Sample code of using Berkeley DB

Berkeley DB is ubiquitous but a bit complicated. Although there are some alternative out there and some wrapper exists, it is good to know about how to manipulate it.

Header and compilation

To use Berkeley DB, of course, you need the header file and the related libraries. If no pre-compiled binary is available, get the source and compile it. For example, I downloaded the db-4.5.20.tar.gz and then,

$ tar zxf db-4.5.20.tar.gz
$ cd db-4.5.20/build_unix
$ ../dist/configure
...
$ make
...
$ cd ../..

So the header file is now db-4.5.20/build_unix/db.h and the compiled libraries is at db-4.5.20/build_unix/libdb.a

Hence, add the following line at the beginning of the code:

#include <db.h>

and invoke the compiler like this:

$ gcc -c -o testdb.o testdb.c -Ipath/to/db-4.5.20/build_unix
$ gcc -o testdb testdb.o -Lpath/to/db-4.5.20/build_unix -ldb

Open the database

DB *dbp;           /* DB structure handle */ 
u_int32_t flags;   /* database open flags */ 
int ret;           /* function return value */ 

void openDB()
{
	/* Initialize the structure. This 
	 * database is not opened in an environment, 
	 * so the environment pointer is NULL. */ 
	ret = db_create(&dbp, NULL, 0); 
	if (ret != 0) { 
		fprintf(stderr, "Error initializing database structure.\n");
		exit(1);
	} 

	/* Database open flags */ 
	flags = DB_CREATE;	/* If the database does not exist, create it.*/ 
		/* Alternative flags: (join by bitwise OR)
		 *   DB_EXCL - Exclusive database creation. Fail if the database already exists
		 *   DB_RDONLY - Open database fo read operations only
		 *   DB_TRUNCATE - Empty the file before opening, i.e. delete everything inside
		 */

	/* open the database */ 
	ret = dbp->open(dbp,		/* DB structure pointer */ 
			NULL,		/* Transaction pointer */ 
			"my_db.db",	/* On-disk file that holds the database. */ 
			NULL,		/* Optional logical database name */ 
			DB_BTREE,	/* Database access method */ 
			flags,		/* Open flags */ 
			0);		/* File mode (using defaults) */ 
	if (ret != 0) { 
		fprintf(stderr, "Error opening database my_db.db\n");
		exit(1);
	};
};

Closing the database

void closeDB()
{
	/* When we're done with the database, close it. */ 
	if (dbp != NULL) 
		dbp->close(dbp, 0); 
};

Delete the database

This means to unlink the database file from the filesystem.

void deleteDB()
{
	dbp->remove(	dbp,			/* Database pointer */
			"mydb.db",		/* Database file to remove */
			NULL,			/* Database to remove. This is
						 * NULL so the entire file is 
						 * removed. */
			0);			/* Flags. None used. */
};

Rename the database

Rename the filename of the database.

void renameDB()
{
	dbp->rename(	dbp,			/* Database pointer */
			"mydb.db",		/* Database file to rename */
			NULL,			/* Database to rename. This is 
						 * NULL so the entire file is 
						 * renamed. */
			"newdb.db",		/* New database file name */
			0);			/* Flags. None used. */
};

Write to the database

void writeDB()
{
	DBT key, data; 
	char *keystr = "One";
	float value = 1.00; 
	int ret; 

	/* Zero out the DBTs before using them. */ 
	memset(&key, 0, sizeof(DBT)); 
	memset(&data, 0, sizeof(DBT)); 
	key.data = keystr; 
	key.size = strlen(keystr)+1;
	data.data = &value;
	data.size = sizeof(float);
	ret = dbp->put(dbp, NULL, &key, &data, DB_NOOVERWRITE); 
	if (ret == DB_KEYEXIST) { 
		dbp->err(dbp, ret, "Failed inserting into database as the key already exists");
	};
};

Read from the database

Below is to retrieve exactly one record for the exact matching of the key:

void readDB()
{
	DBT key, data; 
	char *keystr = "One";
	float value = 5.5; 
	int ret;
	
	/* Zero out the DBTs before using them. */ 
	memset(&key, 0, sizeof(DBT)); 
	memset(&data, 0, sizeof(DBT)); 
	key.data = keystr;
	key.size = strlen(keystr)+1;
	data.data = &value;
	data.ulen = sizeof(float);
	data.flags = DB_DBT_USERMEM;
	ret = dbp->get(dbp, NULL, &key, &data, 0); 
	if (ret == DB_NOTFOUND) {
		dbp->err(dbp, ret, "Data not found in the database");
	};
	printf("%f\n",*(float*)(data.data));
};

Alternatively, we can also retrieve records using a cursor.

Delete from the database

void deleteRecord()
{
	DBT key;
	char *keystr = "One";
	
	/* Zero out the DBTs before using them. */ 
	memset(&key, 0, sizeof(DBT)); 
	key.data = keystr;
	key.size = strlen(keystr)+1;

	dbp->del(dbp, NULL, &key, 0); 
};

Walking through the database

Use of cursor is required for walking through the database:

void walkDB()
{
	DBC *cursorp; 
	DBT key, data; 
	int ret; 

	/* Initialize cursor */ 
	dbp->cursor(dbp, NULL, &cursorp, 0); 

	/* Initialize our DBTs. */ 
	memset(&key, 0, sizeof(DBT)); 
	memset(&data, 0, sizeof(DBT)); 

	/* Iterate over the database, retrieving each record in turn. */
		/* use DB_NEXT for interating forward
		 * and DB_PREV for backward */
	} while (<#condition#>) 
	while (!(ret=cursorp->c_get(cursorp, &key, &data, DB_NEXT))) {
		printf("%s - %f\n", key.data, *(float*)(data.data));
	}; 
	if (ret != DB_NOTFOUND) { 
		fprintf(stderr,"Nothing found in the database.\n");
		exit(1);
	};

	/* Close cursor before exit */
	if (cursorp != NULL) 
		cursorp->c_close(cursorp); 
};