbmerrno.h)
In all of the following calls, the parameter dbf refers to the pointer returned from gdbm_open.
It is important that every file opened is also closed. This is needed to update the reader/writer count on the file. This is done by:
gdbm_close (dbf);
The database is used by 3 primary routines. The first stores data in the database.
ret = gdbm_store ( dbf, key, content, flag )
Dbf is the pointer returned by gdbm_open. Key is the key data. Content is the data to be associated with the key. Flag can have one of the following values:
GDBM_INSERT insert only, generate an error if key exists
GDBM_REPLACE replace contents if key exists.
If a reader calls gdbm_store, the return value will be -1. If called with GDBM_INSERT and key is in the database, the return value will be 1. Otherwise, the return value is 0.
If the data is stored for a key that is already in the data base, gdbm replaces the old data with the new data if called with GDBM_REPLACE. Two data items for the same key are not obtained and there is no error from gdbm_store.
To search for some data:
content = gdbm_fetch ( dbf, key )
Dbf is the pointer returned by gdbm_open. Key is the key data.
If the dptr element of the return value is NULL, no data was found. Otherwise the return value is a pointer to the found data. The storage space for the dptr element is allocated using malloc. Gdbm does not automatically free this data. It is the programmer's responsibility to free this storage when it is no longer needed.
To search for some data, without retrieving it:
ret = gdbm_exists ( dbf, key )
Dbf is the pointer returned by gdbm_open. Key is the key data to search for.
If the key is found within the database, the return value ret will be true. If nothing appropiate is found, ret will be false. This routine is useful for checking for the existance of a record, without performing the memory allocation done by gdbm_fetch.
To remove some data from the database:
ret = gdbm_delete ( dbf, key )
Dbf is the pointer returned by gdbm_open. Key is the key data.
The return value is -1 if the item is not present or the requester is a reader. The return value is 0 if there was a successful delete.
The next two routines allow for accessing all items in the database. This access is not key sequential, but it is guaranteed to visit every key in the database once. (The order has to do with the hash values.)
key = gdbm_firstkey ( dbf )
nextkey = gdbm_nextkey ( dbf, key )
Dbf is the pointer returned by gdbm_open. Key is the key data.
The return values are both of type datum. If the dptr element of the return value is NULL, there is no first key or next key. Again notice that dptr points to data allocated by malloc and gdbm will not free it for the developer.
These functions were intended to visit the database in read-only algorithms, for instance, to validate the database or similar operations.
File `visiting' is based on a `hash table'. gdbm_delete re-arranges the hash table to make sure that any collisions in the table do not leave some item `un-findable'. The original key order is NOT guaranteed to remain unchanged in ALL instances. It is possible that some key will not be visited if a loop like the following is executed:
key = gdbm_firstkey ( dbf );
while ( key.dptr ) {
nextkey = gdbm_nextkey ( dbf, key );
if ( some condition ) {
gdbm_delete ( dbf, key );
free ( key.dptr );
}
key = nextkey;
}
The following routine should be used very infrequently.
ret = gdbm_reorganize ( dbf )
If there are a lot of deletions and the developer would like to shrink the space used by the gdbm file, this routine will reorganize the database. Gdbm will not shorten the length of a gdbm file except by using this reorganization. (Deleted file space will be reused.)
If GDBM_FAST value is used in gdbm_open call, the following routine can be used to guarantee that the database is physically written to the disk file.
gdbm_sync ( dbf )
It will not return until the disk file state is syncronized with the in-memory state of the database.
To convert a gdbm error code into English text, use this routine:
ret = gdbm_strerror ( errno )
Where errno is of type gdbm_error, usually the global variable gdbm_errno. The appropiate phrase is returned.
gdbm now supports the ability to set certain options on an already open database.
ret = gdbm_setopt ( dbf, option, value, size )
Where dbf is the return value from a previous call to gdbm_open, and option specifies which option to set. The valid options are currently:
GDBM_CACHESIZE: Set the size of the internal bucketcache. This option may only be set once on each GDBM_FILE descriptor, and is set automatically to 100 upon the first access to the
database.
GDBM_FASTMODE:
Set fast mode to either on or off. This allows fast mode to be toggled on an already
open and active database. value (see below) should be set to either TRUE or FALSE.
value is the value to set option to, specified as an integer pointer. size is the size of the data pointed to by value. The return value will be -1 upon failure, or 0 upon success. The global variable gdbm_errno will be set upon failure.
For instance, to set a database to use a cache of 10, after opening it with gdbm_open, but prior to accessing it in any way, the following code could be used:
int value = 10;
ret = gdbm_setopt( dbf, GDBM_CACHESIZE, &value, sizeof(int));
The following two external variables may be useful:
gdbm_errno is the variable that contains more information about gdbm errors. (gdbm.h has the definitions of the error values and defines gdbm_errno as an external variable.)
gdbm_version is the string containing the version information
This gives an introduction to GNU Database manager. Embedded Shruti has used a version of GDBM ported into Windows CE so that it can be used by Pocket-PC Software Development Kit. Details of the GDBM functions used in Embedded Shruti will be provided in Chapter 4 which explains the complete design of Embedded Shruti.
Chapter 3
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