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28.3. 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.

28.3.1. Creating a Large Object

The function

Oid lo_creat(PGconn *conn, int mode);

creates a new large object. mode is a bit mask describing several different attributes of the new object. The symbolic constants listed here are defined in the header file libpq/libpq-fs.h. The access type (read, write, or both) is controlled by or'ing together the bits INV_READ and INV_WRITE. The low-order sixteen bits of the mask have historically been used at Berkeley to designate the storage manager number on which the large object should reside. These bits should always be zero now. The return value is the OID that was assigned to the new large object.

An example:

inv_oid = lo_creat(INV_READ|INV_WRITE);

28.3.2. 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.

28.3.3. 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 name of the file.

28.3.4. Opening an Existing Large Object

To open an existing large object, 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. A large object cannot be opened before it is created. lo_open returns a 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.

28.3.5. 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 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.

28.3.6. 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 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.

28.3.7. Seeking on a Large Object

To change the current read or write location on a large object, call

int lo_lseek(PGconn *conn, int fd, int offset, int whence);

This function moves the current location pointer for the large object described 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.

28.3.8. Obtaining the Seek Position of a Large Object

To obtain the current read or write location of a large object, call

int lo_tell(PGconn *conn, int fd);

If there is an error, the return value is negative.

28.3.9. Closing a Large Object Descriptor

A large object 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.

28.3.10. 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. In the event of an error, the return value is negative.

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