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. (This requirement is
strictly enforced as of PostgreSQL 6.5, though it
has been an implicit requirement in previous versions, resulting
in misbehavior if ignored.)
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.
mode
is a bit mask
describing several different attributes of the new
object. The symbolic constants used here are defined
in the header file libpq/libpq-fs.h
.
The access type (read, write, or both) is controlled by
or'ing together the bits INV_READ
and
INV_WRITE
. The low-order sixteen bits of the mask have
historically been used at Berkeley to designate the storage manager number on which the large object
should reside. These bits should always be zero now. (The access type
does not actually do anything anymore either, but one or both flag bits
must be set to avoid an error.)
The return value is the OID that was assigned to the new large object,
or InvalidOid (zero) on failure.
An example:
inv_oid = lo_creat(conn, INV_READ|INV_WRITE);
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 filesystem 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.
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 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.