cdb(3)cdb(3)NAMEcdb - Constant DataBase library
SYNOPSYS
#include <cdb.h>
cc ... -lcdb
DESCRIPTIONcdb is a library to create and access Constant DataBase files. File
stores (key,value) pairs and used to quickly find a value based on a
given key. Cdb files are create-once files, that is, once created,
file cannot be updated but recreated from scratch -- this is why data‐
base is called constant. Cdb file is optimized for quick access. For‐
mat of such file described in cdb(5) manpage. This manual page corre‐
sponds to version 0.78 of tinycdb package.
Library defines two non-interlaced interfaces: for querying existing
cdb file data (read-only mode) and for creating such a file (almost
write-only). Strictly speaking, those modes allows very limited set of
opposite operation as well (i.e. in query mode, it is possible to
update key's value).
All routines in this library are thread-safe as no global data used,
except of errno variable for error indication.
cdb datafiles may be moved between systems safely, since format does
not depend on architecture.
QUERY MODE
There are two query modes available. First uses a structure that rep‐
resents a cdb database, just like FILE structure in stdio library, and
another works with plain filedescriptor. First mode is more sophisti‐
cated and flexible, and usually somewhat faster. It uses mmap(2)
internally. This mode may look more "natural" or object-oriented com‐
pared to second one.
The following routines works with any mode:
unsigned cdb_unpack(buf)
const unsigned char buf[4];
helper routine to convert 32-bit integer from internal represen‐
tation to machine format. May be used to handle application
integers in a portable way. There is no error return.
Query Mode 1
All query operations in first more deals with common data structure,
struct cdb, associated with an open file descriptor. This structure is
opaque to application.
The following routines exists for accessing cdb database:
int cdb_init(cdbp, fd)
struct cdb *cdbp;
int fd;
initializes structure given by cdbp pointer and associates it
with opened file descriptor fd. Memory allocation for structure
itself if needed and file open operation should be done by
application. File fd should be opened at least read-only, and
should be seekable. Routine returns 0 on success or negative
value on error.
void cdb_free(cdbp)
struct cdb *cdbp;
frees internal resources held by structure. Note that this rou‐
tine does not closes a file.
int cdb_fileno(cdbp)
const struct cdb *cdbp;
returns filedescriptor associated with cdb (as was passed to
cdb_init()).
int cdb_read(cdbp, buf, len, pos)
int cdb_readdata(cdbp, buf, len, pos)
int cdb_readkey(cdbp, buf, len, pos)
const struct cdb *cdbp;
void *buf;
unsigned len;
unsigned pos;
reads a data from cdb file, starting at position pos of length
len, placing result to buf. This routine may be used to get
actual value found by cdb_find() or other routines that returns
position and length of a data. Returns 0 on success or negative
value on error. Routines cdb_readdata() and cdb_readkey() are
shorthands to read current (after e.g. cdb_find()) data and key
respectively, using cdb_read().
const void *cdb_get(cdbp, len, pos)
const void *cdb_getdata(cdbp)
const void *cdb_getkey(cdbp)
const struct cdb *cdbp;
unsigned len;
unsigned pos;
Internally, cdb library uses memory-mmaped region to access the
on-disk database. cdb_get() allows to access internal memory in
a way similar to cdb_read() but without extra copying and buffer
allocation. Returns pointer to actual data on success or NULL
on error (position points to outside of the database). Routines
cdb_getdata() and cdb_getkey() are shorthands to access current
(after e.g. cdb_find()) data and key respectively, using
cdb_get().
int cdb_find(cdbp, key, klen)
unsigned cdb_datapos(cdbp)
unsigned cdb_datalen(cdbp)
unsigned cdb_keypos(cdbp)
unsigned cdb_keylen(cdbp)
struct cdb *cdbp;
const void *key;
unsigned klen;
attempts to find a key given by (key,klen) parameters. If key
exists in database, routine returns 1 and places position and
length of value associated with this key to internal fields
inside cdbp structure, to be accessible by cdb_datapos(cdbp) and
cdb_datalen(cdbp) routines. If key is not in database,
cdb_find() returns 0. On error, negative value is returned.
