lobj.sgml

<!-- doc/src/sgml/lobj.sgml -->

 <chapter id="largeObjects">
  <title>Large Objects</title>

  <indexterm zone="largeobjects"><primary>large object</></>
  <indexterm><primary>BLOB</><see>large object</></>

   <para>
    <productname>PostgreSQL</productname> has a <firstterm>large object</>
    facility, which provides stream-style access to user data that is stored
    in a special large-object structure.  Streaming access is useful
    when working with data values that are too large to manipulate
    conveniently as a whole.
   </para>

   <para>
    This chapter describes the implementation and the programming and
    query language interfaces to <productname>PostgreSQL</productname>
    large object data.  We use the <application>libpq</application> C
    library for the examples in this chapter, but most programming
    interfaces native to <productname>PostgreSQL</productname> support
    equivalent functionality.  Other interfaces might use the large
    object interface internally to provide generic support for large
    values.  This is not described here.
   </para>

  <sect1 id="lo-intro">
   <title>Introduction</title>

   <indexterm>
    <primary>TOAST</primary>
    <secondary>versus large objects</secondary>
   </indexterm>

   <para>
    All large objects are stored in a single system table named <link
    linkend="catalog-pg-largeobject"><structname>pg_largeobject</structname></link>.
    Each large object also has an entry in the system table <link
    linkend="catalog-pg-largeobject-metadata"><structname>pg_largeobject_metadata</structname></link>.
    Large objects can be created, modified, and deleted using a read/write API
    that is similar to standard operations on files.
   </para>

   <para>
    <productname>PostgreSQL</productname> also supports a storage system called
    <link
    linkend="storage-toast"><quote><acronym>TOAST</acronym></quote></link>,
    which automatically stores values
    larger than a single database page into a secondary storage area per table.
    This makes the large object facility partially obsolete.  One
    remaining advantage of the large object facility is that it allows values
    up to 4 TB in size, whereas <acronym>TOAST</acronym>ed fields can be at
    most 1 GB.  Also, reading and updating portions of a large object can be
    done efficiently, while most operations on a <acronym>TOAST</acronym>ed
    field will read or write the whole value as a unit.
   </para>

  </sect1>

  <sect1 id="lo-implementation">
   <title>Implementation Features</title>

   <para>
    The large object implementation breaks large
    objects up into <quote>chunks</quote> and stores the chunks in
    rows in the database.  A B-tree index guarantees fast
    searches for the correct chunk number when doing random
    access reads and writes.
   </para>

   <para>
    The chunks stored for a large object do not have to be contiguous.
    For example, if an application opens a new large object, seeks to offset
    1000000, and writes a few bytes there, this does not result in allocation
    of 1000000 bytes worth of storage; only of chunks covering the range of
    data bytes actually written.  A read operation will, however, read out
    zeroes for any unallocated locations preceding the last existing chunk.
    This corresponds to the common behavior of <quote>sparsely allocated</>
    files in <acronym>Unix</acronym> file systems.
   </para>

   <para>
    As of <productname>PostgreSQL</> 9.0, large objects have an owner
    and a set of access permissions, which can be managed using
    <xref linkend="sql-grant"> and
    <xref linkend="sql-revoke">.
    <literal>SELECT</literal> privileges are required to read a large
    object, and
    <literal>UPDATE</literal> privileges are required to write or
    truncate it.
    Only the large object's owner (or a database superuser) can delete,
    comment on, or change the owner of a large object.
    To adjust this behavior for compatibility with prior releases, see the
    <xref linkend="guc-lo-compat-privileges"> run-time parameter.
   </para>
  </sect1>

  <sect1 id="lo-interfaces">
   <title>Client Interfaces</title>

   <para>
    This section describes the facilities that
    <productname>PostgreSQL</productname>'s <application>libpq</>
    client interface library provides for accessing large objects.
    The <productname>PostgreSQL</productname> large object interface is
    modeled after the <acronym>Unix</acronym> file-system interface, with
    analogues of <function>open</function>,  <function>read</function>,
    <function>write</function>,
    <function>lseek</function>, etc.
   </para>

