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>PostgreSQL 8.3.8 Documentation</TH
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>Chapter 31. Large Objects</TD
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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="LO-INTERFACES"
>31.3. Client Interfaces</A
></H1
><P
>    This section describes the facilities that
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> client interface libraries
    provide for accessing large objects.  All large object
    manipulation using these functions <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>must</I
></SPAN
> take
    place within an SQL transaction block.
    The  <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>  large  object interface is modeled after
    the <ACRONYM
CLASS="ACRONYM"
>Unix</ACRONYM
>  file-system  interface,  with  analogues  of
    <CODE
CLASS="FUNCTION"
>open</CODE
>,  <CODE
CLASS="FUNCTION"
>read</CODE
>,
    <CODE
CLASS="FUNCTION"
>write</CODE
>,
    <CODE
CLASS="FUNCTION"
>lseek</CODE
>, etc.
   </P
><P
>    Client applications which use the large object interface in
    <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> should include the header file
    <TT
CLASS="FILENAME"
>libpq/libpq-fs.h</TT
> and link with the
    <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> library.
   </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33038"
>31.3.1. Creating a Large Object</A
></H2
><P
>     The function
</P><PRE
CLASS="SYNOPSIS"
>Oid lo_creat(PGconn *conn, int mode);</PRE
><P>
     <A
NAME="AEN33042"
></A
>
     creates a new large object.  
     The return value is the OID that was assigned to the new large object,
     or <TT
CLASS="SYMBOL"
>InvalidOid</TT
> (zero) on failure.

     <TT
CLASS="REPLACEABLE"
><I
>mode</I
></TT
> is unused and
     ignored as of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.1; however, for
     backwards compatibility with earlier releases it is best to
     set it to <TT
CLASS="SYMBOL"
>INV_READ</TT
>, <TT
CLASS="SYMBOL"
>INV_WRITE</TT
>,
     or <TT
CLASS="SYMBOL"
>INV_READ</TT
> <TT
CLASS="LITERAL"
>|</TT
> <TT
CLASS="SYMBOL"
>INV_WRITE</TT
>.
     (These symbolic constants are defined
     in the header file <TT
CLASS="FILENAME"
>libpq/libpq-fs.h</TT
>.)
    </P
><P
>     An example:
</P><PRE
CLASS="PROGRAMLISTING"
>inv_oid = lo_creat(conn, INV_READ|INV_WRITE);</PRE
><P>
    </P
><P
>     The function
</P><PRE
CLASS="SYNOPSIS"
>Oid lo_create(PGconn *conn, Oid lobjId);</PRE
><P>
     <A
NAME="AEN33057"
></A
>
     also creates a new large object.  The OID to be assigned can be
     specified by <TT
CLASS="REPLACEABLE"
><I
>lobjId</I
></TT
>;
     if so, failure occurs if that OID is already in use for some large
     object.  If <TT
CLASS="REPLACEABLE"
><I
>lobjId</I
></TT
>
     is <TT
CLASS="SYMBOL"
>InvalidOid</TT
> (zero) then <CODE
CLASS="FUNCTION"
>lo_create</CODE
> assigns an unused
     OID (this is the same behavior as <CODE
CLASS="FUNCTION"
>lo_creat</CODE
>).
     The return value is the OID that was assigned to the new large object,
     or <TT
CLASS="SYMBOL"
>InvalidOid</TT
> (zero) on failure.
    </P
><P
>     <CODE
CLASS="FUNCTION"
>lo_create</CODE
> is new as of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
     8.1; if this function is run against an older server version, it will
     fail and return <TT
CLASS="SYMBOL"
>InvalidOid</TT
>.
