struct.so

berkeley db 4.6.21的源码。berkeley db是一个简单的数据库管理系统

m4_comment([$Id: struct.so,v 10.8 2002/12/22 20:42:09 bostic Exp $])

m4_ref_title(Access Methods,
    Storing C/C++ structures/objects,,
    am_misc/partial, am_misc/perm)

m4_p([dnl
m4_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.])

m4_p([dnl
First, you can do some form of run-length encoding and copy your
structure into another piece of memory before storing it:])

m4_indent([dnl
struct {
	char *data1;
	u_int32_t data2;
	...
} info;
size_t len;
u_int8_t *p, data_buffer__LB__1024__RB__;
m4_blank
p = &data_buffer__LB__0__RB__;
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);
...])

m4_p([dnl
and so on, until all the fields of the structure have been loaded into
the byte array.  If you want more examples, see the m4_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:])

m4_indent([dnl
struct {
	char *data1;
	u_int32_t data2;
	...
} info;
size_t len;
u_int8_t *p;
m4_blank
p = &data_buffer__LB__0__RB__;
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);
...])

m4_p([dnl
and so on.])

m4_p([dnl
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:])

m4_indent([dnl
struct {
	int a, b, c;
	u_int8_t buf__LB__1__RB__;
} info;])

m4_p([dnl
Then, let's say you have a string you want to store in this structure.
When you allocate the structure, you allocate it as:])

m4_indent([dnl
malloc(sizeof(struct info) + strlen(string));])

m4_p([dnl
Since the allocated memory is contiguous, you can the initialize the
structure as:])

m4_indent([dnl
info.a = 1;
info.b = 2;
info.c = 3;
memcpy(&info.buf__LB__0__RB__, string, strlen(string) + 1);])

m4_p([dnl
and give it to m4_db to store, with a length of:])

m4_indent([dnl
sizeof(struct info) + strlen(string);])

m4_p([dnl
In this case, the structure can be copied out of the database and used
without any additional work.])

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