Table of Contents
DB records contain two parts — a key and some data. Both the key
and its corresponding data are
encapsulated in
DBT structures.
Therefore, to access a DB record, you need two such
structures,
one for the key and
one for the data.
DBT structures provide a void *
field that you use to point to your data, and another field that identifies
the data length. They can therefore be used to store anything from simple
primitive data to complex structures so long as the information you want to
store resides in a single contiguous block of memory.
This chapter describes
DBT
usage. It also
introduces storing and retrieving key/value pairs from a database.
Each database record is comprised of two
DBT structures
— one for the key and another for the data.
To store a database record where the key and/or the data are primitive
data (int, float, and so forth),
or where the key and/or the data contain an array, we need only to point
to the memory location where that data resides and identify its
length. For example:
#include <db.h> #include <string.h> ... DBT key, data; float money = 122.45; char *description = "Grocery bill."; /* Zero out the DBTs before using them. */ memset(&key, 0, sizeof(DBT)); memset(&data, 0, sizeof(DBT)); key.data = &money; key.size = sizeof(float); data.data = description; data.size = strlen(description) + 1;
To retrieve the record, simply assign the void * returned in the
DBT
to the appropriate variable.
Note that in the following example we do not allow DB to assign the
memory for the retrieval of the money value. The reason why is that some
systems may require float values to have a specific alignment, and the
memory as returned by DB
may not be properly aligned (the same problem may exist for structures
on some systems). We tell DB to use our memory instead of its
own by specifying the DB_DBT_USERMEM flag. Be aware that
when we do this, we must also identify how much user memory is available
through the use of the ulen field.
#include <db.h> #include <string.h> ... float money; DBT key, data; char *description; /* Initialize the DBTs */ memset(&key, 0, sizeof(DBT)); memset(&data, 0, sizeof(DBT)); key.data = &money; key.ulen = sizeof(float); key.flags = DB_DBT_USERMEM; /* Database retrieval code goes here */ /* * Money is set into the memory that we supplied. */ description = data.data;