The facilities PostgreSQL provides to
access large objects, both in the backend as part of user-defined
functions or the front end as part of an application
using the interface, are described below. For users
familiar with POSTGRES 4.2,
PostgreSQL has a new set of
functions providing a more coherent interface.
Note: All large object manipulation must take
place within an SQL transaction. 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(2), read(2),
write(2),
lseek(2), etc. User
functions call these routines to retrieve only the data of
interest from a large object. For example, if a large
object type called mugshot existed that stored
photographs of faces, then a function called beard could
be declared on mugshot data. beard could look at the
lower third of a photograph, and determine the color of
the beard that appeared there, if any. The entire
large-object value need not be buffered, or even
examined, by the beard function.
Large objects may be accessed from dynamically-loaded C
functions or database client programs that link the
library. PostgreSQL provides a set of routines that
support opening, reading, writing, closing, and seeking on
large objects.
The routine
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 listed 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 commands below create a large object:
inv_oid = lo_creat(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.
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 name of the file.
To open an existing large object, 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 large object descriptor
for later use in lo_read, lo_write,
lo_lseek, lo_tell, and
lo_close.
The routine
int lo_write(PGconn *conn, int fd, const char *buf, size_t len)
writes len bytes from buf to large object 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 routine
int lo_read(PGconn *conn, int fd, char *buf, size_t len)
reads len bytes from large object 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 on a large
object, call
int lo_lseek(PGconn *conn, int fd, int offset, int whence)
This routine moves the current location pointer for the
large object described by fd to the new location specified
by offset. The valid values for whence are
SEEK_SET, SEEK_CUR, and SEEK_END.
A large object 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.
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. In the event of an error, the return value is negative.