    </P
><P
>     An example:
</P><PRE
CLASS="PROGRAMLISTING"
>inv_oid = lo_create(conn, desired_oid);</PRE
><P>
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33071"
>31.3.2. Importing a Large Object</A
></H2
><P
>     To import an operating system file as a large object, call
</P><PRE
CLASS="SYNOPSIS"
>Oid lo_import(PGconn *conn, const char *filename);</PRE
><P>
     <A
NAME="AEN33075"
></A
>
     <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
> 
     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 <TT
CLASS="SYMBOL"
>InvalidOid</TT
> (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.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33079"
>31.3.3. Exporting a Large Object</A
></H2
><P
>     To export a large object
     into an operating system file, call
</P><PRE
CLASS="SYNOPSIS"
>int lo_export(PGconn *conn, Oid lobjId, const char *filename);</PRE
><P>
     <A
NAME="AEN33083"
></A
>
     The <TT
CLASS="PARAMETER"
>lobjId</TT
> argument specifies the OID of the large
     object to export and the <TT
CLASS="PARAMETER"
>filename</TT
> 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.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33087"
>31.3.4. Opening an Existing Large Object</A
></H2
><P
>     To open an existing large object for reading or writing, call
</P><PRE
CLASS="SYNOPSIS"
>int lo_open(PGconn *conn, Oid lobjId, int mode);</PRE
><P>
     <A
NAME="AEN33091"
></A
>
     The <TT
CLASS="PARAMETER"
>lobjId</TT
> argument specifies the OID of the large
     object to open.   The <TT
CLASS="PARAMETER"
>mode</TT
> bits control whether the
     object is opened for reading (<TT
CLASS="SYMBOL"
>INV_READ</TT
>), writing
     (<TT
CLASS="SYMBOL"
>INV_WRITE</TT
>), or both.
     (These symbolic constants are defined
     in the header file <TT
CLASS="FILENAME"
>libpq/libpq-fs.h</TT
>.)
     A large object cannot be opened before it is created.
     <CODE
CLASS="FUNCTION"
>lo_open</CODE
> returns a (non-negative) large object
     descriptor for later use in <CODE
CLASS="FUNCTION"
>lo_read</CODE
>,
     <CODE
CLASS="FUNCTION"
>lo_write</CODE
>, <CODE
CLASS="FUNCTION"
>lo_lseek</CODE
>,
     <CODE
CLASS="FUNCTION"
>lo_tell</CODE
>, and <CODE
CLASS="FUNCTION"
>lo_close</CODE
>.
     The descriptor is only valid for 
     the duration of the current transaction.
     On failure, -1 is returned.
    </P
><P
>     The server currently does not distinguish between modes
     <TT
CLASS="SYMBOL"
>INV_WRITE</TT
> and <TT
CLASS="SYMBOL"
>INV_READ</TT
> <TT
CLASS="LITERAL"
>|</TT
>
     <TT
CLASS="SYMBOL"
>INV_WRITE</TT
>: you are allowed to read from the descriptor
     in either case.  However there is a significant difference between
     these modes and <TT
CLASS="SYMBOL"
>INV_READ</TT
> alone: with <TT
CLASS="SYMBOL"
>INV_READ</TT
>
     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 <CODE
CLASS="FUNCTION"
>lo_open</CODE
> was executed,
     regardless of later writes by this or other transactions.  Reading
     from a descriptor opened with <TT
CLASS="SYMBOL"
>INV_WRITE</TT
> 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 <TT
CLASS="LITERAL"
>SERIALIZABLE</TT
> versus <TT
CLASS="LITERAL"
>READ COMMITTED</TT
> transaction
     modes for ordinary SQL <TT
CLASS="COMMAND"
>SELECT</TT
> commands.
