35.3. Client Interfaces
- 35.3.1. Creating a Large Object
- 35.3.2. Importing a Large Object
- 35.3.3. Exporting a Large Object
- 35.3.4. Opening an Existing Large Object
- 35.3.5. Writing Data to a Large Object
- 35.3.6. Reading Data from a Large Object
- 35.3.7. Seeking in a Large Object
- 35.3.8. Obtaining the Seek Position of a Large Object
- 35.3.9. Truncating a Large Object
- 35.3.10. Closing a Large Object Descriptor
- 35.3.11. Removing a Large Object
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.
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.
35.3.1. Creating a Large Object
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);
35.3.2. Importing a Large Object
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
.
35.3.3. Exporting 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 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.
35.3.4. Opening an Existing Large Object
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);
35.3.5. Writing Data to a Large Object
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.
35.3.6. Reading Data from a Large Object
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.
35.3.7. Seeking in a Large Object
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.
35.3.8. Obtaining the Seek Position of a Large Object
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.
35.3.9. Truncating a Large Object
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.
35.3.10. Closing a Large Object Descriptor
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.