| PostgreSQL 8.2.6 Documentation | ||||
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This section describes the facilities that
PostgreSQL
client interface libraries provide for accessing large objects. All large object manipulation using these functions
must
take place within an SQL transaction block. The
PostgreSQL
large object interface is modeled after the
Unix
file-system interface, with analogues of
open
,
read
,
write
,
lseek
, etc.
Client applications which use the large object interface in libpq should include the header file libpq/libpq-fs.h and link with the libpq library.
The function
Oid lo_creat(PGconn *conn, int mode);
creates a new large object. The return value is the OID that was assigned to the new large object, or InvalidOid (zero) on failure. mode is unused and ignored as of PostgreSQL 8.1; however, for backwards compatibility with earlier releases it is best to set it to INV_READ , INV_WRITE , or INV_READ | INV_WRITE . (These symbolic constants are defined in the header file libpq/libpq-fs.h .)
An example:
inv_oid = lo_creat(conn, INV_READ|INV_WRITE);
The function
Oid lo_create(PGconn *conn, Oid lobjId);
also creates a new large object. The OID to be assigned can be specified by
lobjId
; if so, failure occurs if that OID is already in use for some large object. If
lobjId
is
InvalidOid
(zero) then
lo_create
assigns an unused OID (this is the same behavior as
lo_creat
). The return value is the OID that was assigned to the new large object, or
InvalidOid
(zero) on failure.
lo_create
is new as of
PostgreSQL
8.1; if this function is run against an older server version, it will fail and return
InvalidOid
.
An example:
inv_oid = lo_create(conn, desired_oid);
To import an operating system file as a large object, call
Oid lo_import(PGconn *conn, const char *filename);
filename specifies the operating system name of the file to be imported as a large object. The return value is the OID that was assigned to the new large object, or InvalidOid (zero) on failure. Note that the file is read by the client interface library, not by the server; so it must exist in the client file system and be readable by the client application.
To export a large object into an operating system file, call
int lo_export(PGconn *conn, Oid lobjId, const char *filename);
The lobjId argument specifies the OID of the large object to export and the filename argument specifies the operating system name of the file. Note that the file is written by the client interface library, not by the server. Returns 1 on success, -1 on failure.
To open an existing large object for reading or writing, call
int lo_open(PGconn *conn, Oid lobjId, int mode);
The
lobjId
argument specifies the OID of the large object to open. The
mode
bits control whether the object is opened for reading (
INV_READ
), writing (
INV_WRITE
), or both. (These symbolic constants are defined in the header file
libpq/libpq-fs.h
.) A large object cannot be opened before it is created.
lo_open
returns a (non-negative) large object descriptor for later use in
lo_read
,
lo_write
,
lo_lseek
,
lo_tell
, and
lo_close
. The descriptor is only valid for the duration of the current transaction. On failure, -1 is returned.
The server currently does not distinguish between modes
INV_WRITE
and
INV_READ
|
INV_WRITE
: you are allowed to read from the descriptor in either case. However there is a significant difference between these modes and
INV_READ
alone: with
INV_READ
you cannot write on the descriptor, and the data read from it will reflect the contents of the large object at the time of the transaction snapshot that was active when
lo_open
was executed, regardless of later writes by this or other transactions. Reading from a descriptor opened with
INV_WRITE
returns data that reflects all writes of other committed transactions as well as writes of the current transaction. This is similar to the behavior of
SERIALIZABLE
versus
READ COMMITTED
transaction modes for ordinary SQL
SELECT
commands.
An example:
inv_fd = lo_open(conn, inv_oid, INV_READ|INV_WRITE);
The function
int lo_write(PGconn *conn, int fd, const char *buf, size_t len);
writes
len
bytes from
buf
to large object descriptor
fd
. The
fd
argument must have been returned by a previous
lo_open
. The number of bytes actually written is returned. In the event of an error, the return value is negative.
The function
int lo_read(PGconn *conn, int fd, char *buf, size_t len);
reads
len
bytes from large object descriptor
fd
into
buf
. The
fd
argument must have been returned by a previous
lo_open
. The number of bytes actually read is returned. In the event of an error, the return value is negative.
To change the current read or write location associated with a large object descriptor, call
int lo_lseek(PGconn *conn, int fd, int offset, int whence);
This function moves the current location pointer for the large object descriptor identified by fd to the new location specified by offset . The valid values for whence are SEEK_SET (seek from object start), SEEK_CUR (seek from current position), and SEEK_END (seek from object end). The return value is the new location pointer, or -1 on error.
To obtain the current read or write location of a large object descriptor, call
int lo_tell(PGconn *conn, int fd);
A large object descriptor may be closed by calling
int lo_close(PGconn *conn, int fd);
where
fd
is a large object descriptor returned by
lo_open
. On success,
lo_close
returns zero. On error, the return value is negative.
Any large object descriptors that remain open at the end of a transaction will be closed automatically.
To remove a large object from the database, call
int lo_unlink(PGconn *conn, Oid lobjId);
The lobjId argument specifies the OID of the large object to remove. Returns 1 if successful, -1 on failure.