    </P
><P
>     An example:
</P><PRE
CLASS="PROGRAMLISTING"
>inv_fd = lo_open(conn, inv_oid, INV_READ|INV_WRITE);</PRE
><P>
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33118"
>31.3.5. Writing Data to a Large Object</A
></H2
><P
>     The function
</P><PRE
CLASS="SYNOPSIS"
>int lo_write(PGconn *conn, int fd, const char *buf, size_t len);</PRE
><P>
     <A
NAME="AEN33122"
></A
> writes
     <TT
CLASS="PARAMETER"
>len</TT
> bytes from <TT
CLASS="PARAMETER"
>buf</TT
>
     to large object descriptor <TT
CLASS="PARAMETER"
>fd</TT
>.  The <TT
CLASS="PARAMETER"
>fd</TT
>
     argument must have been returned by a previous
     <CODE
CLASS="FUNCTION"
>lo_open</CODE
>.  The number of bytes actually
     written is returned.  In the event of an error, the return value
     is negative.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33129"
>31.3.6. Reading Data from a Large Object</A
></H2
><P
>     The function
</P><PRE
CLASS="SYNOPSIS"
>int lo_read(PGconn *conn, int fd, char *buf, size_t len);</PRE
><P>
     <A
NAME="AEN33133"
></A
> reads
     <TT
CLASS="PARAMETER"
>len</TT
> bytes from large object descriptor
     <TT
CLASS="PARAMETER"
>fd</TT
> into <TT
CLASS="PARAMETER"
>buf</TT
>. The
     <TT
CLASS="PARAMETER"
>fd</TT
> argument must have been returned by a
     previous <CODE
CLASS="FUNCTION"
>lo_open</CODE
>.  The number of bytes
     actually read is returned. In the event of an error, the return
     value is negative.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33140"
>31.3.7. Seeking in a Large Object</A
></H2
><P
>     To change the current read or write location associated with a
     large object descriptor, call
</P><PRE
CLASS="SYNOPSIS"
>int lo_lseek(PGconn *conn, int fd, int offset, int whence);</PRE
><P>
     <A
NAME="AEN33144"
></A
> This function moves the
     current location pointer for the large object descriptor identified by
     <TT
CLASS="PARAMETER"
>fd</TT
> to the new location specified by
     <TT
CLASS="PARAMETER"
>offset</TT
>.  The valid values for <TT
CLASS="PARAMETER"
>whence</TT
>
     are <TT
CLASS="SYMBOL"
>SEEK_SET</TT
> (seek from object start),
     <TT
CLASS="SYMBOL"
>SEEK_CUR</TT
> (seek from current position), and
     <TT
CLASS="SYMBOL"
>SEEK_END</TT
> (seek from object end).  The return value is
     the new location pointer, or -1 on error.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33152"
>31.3.8. Obtaining the Seek Position of a Large Object</A
></H2
><P
>     To obtain the current read or write location of a large object descriptor,
     call
</P><PRE
CLASS="SYNOPSIS"
>int lo_tell(PGconn *conn, int fd);</PRE
><P>
     <A
NAME="AEN33156"
></A
> If there is an error, the
     return value is negative.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33158"
>31.3.9. Truncating a Large Object</A
></H2
><P
>     To truncate a large object to a given length, call
</P><PRE
CLASS="SYNOPSIS"
>int lo_truncate(PGcon *conn, int fd, size_t len);</PRE
><P>
     <A
NAME="AEN33162"
></A
> truncates the large object
     descriptor <TT
CLASS="PARAMETER"
>fd</TT
> to length <TT
CLASS="PARAMETER"
>len</TT
>.  The
     <TT
CLASS="PARAMETER"
>fd</TT
> argument must have been returned by a
     previous <CODE
CLASS="FUNCTION"
>lo_open</CODE
>.  If <TT
CLASS="PARAMETER"
>len</TT
> is
     greater than the current large object length, the large object
     is extended with null bytes ('\0').</P
><P
>     The file offset is not changed.</P
><P
>     On success <CODE
CLASS="FUNCTION"
>lo_truncate</CODE
> returns
     zero.  On error, the return value is negative.</P
><P
>     <CODE
CLASS="FUNCTION"
>lo_truncate</CODE
> is new as of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
     8.3; if this function is run against an older server version, it will
     fail and return a negative value.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33175"
>31.3.10. Closing a Large Object Descriptor</A
></H2
><P
>     A large object descriptor can be closed by calling
</P><PRE
CLASS="SYNOPSIS"
>int lo_close(PGconn *conn, int fd);</PRE
><P>
     <A
NAME="AEN33179"
></A
> where <TT
CLASS="PARAMETER"
>fd</TT
> is a
     large object descriptor returned by <CODE
CLASS="FUNCTION"
>lo_open</CODE
>.
     On success, <CODE
CLASS="FUNCTION"
>lo_close</CODE
> returns zero.  On
     error, the return value is negative.</P
><P
>     Any large  object  descriptors that remain open at the end of a
     transaction will be closed automatically.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33185"
>31.3.11. Removing a Large Object</A
></H2
><P
>     To remove a large object from the database, call
</P><PRE
CLASS="SYNOPSIS"
>int lo_unlink(PGconn *conn, Oid lobjId);</PRE
><P>
     <A
NAME="AEN33189"
></A
> The
     <TT
CLASS="PARAMETER"
>lobjId</TT
> argument specifies the OID of the
     large object to remove.  Returns 1 if successful, -1 on failure.
    </P
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