Data pointers (available via cdb_datapos() and cdb_datalen())
gets updated only in case of successful search. Note that using
cdb_find() it is possible to lookup only first record with a
given key.
int cdb_findinit(cdbfp, cdbp, key, klen)
int cdb_findnext(cdbfp)
struct cdb_find *cdbfp;
const struct cdb *cdbp;
const void *key;
unsigned klen;
sequential-find routines that used separate structure. It is
possible to have more than one record with the same key in a
database, and these routines allows to enumerate all them.
cdb_findinit() initializes search structure pointed to by cdbfp.
It will return negative value on error or non-negative value on
success. cdb_findnext() attempts to find next (first when
called right after cdb_findinit()) matching key, setting value
position and length in cdbfp structure. It will return positive
value if given key was found, 0 if there is no more such key(s),
or negative value on error. To access value position and length
after successful call to cdb_findnext() (when it returned posi‐
tive result), use cdb_datapos(cdbp) and cdb_datalen(cdbp) rou‐
tines. It is error to continue using cdb_findnext() after it
returned 0 or error condition (cdb_findinit() should be called
again). Current data pointers (available via cdb_datapos() and
cdb_datalen()) gets updated only on successful search.
void cdb_seqinit(cptr, cdbp)
int cdb_seqnext(cptr, cdbp)
unsigned *cptr;
struct cdb *cdbp;
sequential enumeration of all records stored in cdb file.
cdb_seqinit() initializes access current data pointer cptr to
point before first record in a cdb file. cdb_seqnext() updates
data pointers in cdbp to point to the next record and updates
cptr, returning positive value on success, 0 on end of data con‐
dition and negative value on error. Current record will be
available after successful operation using cdb_datapos(cdbp) and
cdb_datalen(cdbp) (for the data) and cdb_keypos(cdbp) and
cdb_keylen(cdbp) (for the key of the record). Data pointers
gets updated only in case of successful operation.
Query Mode 2
In this mode, one need to open a cdb file using one of standard system
calls (such as open(2)) to obtain a filedescriptor, and then pass that
filedescriptor to cdb routines. Available methods to query a cdb data‐
base using only a filedescriptor include:
int cdb_seek(fd, key, klen, dlenp)
int fd;
const void *key;
unsigned klen;
unsigned *dlenp;
searches a cdb database (as pointed to by fd filedescriptor) for
a key given by (key, klen), and positions file pointer to start
of data associated with that key if found, so that next read
operation from this filedescriptor will read that value, and
places length of value, in bytes, to variable pointed to by
dlenp. Returns positive value if operation was successful, 0 if
key was not found, or negative value on error. To read the data
from a cdb file, cdb_bread() routine below can be used.
int cdb_bread(fd, buf, len)
int fd;
void *buf;
int len;
reads data from a file (as pointed to by fd filedescriptor) and
places len bytes from this file to a buffer pointed to by buf.
Returns 0 if exactly len bytes was read, or a negative value in
case of error or end-of-file. This routine ignores interrupt
errors (EINTR). Sets errno variable to EIO in case of end-of-
file condition (when there is less than len bytes available to
read).
Notes
Note that value of any given key may be updated in place by another
value of the same size, by writing to file at position found by
cdb_find() or cdb_seek(). However one should be very careful when
doing so, since write operation may not succeed in case of e.g. power
failure, thus leaving corrupted data. When database is (re)created,
one can guarantee that no incorrect data will be written to database,
but not with inplace update. Note also that it is not possible to
update any key or to change length of value.
CREATING MODEcdb database file should usually be created in two steps: first, tempo‐
rary file created and written to disk, and second, that temporary file
is renamed to permanent place. Unix rename(2) call is atomic opera‐
tion, it removes destination file if any AND renaes another file in one
step. This way it is guaranteed that readers will not see incomplete
database. To prevent multiple simultaneous updates, locking may also
be used.
All routines used to create cdb database works with struct cdb_make
object that is opaque to application. Application may assume that
struct cdb_make has at least the same member(s) as published in struct
cdb above.
int cdb_make_start(cdbmp, fd)
struct cdb_make *cdbmp;
int fd;
initializes structure to create a database. File fd should be
opened read-write and should be seekable. Returns 0 on success
or negative value on error.
int cdb_make_add(cdbmp, key, klen, val, vlen)
struct cdb_make *cdbmp;
const void *key, *val;
unsigned klen, vlen;
adds record with key (key,klen) and value (val,vlen) to a data‐
base. Returns 0 on success or negative value on error. Note
that this routine does not checks if given key already exists,
but cdb_find() will not see second record with the same key. It
is not possible to continue building a database if
cdb_make_add() returned error indicator.
int cdb_make_finish(cdbmp)
struct cdb_make *cdbmp;
finalizes database file, constructing all needed indexes, and
frees memory structures. It does not closes filedescriptor.
