Standard C Library Functions ndbm(3C)
NAME
ndbm, dbm_clearerr, dbm_close, dbm_delete, dbm_error,
dbm_fetch, dbm_firstkey, dbm_nextkey, dbm_open, dbm_store -
database functionsSYNOPSIS
#include
int dbm_clearerr(DBM *db);
void dbm_close(DBM *db);
int dbm_delete(DBM *db, datum key);
int dbm_error(DBM *db);
datum dbm_fetch(DBM *db, datum key);
datum dbm_firstkey(DBM *db);
datum dbm_nextkey(DBM *db);
DBM *dbm_open(const char *file, int open_flags, mode_t file_mode);
int dbm_store(DBM *db, datum key, datum content, int store_mode);
DESCRIPTION
These functions create, access and modify a database. They maintain key/content pairs in a database. The functions will handle large databases (up to a billion blocks) and will access a keyed item in one or two file system accesses. This package replaces the earlier dbm library, which managed only a single database. keys and contents are described by the datum typedef. A datum consists of at least two members, dptr and dsize. The dptr member points to an object that is dsize bytes in length. Arbitrary binary data, as well as ASCII character strings, may be stored in the object pointed to by dptr.SunOS 5.11 Last change: 17 Sep 2001 1
Standard C Library Functions ndbm(3C) The database is stored in two files. One file is a directory containing a bit map of keys and has .dir as its suffix.The second file contains all data and has .pag as its suf-
fix.The dbm_open() function opens a database. The file argument
to the function is the pathname of the database. The func-
tion opens two files named file.dir and file.pag. Theopen_flags argument has the same meaning as the flags argu-
ment of open(2) except that a database opened for write-only
access opens the files for read and write access. Thefile_mode argument has the same meaning as the third argu-
ment of open(2).The dbm_close() function closes a database. The argument db
must be a pointer to a dbm structure that has been returnedfrom a call to dbm_open().
The dbm_fetch() function reads a record from a database.
The argument db is a pointer to a database structure thathas been returned from a call to dbm_open(). The argument
key is a datum that has been initialized by the application program to the value of the key that matches the key of the record the program is fetching.The dbm_store() function writes a record to a database. The
argument db is a pointer to a database structure that hasbeen returned from a call to dbm_open(). The argument key is
a datum that has been initialized by the application program to the value of the key that identifies (for subsequent reading, writing or deleting) the record the program iswriting. The argument content is a datum that has been ini-
tialized by the application program to the value of therecord the program is writing. The argument store_mode con-
trols whether dbm_store() replaces any pre-existing record
that has the same key that is specified by the key argument.The application program must set store_mode to either
DBM_INSERT or DBM_REPLACE. If the database contains a
record that matches the key argument and store_mode is
DBM_REPLACE, the existing record is replaced with the new
record. If the database contains a record that matches thekey argument and store_mode is DBM_INSERT, the existing
record is not replaced with the new record. If the database does not contain a record that matches the key argument andstore_mode is either DBM_INSERT or DBM_REPLACE, the new
record is inserted in the database.SunOS 5.11 Last change: 17 Sep 2001 2
Standard C Library Functions ndbm(3C)The dbm_delete() function deletes a record and its key from
the database. The argument db is a pointer to a databasestructure that has been returned from a call to dbm_open().
The argument key is a datum that has been initialized by the application program to the value of the key that identifies the record the program is deleting.The dbm_firstkey() function returns the first key in the
database. The argument db is a pointer to a database struc-
ture that has been returned from a call to dbm_open().
The dbm_nextkey() function returns the next key in the data-
base. The argument db is a pointer to a database structurethat has been returned from a call to dbm_open(). The
dbm_firstkey() function must be called before calling
dbm_nextkey(). Subsequent calls to dbm_nextkey() return the
next key until all of the keys in the database have been returned.The dbm_error() function returns the error condition of the
database. The argument db is a pointer to a database struc-
ture that has been returned from a call to dbm_open().
The dbm_clearerr() function clears the error condition of
the database. The argument db is a pointer to a databasestructure that has been returned from a call to dbm_open().