   <para>
    All large object manipulation using these functions
    <emphasis>must</emphasis> take place within an SQL transaction block,
    since large object file descriptors are only valid for the duration of
    a transaction.
   </para>

   <para>
    If an error occurs while executing any one of these functions, the
    function will return an otherwise-impossible value, typically 0 or -1.
    A message describing the error is stored in the connection object and
    can be retrieved with <function>PQerrorMessage</>.
   </para>

   <para>
    Client applications that use these functions should include the header file
    <filename>libpq/libpq-fs.h</filename> and link with the
    <application>libpq</application> library.
   </para>

   <sect2 id="lo-create">
    <title>Creating a Large Object</title>

    <para>
     <indexterm><primary>lo_creat</></>
     The function
<synopsis>
Oid lo_creat(PGconn *conn, int mode);
</synopsis>
     creates a new large object.
     The return value is the OID that was assigned to the new large object,
     or <symbol>InvalidOid</symbol> (zero) on failure.

     <replaceable class="parameter">mode</replaceable> is unused and
     ignored as of <productname>PostgreSQL</productname> 8.1; however, for
     backward compatibility with earlier releases it is best to
     set it to <symbol>INV_READ</symbol>, <symbol>INV_WRITE</symbol>,
     or <symbol>INV_READ</symbol> <literal>|</> <symbol>INV_WRITE</symbol>.
     (These symbolic constants are defined
     in the header file <filename>libpq/libpq-fs.h</filename>.)
    </para>

    <para>
     An example:
<programlisting>
inv_oid = lo_creat(conn, INV_READ|INV_WRITE);
</programlisting>
    </para>

    <para>
     <indexterm><primary>lo_create</></>
     The function
<synopsis>
Oid lo_create(PGconn *conn, Oid lobjId);
</synopsis>
     also creates a new large object.  The OID to be assigned can be
     specified by <replaceable class="parameter">lobjId</replaceable>;
     if so, failure occurs if that OID is already in use for some large
     object.  If <replaceable class="parameter">lobjId</replaceable>
     is <symbol>InvalidOid</symbol> (zero) then <function>lo_create</> assigns an unused
     OID (this is the same behavior as <function>lo_creat</>).
     The return value is the OID that was assigned to the new large object,
     or <symbol>InvalidOid</symbol> (zero) on failure.
    </para>

    <para>
     <function>lo_create</> is new as of <productname>PostgreSQL</productname>
     8.1; if this function is run against an older server version, it will
     fail and return <symbol>InvalidOid</symbol>.
    </para>

    <para>
     An example:
<programlisting>
inv_oid = lo_create(conn, desired_oid);
</programlisting>
    </para>
   </sect2>

   <sect2 id="lo-import">
    <title>Importing a Large Object</title>

    <para>
     <indexterm><primary>lo_import</></>
     To import an operating system file as a large object, call
<synopsis>
Oid lo_import(PGconn *conn, const char *filename);
</synopsis>
     <replaceable class="parameter">filename</replaceable>
     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 <symbol>InvalidOid</symbol> (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.
    </para>

    <para>
     <indexterm><primary>lo_import_with_oid</></>
     The function
<synopsis>
Oid lo_import_with_oid(PGconn *conn, const char *filename, Oid lobjId);
</synopsis>
     also imports a new large object.  The OID to be assigned can be
     specified by <replaceable class="parameter">lobjId</replaceable>;
     if so, failure occurs if that OID is already in use for some large
     object.  If <replaceable class="parameter">lobjId</replaceable>
     is <symbol>InvalidOid</symbol> (zero) then <function>lo_import_with_oid</> assigns an unused
     OID (this is the same behavior as <function>lo_import</>).
     The return value is the OID that was assigned to the new large object,
     or <symbol>InvalidOid</symbol> (zero) on failure.
    </para>