Returns 0 on success or negative value on error.
int cdb_make_exists(cdbmp, key, klen)
struct cdb_make *cdbmp;
const void *key;
unsigned klen;
This routine attempts to find given by (key,klen) key in a not-
yet-complete database. It may significantly slow down the whole
process, and currently it flushes internal buffer to disk on
every call with key those hash value already exists in db.
Returns 0 if such key doesn't exists, 1 if it is, or negative
value on error. Note that database file should be opened read-
write (not write-only) to use this routine. If
cdb_make_exists() returned error, it may be not possible to con‐
tinue constructing database.
int cdb_make_find(cdbmp, key, klen, mode)
struct cdb_make *cdbmp;
const void *key;
unsigned klen;
int mode;
This routine attempts to find given by (key,klen) key in the
database being created. If the given key is already exists, it
an action specified by mode will be performed:
CDB_FIND
checks whenever the given record is already in the data‐
base.
CDB_FIND_REMOVE
removes all matching records by re-writing the database
file accordingly.
CDB_FIND_FILL0
fills all matching records with zeros and removes them
from index so that the records in question will not be
findable with cdb_find(). This is faster than
CDB_FIND_REMOVE, but leaves zero "gaps" in the database.
Lastly inserted records, if matched, are always removed.
If no matching keys was found, routine returns 0. In case at
least one record has been found/removed, positive value will be
returned. On error, negative value will be returned and errno
will be set appropriately. When cdb_make_find() returned nega‐
tive value in case of error, it is not possible to continue con‐
structing the database.
cdb_make_exists() is the same as calling cdb_make_find() with
mode set to CDB_FIND.
int cdb_make_put(cdbmp, key, klen, val, vlen, mode)
struct cdb_make *cdbmp;
const void *key, *val;
unsigned klen, vlen;
int mode;
This is a somewhat combined cdb_make_exists() and cdb_make_add()
routines. mode argument controls how repeated (already exist‐
ing) keys will be treated:
CDB_PUT_ADD
no duplicate checking will be performed. This mode is
the same as cdb_make_add() routine does.
CDB_PUT_REPLACE
If the key already exists, it will be removed from the
database before adding new key,value pair. This requires
moving data in the file, and can be quite slow if the
file is large. All matching old records will be removed
this way. This is the same as calling cdb_make_find()
with CDB_FIND_REMOVE mode argument followed by calling
cdb_make_add().
CDB_PUT_REPLACE0
If the key already exists and it isn't the last record in
the file, old record will be zeroed out before adding new
key,value pair. This is alot faster than
CDB_PUT_REPLACE, but some extra data will still be
present in the file. The data -- old record -- will not
be accessible by normal searches, but will appear in
sequential database traversal. This is the same as call‐
ing cdb_make_find() with CDB_FIND_FILL0 mode argument
followed by cdb_make_add().
CDB_PUT_INSERT
add key,value pair only if such key does not exists in a
database. Note that since query (see query mode above)
will find first added record, this mode is somewhat use‐
less (but allows to reduce database size in case of
repeated keys). This is the same as calling
cdb_make_exists(), followed by cdb_make_add() if the key
was not found.
CDB_PUT_WARN
add key,value pair unconditionally, but also check if
this key already exists. This is equivalent of
cdb_make_exists() to check existence of the given key,
unconditionally followed by cdb_make_add().
If any error occurred during operations, the routine will return
negative integer and will set global variable errno to indicate
reason of failure. In case of successful operation and no
duplicates found, routine will return 0. If any duplicates has
been found or removed (which, in case of CDB_PUT_INSERT mode,
indicates that the new record was not added), routine will
return positive value. If an error occurred and cdb_make_put()
returned negative error, it is not possible to continue database
construction process.