These database functions support key/content pairs of at least 1024 bytes.RETURN VALUES
The dbm_store() and dbm_delete() functions return 0 when
they succeed and a negative value when they fail.The dbm_store() function returns 1 if it is called with a
flags value of DBM_INSERT and the function finds an existing
record with the same key.The dbm_error() function returns 0 if the error condition is
not set and returns a non-zero value if the error condition
is set.The return value of dbm_clearerr() is unspecified .
SunOS 5.11 Last change: 17 Sep 2001 3
Standard C Library Functions ndbm(3C)The dbm_firstkey() and dbm_nextkey() functions return a key
datum. When the end of the database is reached, the dptr member of the key is a null pointer. If an error is detected, the dptr member of the key is a null pointer and the error condition of the database is set.The dbm_fetch() function returns a content datum. If no
record in the database matches the key or if an error condi-
tion has been detected in the database, the dptr member of the content is a null pointer.The dbm_open() function returns a pointer to a database
structure. If an error is detected during the operation,dbm_open() returns a (DBM *)0.
ERRORS
No errors are defined.USAGE
The following code can be used to traverse the database:for(key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))
The dbm_ functions provided in this library should not be
confused in any way with those of a general-purpose database
management system. These functions do not provide for mul-
tiple search keys per entry, they do not protect againstmulti-user access (in other words they do not lock records
or files), and they do not provide the many other useful database functions that are found in more robust database management systems. Creating and updating databases by use of these functions is relatively slow because of data copies that occur upon hash collisions. These functions are useful for applications requiring fast lookup of relatively static information that is to be indexed by a single key. The dptr pointers returned by these functions may point into static storage that may be changed by subsequent calls.The dbm_delete() function does not physically reclaim file
space, although it does make it available for reuse.After calling dbm_store() or dbm_delete() during a pass
through the keys by dbm_firstkey() and dbm_nextkey(), the
application should reset the database by callingSunOS 5.11 Last change: 17 Sep 2001 4
Standard C Library Functions ndbm(3C)dbm_firstkey() before again calling dbm_nextkey().
EXAMPLES
Example 1 Using the Database Functions The following example stores and retrieves a phone number, using the name as the key. Note that this example does not include error checking.#include
#include
#include
#define NAME "Bill"
#define PHONE_NO "123-4567"
#define DB_NAME "phones"
main() { DBM *db;datum name = {NAME, sizeof (NAME)};
datum put_phone_no = {PHONE_NO, sizeof (PHONE_NO)};
datum get_phone_no;
/* Open the database and store the record */db = dbm_open(DB_NAME, O_RDWR | O_CREAT, 0660);
(void) dbm_store(db, name, put_phone_no, DBM_INSERT);
/* Retrieve the record */get_phone_no = dbm_fetch(db, name);
(void) printf("Name: %s, Phone Number: %s\n", name.dptr,
get_phone_no.dptr);
/* Close the database */dbm_close(db);
return (0); }ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:SunOS 5.11 Last change: 17 Sep 2001 5
Standard C Library Functions ndbm(3C)____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Committed ||_____________________________|_____________________________|
| MT-Level | Unsafe |
|_____________________________|_____________________________|
| Standard | See standards(5). ||_____________________________|_____________________________|
SEE ALSO
ar(1), cat(1), cp(1), tar(1), open(2), netconfig(4), attri-
butes(5), standards(5) NOTES The .pag file will contain holes so that its apparent size may be larger than its actual content. Older versions of the UNIX operating system may create real file blocks for these holes when touched. These files cannot be copied by normal means ( cp(1), cat(1), tar(1), ar(1)) without filling in the holes. The sum of the sizes of a key/content pair must not exceed the internal block size (currently 1024 bytes). Moreover all key/content pairs that hash together must fit on a singleblock. dbm_store() will return an error in the event that a
disk block fills with inseparable data.The order of keys presented by dbm_firstkey() and
dbm_nextkey() depends on a hashing function.
There are no interlocks and no reliable cache flushing; thus concurrent updating and reading is risky. The database files (file.dir and file.pag) are binary andare architecture-specific (for example, they depend on the
architecture's byte order.) These files are not guaranteed to be portable across architectures.SunOS 5.11 Last change: 17 Sep 2001 6