The facilities PostgreSQL provides to access large objects, both in the backend as part of user-defined functions or the front end as part of an application using the interface, are described below. For users familiar with POSTGRES 4.2, PostgreSQL has a new set of functions providing a more coherent interface.
Note: All large object manipulation must take place within an SQL transaction. 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(2), read(2), write(2), lseek(2), etc. User functions call these routines to retrieve only the data of interest from a large object. For example, if a large object type called mugshot existed that stored photographs of faces, then a function called beard could be declared on mugshot data. beard could look at the lower third of a photograph, and determine the color of the beard that appeared there, if any. The entire large-object value need not be buffered, or even examined, by the beard function. Large objects may be accessed from dynamically-loaded C functions or database client programs that link the library. PostgreSQL provides a set of routines that support opening, reading, writing, closing, and seeking on large objects.
The routine
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 commands below create a large object:
inv_oid = lo_creat(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.
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.
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 routine
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.
The routine
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.
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 routine 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_CUR, and SEEK_END.
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.
To remove a large object from the database, call
Oid lo_unlink(PGconn *conn, Oid lobjId)
The lobjId argument specifies the OID of the large object to remove.