    <para>
     <function>lo_import_with_oid</> is new as of <productname>PostgreSQL</productname>
     8.4 and uses <function>lo_create</function> internally which is new in 8.1; if this function is run against 8.0 or before, it will
     fail and return <symbol>InvalidOid</symbol>.
    </para>
   </sect2>

   <sect2 id="lo-export">
    <title>Exporting a Large Object</title>

    <para>
     <indexterm><primary>lo_export</></>
     To export a large object
     into an operating system file, call
<synopsis>
int lo_export(PGconn *conn, Oid lobjId, const char *filename);
</synopsis>
     The <parameter>lobjId</parameter> argument specifies the OID of the large
     object to export and the <parameter>filename</parameter> 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.
    </para>
   </sect2>

   <sect2 id="lo-open">
    <title>Opening an Existing Large Object</title>

    <para>
     <indexterm><primary>lo_open</></>
     To open an existing large object for reading or writing, call
<synopsis>
int lo_open(PGconn *conn, Oid lobjId, int mode);
</synopsis>
     The <parameter>lobjId</parameter> argument specifies the OID of the large
     object to open.   The <parameter>mode</parameter> bits control whether the
     object is opened for reading (<symbol>INV_READ</>), writing
     (<symbol>INV_WRITE</symbol>), or both.
     (These symbolic constants are defined
     in the header file <filename>libpq/libpq-fs.h</filename>.)
     <function>lo_open</function> returns a (non-negative) large object
     descriptor for later use in <function>lo_read</function>,
     <function>lo_write</function>, <function>lo_lseek</function>,
     <function>lo_lseek64</function>, <function>lo_tell</function>,
     <function>lo_tell64</function>, <function>lo_truncate</function>,
     <function>lo_truncate64</function>, and <function>lo_close</function>.
     The descriptor is only valid for
     the duration of the current transaction.
     On failure, -1 is returned.
    </para>

    <para>
     The server currently does not distinguish between modes
     <symbol>INV_WRITE</symbol> and <symbol>INV_READ</> <literal>|</>
     <symbol>INV_WRITE</symbol>: you are allowed to read from the descriptor
     in either case.  However there is a significant difference between
     these modes and <symbol>INV_READ</> alone: with <symbol>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 <function>lo_open</> was executed,
     regardless of later writes by this or other transactions.  Reading
     from a descriptor opened with <symbol>INV_WRITE</symbol> 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 <literal>REPEATABLE READ</> versus <literal>READ COMMITTED</> transaction
     modes for ordinary SQL <command>SELECT</> commands.
    </para>

    <para>
     An example:
<programlisting>
inv_fd = lo_open(conn, inv_oid, INV_READ|INV_WRITE);
</programlisting>
    </para>
</sect2>

<sect2 id="lo-write">
<title>Writing Data to a Large Object</title>

<para>
     <indexterm><primary>lo_write</></>
     The function
<synopsis>
int lo_write(PGconn *conn, int fd, const char *buf, size_t len);
</synopsis>
     writes <parameter>len</parameter> bytes from <parameter>buf</parameter>
     (which must be of size <parameter>len</parameter>) to large object
     descriptor <parameter>fd</>.  The <parameter>fd</parameter> argument must
     have been returned by a previous <function>lo_open</function>.  The
     number of bytes actually written is returned (in the current
     implementation, this will always equal <parameter>len</parameter> unless
     there is an error).  In the event of an error, the return value is -1.
</para>

<para>
     Although the <parameter>len</parameter> parameter is declared as
     <type>size_t</>, this function will reject length values larger than
     <literal>INT_MAX</>.  In practice, it's best to transfer data in chunks
     of at most a few megabytes anyway.
</para>
</sect2>

