функции базы данных (database functions)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
dbm_clearerr, dbm_close, dbm_delete, dbm_error, dbm_fetch,
dbm_firstkey, dbm_nextkey, dbm_open, dbm_store — database
functions
Синопсис (Synopsis)
#include <ndbm.h>
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);
Описание (Description)
These functions create, access, and modify a database.
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 character strings, may be
stored in the object pointed to by dptr.
A database shall be stored in one or two files. When one file is
used, the name of the database file shall be formed by appending
the suffix .db to the file argument given to dbm_open(). When
two files are used, the names of the database files shall be
formed by appending the suffixes .dir and .pag respectively to
the file argument.
The dbm_open() function shall open a database. The file argument
to the function is the pathname of the database. The open_flags
argument has the same meaning as the flags argument of open()
except that a database opened for write-only access opens the
files for read and write access and the behavior of the O_APPEND
flag is unspecified. The file_mode argument has the same meaning
as the third argument of open().
The dbm_open() function need not accept pathnames longer than
{PATH_MAX}-4 bytes (including the terminating null), or pathnames
with a last component longer than {NAME_MAX}-4 bytes (excluding
the terminating null).
The dbm_close() function shall close a database. The application
shall ensure that argument db is a pointer to a dbm structure
that has been returned from a call to dbm_open().
These database functions shall support an internal block size
large enough to support key/content pairs of at least 1023 bytes.
The dbm_fetch() function shall read a record from a database. The
argument db is a pointer to a database structure that has been
returned from a call to dbm_open(). The argument key is a datum
that has been initialized by the application to the value of the
key that matches the key of the record the program is fetching.
The dbm_store() function shall write a record to a database. The
argument db is a pointer to a database structure that has been
returned from a call to dbm_open(). The argument key is a datum
that has been initialized by the application to the value of the
key that identifies (for subsequent reading, writing, or
deleting) the record the application is writing. The argument
content is a datum that has been initialized by the application
to the value of the record the program is writing. The argument
store_mode controls whether dbm_store() replaces any pre-existing
record that has the same key that is specified by the key
argument. The application shall 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 shall be replaced with the new record. If the
database contains a record that matches the key argument and
store_mode is DBM_INSERT, the existing record shall be left
unchanged and the new record ignored. If the database does not
contain a record that matches the key argument and store_mode is
either DBM_INSERT or DBM_REPLACE, the new record shall be
inserted in the database.
If the sum of a key/content pair exceeds the internal block size,
the result is unspecified. Moreover, the application shall ensure
that all key/content pairs that hash together fit on a single
block. The dbm_store() function shall return an error in the
event that a disk block fills with inseparable data.
The dbm_delete() function shall delete a record and its key from
the database. The argument db is a pointer to a database
structure that has been returned from a call to dbm_open(). The
argument key is a datum that has been initialized by the
application to the value of the key that identifies the record
the program is deleting.
The dbm_firstkey() function shall return the first key in the
database. The argument db is a pointer to a database structure
that has been returned from a call to dbm_open().
The dbm_nextkey() function shall return the next key in the
database. The argument db is a pointer to a database structure
that has been returned from a call to dbm_open(). The
application shall ensure that the dbm_firstkey() function is
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 shall return the error condition of the
database. The argument db is a pointer to a database structure
that has been returned from a call to dbm_open().
The dbm_clearerr() function shall clear the error condition of
the database. The argument db is a pointer to a database
structure that has been returned from a call to dbm_open().
The dptr pointers returned by these functions may point into
static storage that may be changed by subsequent calls.
These functions need not be thread-safe.
Возвращаемое значение (Return value)
The dbm_store() and dbm_delete() functions shall return 0 when
they succeed and a negative value when they fail.
The dbm_store() function shall return 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 shall return 0 if the error condition is
not set and return a non-zero value if the error condition is
set.
The return value of dbm_clearerr() is unspecified.
The dbm_firstkey() and dbm_nextkey() functions shall 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 shall be a null pointer and the error condition
of the database shall be set.
The dbm_fetch() function shall return a content datum. If no
record in the database matches the key or if an error condition
has been detected in the database, the dptr member of the content
shall be a null pointer.
The dbm_open() function shall return a pointer to a database
structure. If an error is detected during the operation,
dbm_open() shall return a (DBM *)0.
Ошибки (Error)
No errors are defined.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application 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 multiple
search keys per entry, they do not protect against multi-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.
Note that a strictly conforming application is extremely limited
by these functions: since there is no way to determine that the
keys in use do not all hash to the same value (although that
would be rare), a strictly conforming application cannot be
guaranteed that it can store more than one block's worth of data
in the database. As long as a key collision does not occur,
additional data may be stored, but because there is no way to
determine whether an error is due to a key collision or some
other error condition (dbm_error() being effectively a Boolean),
once an error is detected, the application is effectively limited
to guessing what the error might be if it wishes to continue
using these functions.
The dbm_delete() function need not physically reclaim file space,
although it does make it available for reuse by the database.
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 calling dbm_firstkey() before again
calling dbm_nextkey(). The contents of these files are
unspecified and may not be portable.
Applications should take care that database pathname arguments
specified to dbm_open() are not prefixes of unrelated files. This
might be done, for example, by placing databases in a separate
directory.
Since some implementations use three characters for a suffix and
others use four characters for a suffix, applications should
ensure that the maximum portable pathname length passed to
dbm_open() is no greater than {PATH_MAX}-4 bytes, with the last
component of the pathname no greater than {NAME_MAX}-4 bytes.
Обоснование (Rationale)
Previously the standard required the database to be stored in two
files, one file being a directory containing a bitmap of keys and
having .dir as its suffix. The second file containing all data
and having .pag as its suffix. This has been changed not to
specify the use of the files and to allow newer implementations
of the Berkeley DB interface using a single file that have
evolved while remaining compatible with the application
programming interface. The standard developers considered
removing the specific suffixes altogether but decided to retain
them so as not to pollute the application file name space more
than necessary and to allow for portable backups of the database.
Будущие направления (Future directions)
None.
Смотри также (See also)
open(3p)
The Base Definitions volume of POSIX.1‐2017, ndbm.h(0p)
