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)
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
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
lo_close. The descriptor is only valid for
the duration of the current transaction. On failure, -1 is
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
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);
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.
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
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);