As with cdb_make_exists() and cdb_make_find(), usage of this
routine with any but CDB_PUT_ADD mode can significantly slow
down database creation process, especially when mode is equal to
CDB_PUT_REPLACE0.
void cdb_pack(num, buf)
unsigned num;
unsigned char buf[4];
helper routine that used internally to convert machine integer n
to internal form to be stored in datafile. 32-bit integer is
stored in 4 bytes in network byte order. May be used to handle
application data. There is no error return.
unsigned cdb_hash(buf, len)
const void *buf;
unsigned len;
helper routine that calculates cdb hash value of given bytes.
CDB hash function is
hash[n] = (hash[n-1] + (hash[n-1] << 5)) ^ buf[n]
starting with
hash[-1] = 5381
ERRORScdb library may set errno to following on error:
EPROTO database file is corrupted in some way
EINVAL the same as EPROTO above if system lacks EPROTO constant
EINVAL flag argument for cdb_make_put() is invalid
EEXIST flag argument for cdb_make_put() is CDB_PUT_INSERT, and key
already exists
ENOMEM not enough memory to complete operation (cdb_make_finish and
cdb_make_add)
EIO set by cdb_bread and cdb_seek if a cdb file is shorter than
expected or corrupted in some other way.
EXAMPLES
Note: in all examples below, error checking is not shown for brewity.
Query Mode
int fd;
struct cdb cdb;
char *key, *data;
unsigned keylen, datalen;
/* opening the database */
fd = open(filename, O_RDONLY);
cdb_init(&cdb, fd);
/* initialize key and keylen here */
/* single-record search. */
if (cdb_find(&cdb, key, keylen) > 0) {
datalen = cdb_datalen(&cdb);
data = malloc(datalen + 1);
cdb_read(&cdb, data, datalen, cdb_datapos(&cdb));
data[datalen] = '\0';
printf("key=%s data=%s\n", key, data);
free(data);
}
else
printf("key=%s not found\n", key);
/* multiple record search */
struct cdb_find cdbf;
int n;
cdb_findinit(&cdbf, &cdb, key, keylen);
n = 0;
while(cdb_findnext(&cdbf) > 0) {
datalen = cdb_datalen(&cdb);
data = malloc(datalen + 1);
cdb_read(&cdb, data, datalen, cdb_datapos(&cdb));
data[datalen] = '\0';
printf("key=%s data=%s\n", key, data);
free(data);
++n;
}
printf("key=%s %d records found\n", n);
/* sequential database access */
unsigned pos;
int n;
cdb_seqinit(&pos, &cdb);
n = 0;
while(cdb_seqnext(&pos, &cdb) > 0) {
keylen = cdb_keylen(&cdb);
key = malloc(keylen + 1);
cdb_read(&cdb, key, keylen, cdb_keypos(&cdb));
key[keylen] = '\0';
datalen = cdb_datalen(&cdb);
data = malloc(datalen + 1);
cdb_read(&cdb, data, datalen, cdb_datapos(&cdb));
data[datalen] = '\0';
++n;
printf("record %n: key=%s data=%s\n", n, key, data);
free(data); free(key);
}
printf("total records found: %d\n", n);
/* close the database */
cdb_free(&cdb);
close(fd);
/* simplistic query mode */
fd = open(filename, O_RDONLY);
if (cdb_seek(fd, key, keylen, &datalen) > 0) {
data = malloc(datalen + 1);
cdb_bread(fd, data, datalen);
data[datalen] = '\0';
printf("key=%s data=%s\n", key, data);
}
else
printf("key=%s not found\n", key);
close(fd);
Create Mode
int fd;
struct cdb_make cdbm;
char *key, *data;
unsigned keylen, datalen;
/* initialize the database */
fd = open(filename, O_RDWR|O_CREAT|O_TRUNC, 0644);
cdb_make_start(&cdbm, fd);
while(have_more_data()) {
/* initialize key and data */
if (cdb_make_exists(&cdbm, key, keylen) == 0)
cdb_make_add(&cdbm, key, keylen, data, datalen);
/* or use cdb_make_put() with appropriate flags */
}
/* finalize and close the database */
cdb_make_finish(&cdbm);
close(fd);
SEE ALSOcdb(5), cdb(1), dbm(3), db(3), open(2).
AUTHOR
The tinycdb package written by Michael Tokarev <mjt@corpit.ru>, based
on ideas and shares file format with original cdb library by Dan Bern‐
stein.
LICENSE
Public domain.
Jun 2006 cdb(3)
