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
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.