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33.3.1. Creating a Large Object

33.3.2. Importing a Large Object

33.3.3. Exporting a Large Object

33.3.4. Opening an Existing Large Object

33.3.5. Writing Data to a Large Object

33.3.6. Reading Data from a Large Object

33.3.7. Seeking in a Large Object

33.3.8. Obtaining the Seek Position of a Large Object

33.3.9. Truncating a Large Object

33.3.10. Closing a Large Object Descriptor

33.3.11. Removing a Large Object

The function

Oid lo_create(PGconn *conn, Oid lobjId);

creates a new large object. The OID to be assigned can be specified bylobjId; if so, failure occurs if that OID is already in use for some large object. IflobjId isInvalidOid (zero) thenlo_create assigns an unused OID. The return value is the OID that was assigned to the new large object, orInvalidOid (zero) on failure.

An example:

inv_oid = lo_create(conn, desired_oid);

The older function

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, orInvalidOid (zero) on failure.

InPostgreSQL releases 8.1 and later, themode is ignored, so thatlo_creat is exactly equivalent tolo_create with a zero second argument. However, there is little reason to uselo_creat unless you need to work with servers older than 8.1. To work with such an old server, you must uselo_creat notlo_create, and you must setmode to one ofINV_READ,INV_WRITE, orINV_READ|INV_WRITE. (These symbolic constants are defined in the header filelibpq/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, orInvalidOid (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.

The function

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 bylobjId; if so, failure occurs if that OID is already in use for some large object. IflobjId isInvalidOid (zero) thenlo_import_with_oid assigns an unused OID (this is the same behavior aslo_import). The return value is the OID that was assigned to the new large object, orInvalidOid (zero) on failure.

lo_import_with_oid is new as ofPostgreSQL 8.4 and useslo_create internally which is new in 8.1; if this function is run against 8.0 or before, it will fail and returnInvalidOid.

To export a large object into an operating system file, call

int lo_export(PGconn *conn, Oid lobjId, const char *filename);

ThelobjId argument specifies the OID of the large object to export and thefilename 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);

ThelobjId argument specifies the OID of the large object to open. Themode bits control whether the object is opened for reading (INV_READ), writing (INV_WRITE), or both. (These symbolic constants are defined in the header filelibpq/libpq-fs.h.)lo_open returns a (non-negative) large object descriptor for later use inlo_read,lo_write,lo_lseek,lo_lseek64,lo_tell,lo_tell64,lo_truncate,lo_truncate64, andlo_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 modesINV_WRITE andINV_READ|INV_WRITE: you are allowed to read from the descriptor in either case. However there is a significant difference between these modes andINV_READ alone: withINV_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 whenlo_open was executed, regardless of later writes by this or other transactions. Reading from a descriptor opened withINV_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 ofREPEATABLE READ versusREAD COMMITTED transaction modes for ordinary SQLSELECT commands.

lo_open will fail ifSELECT privilege is not available for the large object, or ifINV_WRITE is specified andUPDATE privilege is not available. (Prior toPostgreSQL 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 thelo_compat_privileges run-time parameter.

An example:

inv_fd = lo_open(conn, inv_oid, INV_READ|INV_WRITE);

The function

int lo_write(PGconn *conn, int fd, const char *buf, size_t len);

writeslen bytes frombuf (which must be of sizelen) to large object descriptorfd. Thefd argument must have been returned by a previouslo_open. The number of bytes actually written is returned (in the current implementation, this will always equallen unless there is an error). In the event of an error, the return value is -1.

Although thelen parameter is declared assize_t, this function will reject length values larger thanINT_MAX. In practice, it's best to transfer data in chunks of at most a few megabytes anyway.

The function

int lo_read(PGconn *conn, int fd, char *buf, size_t len);

reads up tolen bytes from large object descriptorfd intobuf (which must be of sizelen). Thefd argument must have been returned by a previouslo_open. The number of bytes actually read is returned; this will be less thanlen if the end of the large object is reached first. In the event of an error, the return value is -1.

Although thelen parameter is declared assize_t, this function will reject length values larger thanINT_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 byfd to the new location specified byoffset. The valid values forwhence areSEEK_SET (seek from object start),SEEK_CUR (seek from current position), andSEEK_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 aslo_lseek, but it can accept anoffset larger than 2GB and/or deliver a result larger than 2GB. Note thatlo_lseek will fail if the new location pointer would be greater than 2GB.

lo_lseek64 is new as ofPostgreSQL 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 aslo_tell, but it can deliver a result larger than 2GB. Note thatlo_tell will fail if the current read/write location is greater than 2GB.

lo_tell64 is new as ofPostgreSQL 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 descriptorfd to lengthlen. Thefd argument must have been returned by a previouslo_open. Iflen 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 descriptorfd is not changed.

Although thelen parameter is declared assize_t,lo_truncate will reject length values larger thanINT_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 aslo_truncate, but it can accept alen value exceeding 2GB.

lo_truncate is new as ofPostgreSQL 8.3; if this function is run against an older server version, it will fail and return -1.

lo_truncate64 is new as ofPostgreSQL 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);

wherefd is a large object descriptor returned bylo_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);

ThelobjId argument specifies the OID of the large object to remove. Returns 1 if successful, -1 on failure.