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<h2 class="title" style="clear: both"><a id="am_misc_struct"></a>Storing C/C++
structures/objects</h2>
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<p>
Berkeley DB can store any kind of data, that is, it is
entirely 8-bit clean. How you use this depends, to some
extent, on the application language you are using. In the
C/C++ languages, there are a couple of different ways to store
structures and objects.
</p>
<p>
First, you can do some form of run-length encoding and copy
your structure into another piece of memory before storing
it:
</p>
<a id="prog_am23"></a>
<pre class="programlisting">struct {
char *data1;
u_int32_t data2;
...
} info;
size_t len;
u_int8_t *p, data_buffer[1024];
p = &data_buffer[0];
len = strlen(info.data1);
memcpy(p, &len, sizeof(len));
p += sizeof(len);
memcpy(p, info.data1, len);
p += len;
memcpy(p, &info.data2, sizeof(info.data2));
p += sizeof(info.data2);
...
</pre>
<p>
and so on, until all the fields of the structure have been
loaded into the byte array. If you want more examples, see the
Berkeley DB logging routines (for example,
btree/btree_auto.c:__bam_split_log()). This technique is
generally known as "marshalling". If you use this technique,
you must then un-marshall the data when you read it
back:
</p>
<a id="prog_am24"></a>
<pre class="programlisting">struct {
char *data1;
u_int32_t data2;
...
} info;
size_t len;
u_int8_t *p, data_buffer[1024];
...
p = &data_buffer[0];
memcpy(&len, p, sizeof(len));
p += sizeof(len);
info.data1 = malloc(len);
memcpy(info.data1, p, len);
p += len;
memcpy(&info.data2, p, sizeof(info.data2));
p += sizeof(info.data2);
...
</pre>
<p>
and so on.
</p>
<p>
The second way to solve this problem only works if you have
just one variable length field in the structure. In that case,
you can declare the structure as follows:
</p>
<pre class="programlisting">struct {
int a, b, c;
u_int8_t buf[1];
} info;</pre>
<p>
Then, let's say you have a string you want to store in this
structure. When you allocate the structure, you allocate it
as:
</p>
<pre class="programlisting">malloc(sizeof(struct info) + strlen(string));</pre>
<p>
Since the allocated memory is contiguous, you can the
initialize the structure as:
</p>
<pre class="programlisting">info.a = 1;
info.b = 2;
info.c = 3;
memcpy(&info.buf[0], string, strlen(string) + 1);</pre>
<p>
and give it to Berkeley DB to store, with a length
of:
</p>
<pre class="programlisting">sizeof(struct info) + strlen(string);</pre>
<p>
In this case, the structure can be copied out of the
database and used without any additional work.
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