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29.3. Client Interfaces

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 open, read, write, lseek, etc.

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.

29.3.1. Creating a Large Object

The function

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.)

An example:

inv_oid = lo_creat(conn, INV_READ|INV_WRITE);

The function

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 lo_creat). 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.

An example:

inv_oid = lo_create(conn, desired_oid);

29.3.2. 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 filesystem and be readable by the client application.

29.3.3. 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.

29.3.4. 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.) A large object cannot be opened before it is created. lo_open returns a (non-negative) large object descriptor for later use in lo_read, lo_write, lo_lseek, lo_tell, 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 SERIALIZABLE versus READ COMMITTED transaction modes for ordinary SQL SELECT commands.

An example:

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

29.3.5. Writing Data to a Large Object

The function

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.

29.3.6. Reading Data from a Large Object

The function

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 negative.

29.3.7. 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.

29.3.8. 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 negative.

29.3.9. Closing a Large Object Descriptor

A large object descriptor may 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 negative.

Any large object descriptors that remain open at the end of a transaction will be closed automatically.

29.3.10. Removing a Large Object

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.

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