partial.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: partial.so,v 10.25 2003/10/18 19:15:56 bostic Exp $])

m4_ref_title(Access Methods,
    Partial record storage and retrieval,
    @partial record storage and retrieval, am_misc/get_bulk, am_misc/struct)

m4_p([dnl
It is possible to both store and retrieve parts of data items in all
m4_db access methods.  This is done by setting the
m4_ref(DB_DBT_PARTIAL) flag in the m4_ref(Dbt) structure passed to the
m4_db method.])

m4_p([dnl
The m4_ref(DB_DBT_PARTIAL) flag is based on the values of two fields
of the m4_ref(Dbt) structure: m4_arg(dlen) and m4_arg(doff).  The value
of m4_arg(dlen) is the number of bytes of the record in which the
application is interested.  The value of m4_arg(doff) is the offset from
the beginning of the data item where those bytes start.])

m4_p([dnl
For example, if the data item were m4_bold(ABCDEFGHIJKL), a m4_arg(doff)
value of 3 would indicate that the bytes of interest started at
m4_bold(D), and a m4_arg(dlen) value of 4 would indicate that the bytes
of interest were m4_bold(DEFG).])

m4_p([dnl
When retrieving a data item from a database, the m4_arg(dlen) bytes
starting m4_arg(doff) bytes from the beginning of the record are
returned, as if they comprised the entire record.  If any or all of the
specified bytes do not exist in the record, the retrieval is still
successful and any existing bytes are returned.])

m4_p([dnl
When storing a data item into the database, the m4_arg(dlen) bytes
starting m4_arg(doff) bytes from the beginning of the specified key's
data record are replaced by the data specified by the m4_arg(data) and
m4_arg(size) fields.  If m4_arg(dlen) is smaller than m4_arg(size), the
record will grow, and if m4_arg(dlen) is larger than m4_arg(size), the
record will shrink. If the specified bytes do not exist, the record will
be extended using nul bytes as necessary, and the store call will still
succeed.])

m4_p([dnl
The following are various examples of the put case for the
m4_ref(DB_DBT_PARTIAL) flag. In all examples, the initial data item is 20
bytes in length:])

m4_p([m4_bold([ABCDEFGHIJ0123456789])])

m4_nlistbegin
m4_nlist([m4_indent([dnl
size = 20
doff = 0
dlen = 20
data = abcdefghijabcdefghij
m4_blank
Result: The 20 bytes at offset 0 are replaced by the 20 bytes of data;
that is, the entire record is replaced.
m4_blank
ABCDEFGHIJ0123456789 -__GT__ abcdefghijabcdefghij
])])

m4_nlist([m4_indent([dnl
size = 10
doff = 20
dlen = 0
data = abcdefghij
m4_blank
Result: The 0 bytes at offset 20 are replaced by the 10 bytes of data;
that is, the record is extended by 10 bytes.
m4_blank
ABCDEFGHIJ0123456789 -__GT__ ABCDEFGHIJ0123456789abcdefghij
])])

m4_nlist([m4_indent([dnl
size = 10
doff = 10
dlen = 5
data = abcdefghij
m4_blank
Result: The 5 bytes at offset 10 are replaced by the 10 bytes of data.
m4_blank
ABCDEFGHIJ0123456789 -__GT__ ABCDEFGHIJabcdefghij56789
])])

m4_nlist([m4_indent([dnl
size = 10
doff = 10
dlen = 0
data = abcdefghij
m4_blank
Result: The 0 bytes at offset 10 are replaced by the 10 bytes of data;
that is, 10 bytes are inserted into the record.
m4_blank
ABCDEFGHIJ0123456789 -__GT__ ABCDEFGHIJabcdefghij0123456789
])])

m4_nlist([m4_indent([dnl
size = 10
doff = 2
dlen = 15
data = abcdefghij
m4_blank
Result: The 15 bytes at offset 2 are replaced by the 10 bytes of data.
m4_blank
ABCDEFGHIJ0123456789 -__GT__ ABabcdefghij789
])])

m4_nlist([m4_indent([dnl
size = 10
doff = 0
dlen = 0
data = abcdefghij
m4_blank
Result: The 0 bytes at offset 0 are replaced by the 10 bytes of data;
that is, the 10 bytes are inserted at the beginning of the record.
m4_blank
ABCDEFGHIJ0123456789 -__GT__ abcdefghijABCDEFGHIJ0123456789
])])

m4_nlist([m4_indent([dnl
size = 0
doff = 0
dlen = 10
data = ""
m4_blank
Result: The 10 bytes at offset 0 are replaced by the 0 bytes of data;
that is, the first 10 bytes of the record are discarded.
m4_blank
ABCDEFGHIJ0123456789 -__GT__ 0123456789
])])

m4_nlist([m4_indent([dnl
size = 10
doff = 25
dlen = 0
data = abcdefghij
m4_blank
Result: The 0 bytes at offset 25 are replaced by the 10 bytes of data;
that is, 10 bytes are inserted into the record past the end of the
current data (\0 represents a nul byte).
m4_blank
ABCDEFGHIJ0123456789 -__GT__ ABCDEFGHIJ0123456789\0\0\0\0\0abcdefghij
])])

m4_nlistend

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