<sect2 id="lo-read">
<title>Reading Data from a Large Object</title>

<para>
     <indexterm><primary>lo_read</></>
     The function
<synopsis>
int lo_read(PGconn *conn, int fd, char *buf, size_t len);
</synopsis>
     reads up to <parameter>len</parameter> bytes from large object descriptor
     <parameter>fd</parameter> into <parameter>buf</parameter> (which must be
     of size <parameter>len</parameter>).  The <parameter>fd</parameter>
     argument must have been returned by a previous
     <function>lo_open</function>.  The number of bytes actually read is
     returned; this will be less than <parameter>len</parameter> if the end of
     the large object is reached first.  In the event of an error, the return
     value is -1.
</para>

<para>
     Although the <parameter>len</parameter> parameter is declared as
     <type>size_t</>, this function will reject length values larger than
     <literal>INT_MAX</>.  In practice, it's best to transfer data in chunks
     of at most a few megabytes anyway.
</para>
</sect2>

<sect2 id="lo-seek">
<title>Seeking in a Large Object</title>

<para>
     <indexterm><primary>lo_lseek</></>
     To change the current read or write location associated with a
     large object descriptor, call
<synopsis>
int lo_lseek(PGconn *conn, int fd, int offset, int whence);
</synopsis>
     This function moves the
     current location pointer for the large object descriptor identified by
     <parameter>fd</> to the new location specified by
     <parameter>offset</>.  The valid values for <parameter>whence</>
     are <symbol>SEEK_SET</> (seek from object start),
     <symbol>SEEK_CUR</> (seek from current position), and
     <symbol>SEEK_END</> (seek from object end).  The return value is
     the new location pointer, or -1 on error.
</para>

<para>
     <indexterm><primary>lo_lseek64</></>
     When dealing with large objects that might exceed 2GB in size,
     instead use
<synopsis>
pg_int64 lo_lseek64(PGconn *conn, int fd, pg_int64 offset, int whence);
</synopsis>
     This function has the same behavior
     as <function>lo_lseek</function>, but it can accept an
     <parameter>offset</> larger than 2GB and/or deliver a result larger
     than 2GB.
     Note that <function>lo_lseek</function> will fail if the new location
     pointer would be greater than 2GB.
</para>

<para>
     <function>lo_lseek64</> is new as of <productname>PostgreSQL</productname>
     9.3.  If this function is run against an older server version, it will
     fail and return -1.
</para>

</sect2>

<sect2 id="lo-tell">
<title>Obtaining the Seek Position of a Large Object</title>

<para>
     <indexterm><primary>lo_tell</></>
     To obtain the current read or write location of a large object descriptor,
     call
<synopsis>
int lo_tell(PGconn *conn, int fd);
</synopsis>
     If there is an error, the return value is -1.
</para>

<para>
     <indexterm><primary>lo_tell64</></>
     When dealing with large objects that might exceed 2GB in size,
     instead use
<synopsis>
pg_int64 lo_tell64(PGconn *conn, int fd);
</synopsis>
     This function has the same behavior
     as <function>lo_tell</function>, but it can deliver a result larger
     than 2GB.
     Note that <function>lo_tell</function> will fail if the current
     read/write location is greater than 2GB.
</para>

<para>
     <function>lo_tell64</> is new as of <productname>PostgreSQL</productname>
     9.3.  If this function is run against an older server version, it will
     fail and return -1.
</para>
</sect2>

<sect2 id="lo-truncate">
<title>Truncating a Large Object</title>

<para>
     <indexterm><primary>lo_truncate</></>
     To truncate a large object to a given length, call
<synopsis>
int lo_truncate(PGcon *conn, int fd, size_t len);
</synopsis>
     This function truncates the large object
     descriptor <parameter>fd</> to length <parameter>len</>.  The
     <parameter>fd</parameter> argument must have been returned by a
     previous <function>lo_open</function>.  If <parameter>len</> is
     greater than the large object's current length, the large object
     is extended to the specified length with null bytes ('\0').
     On success, <function>lo_truncate</function> returns
     zero.  On error, the return value is -1.
</para>

