This section describes the facilities that
PostgreSQL's libpq
client interface library provides for accessing large objects.
The PostgreSQL large object interface is
modeled after the Unix file-system interface, with
analogues of open, read,
write,
lseek, etc.
All large object manipulation using these functions
must take place within an SQL transaction block,
since large object file descriptors are only valid for the duration of
a transaction. Write operations, including lo_open
with the INV_WRITE mode, are not allowed in a read-only
transaction.
If an error occurs while executing any one of these functions, the
function will return an otherwise-impossible value, typically 0 or -1.
A message describing the error is stored in the connection object and
can be retrieved with
PQerrorMessage
.
Client applications that use these functions should include the header file
libpq/libpq-fs.h and link with the
libpq library.
Client applications cannot use these functions while a libpq connection is in pipeline mode.
Oid lo_create(PGconn *conn, Oid lobjId);
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.
The return value is the OID that was assigned to the new large object,
or InvalidOid (zero) on failure.
An example:
inv_oid = lo_create(conn, desired_oid);
Oid lo_creat(PGconn *conn, int mode);
also creates a new large object, always assigning an unused OID.
The return value is the OID that was assigned to the new large object,
or InvalidOid (zero) on failure.
In PostgreSQL releases 8.1 and later,
the mode is ignored,
so that lo_creat is exactly equivalent to
lo_create with a zero second argument.
However, there is little reason to use lo_creat
unless you need to work with servers older than 8.1.
To work with such an old server, you must
use lo_creat not lo_create,
and you must set mode to
one of 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);
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.
Oid lo_import_with_oid(PGconn *conn, const char *filename, Oid lobjId);
also imports 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_import_with_oid assigns an unused
OID (this is the same behavior as lo_import).
The return value is the OID that was assigned to the new large object,
or InvalidOid (zero) on failure.
lo_import_with_oid is new as of PostgreSQL
8.4 and uses lo_create internally which is new in 8.1; if this function is run against 8.0 or before, it will
fail and return InvalidOid.
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.)
lo_open returns a (non-negative) large object
descriptor for later use in lo_read,
lo_write, lo_lseek,
lo_lseek64, lo_tell,
lo_tell64, lo_truncate,
lo_truncate64, 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 REPEATABLE READ versus READ COMMITTED transaction
modes for ordinary SQL SELECT commands.
lo_open will fail if SELECT
privilege is not available for the large object, or
if INV_WRITE is specified and UPDATE
privilege is not available.
(Prior to PostgreSQL 11, these privilege
checks were instead performed at the first actual read or write call
using the descriptor.)
These privilege checks can be disabled with the
lo_compat_privileges run-time parameter.
An example:
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
(which must be of size len) 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 current
implementation, this will always equal len unless
there is an error). In the event of an error, the return value is -1.
Although the len parameter is declared as
size_t, this function will reject length values larger than
INT_MAX. In practice, it's best to transfer data in chunks
of at most a few megabytes anyway.
int lo_read(PGconn *conn, int fd, char *buf, size_t len);
reads up to len bytes from large object descriptor
fd into buf (which must be
of size len). The fd
argument must have been returned by a previous
lo_open. The number of bytes actually read is
returned; this will be less than len if the end of
the large object is reached first. In the event of an error, the return
value is -1.
Although the len parameter is declared as
size_t, this function will reject length values larger than
INT_MAX. In practice, it's best to transfer data in chunks
of at most a few megabytes anyway.
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.
When dealing with large objects that might exceed 2GB in size, instead use
pg_int64 lo_lseek64(PGconn *conn, int fd, pg_int64 offset, int whence);
This function has the same behavior
as lo_lseek, but it can accept an
offset larger than 2GB and/or deliver a result larger
than 2GB.
Note that lo_lseek will fail if the new location
pointer would be greater than 2GB.
lo_lseek64 is new as of PostgreSQL
9.3. If this function is run against an older server version, it will
fail and return -1.
To obtain the current read or write location of a large object descriptor, call
int lo_tell(PGconn *conn, int fd);
If there is an error, the return value is -1.
When dealing with large objects that might exceed 2GB in size, instead use
pg_int64 lo_tell64(PGconn *conn, int fd);
This function has the same behavior
as lo_tell, but it can deliver a result larger
than 2GB.
Note that lo_tell will fail if the current
read/write location is greater than 2GB.
lo_tell64 is new as of PostgreSQL
9.3. If this function is run against an older server version, it will
fail and return -1.
To truncate a large object to a given length, call
int lo_truncate(PGconn *conn, int fd, size_t len);
This function truncates the large object
descriptor fd to length len. The
fd argument must have been returned by a
previous lo_open. If len is
greater than the large object's current length, the large object
is extended to the specified length with null bytes ('\0').
On success, lo_truncate returns
zero. On error, the return value is -1.
The read/write location associated with the descriptor
fd is not changed.
Although the len parameter is declared as
size_t, lo_truncate will reject length
values larger than INT_MAX.
When dealing with large objects that might exceed 2GB in size, instead use
int lo_truncate64(PGconn *conn, int fd, pg_int64 len);
This function has the same
behavior as lo_truncate, but it can accept a
len value exceeding 2GB.
lo_truncate is new as of PostgreSQL
8.3; if this function is run against an older server version, it will
fail and return -1.
lo_truncate64 is new as of PostgreSQL
9.3; if this function is run against an older server version, it will
fail and return -1.
A large object descriptor can 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 -1.
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.