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
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.
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.)
inv_oid = lo_creat(conn, INV_READ|INV_WRITE);
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
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.
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 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.
(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
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.
inv_fd = lo_open(conn, inv_oid, INV_READ|INV_WRITE);
int lo_write(PGconn *conn, int fd, const char *buf, size_t len);
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
int lo_read(PGconn *conn, int fd, char *buf, size_t len);
len bytes from large object descriptor
fd into buf. The
fd argument must have been returned by a
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);
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);