<para>
     The read/write location associated with the descriptor
     <parameter>fd</parameter> is not changed.
</para>

<para>
     Although the <parameter>len</parameter> parameter is declared as
     <type>size_t</>, <function>lo_truncate</function> will reject length
     values larger than <literal>INT_MAX</>.
</para>

<para>
     <indexterm><primary>lo_truncate64</></>
     When dealing with large objects that might exceed 2GB in size,
     instead use
<synopsis>
int lo_truncate64(PGcon *conn, int fd, pg_int64 len);
</synopsis>
     This function has the same
     behavior as <function>lo_truncate</function>, but it can accept a
     <parameter>len</> value exceeding 2GB.
</para>

<para>
     <function>lo_truncate</> is new as of <productname>PostgreSQL</productname>
     8.3; if this function is run against an older server version, it will
     fail and return -1.
</para>

<para>
     <function>lo_truncate64</> is new as of <productname>PostgreSQL</productname>
     9.3; if this function is run against an older server version, it will
     fail and return -1.
</para>
</sect2>

<sect2 id="lo-close">
<title>Closing a Large Object Descriptor</title>

<para>
     <indexterm><primary>lo_close</></>
     A large object descriptor can be closed by calling
<synopsis>
int lo_close(PGconn *conn, int fd);
</synopsis>
     where <parameter>fd</> is a
     large object descriptor returned by <function>lo_open</function>.
     On success, <function>lo_close</function> returns zero.  On
     error, the return value is -1.
</para>

<para>
     Any large  object  descriptors that remain open at the end of a
     transaction will be closed automatically.
</para>
</sect2>

   <sect2 id="lo-unlink">
    <title>Removing a Large Object</title>

    <para>
     <indexterm><primary>lo_unlink</></>
     To remove a large object from the database, call
<synopsis>
int lo_unlink(PGconn *conn, Oid lobjId);
</synopsis>
     The <parameter>lobjId</parameter> argument specifies the OID of the
     large object to remove.  Returns 1 if successful, -1 on failure.
    </para>
   </sect2>

</sect1>

<sect1 id="lo-funcs">
<title>Server-side Functions</title>

  <para>
   Server-side functions tailored for manipulating large objects from SQL are
   listed in <xref linkend="lo-funcs-table">.
  </para>

  <table id="lo-funcs-table">
   <title>SQL-oriented Large Object Functions</title>
   <tgroup cols="5">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
      <entry>Example</entry>
      <entry>Result</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>
       <indexterm>
        <primary>lo_from_bytea</primary>
       </indexterm>
       <literal><function>lo_from_bytea(<parameter>loid</parameter> <type>oid</type>, <parameter>string</parameter> <type>bytea</type>)</function></literal>
      </entry>
      <entry><type>oid</type></entry>
      <entry>
       Create a large object and store data there, returning its OID.
       Pass <literal>0</> to have the system choose an OID.
      </entry>
      <entry><literal>lo_from_bytea(0, E'\\xffffff00')</literal></entry>
      <entry><literal>24528</literal></entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>lo_put</primary>
       </indexterm>
       <literal><function>lo_put(<parameter>loid</parameter> <type>oid</type>, <parameter>offset</parameter> <type>bigint</type>, <parameter>str</parameter> <type>bytea</type>)</function></literal>
      </entry>
      <entry><type>void</type></entry>
      <entry>
       Write data at the given offset.
      </entry>
      <entry><literal>lo_put(24528, 1, E'\\xaa')</literal></entry>
      <entry></entry>
     </row>

     <row>
      <entry>
       <indexterm>
        <primary>lo_get</primary>
       </indexterm>
       <literal><function>lo_get(<parameter>loid</parameter> <type>oid</type> <optional>, <parameter>from</parameter> <type>bigint</type>, <parameter>for</parameter> <type>int</type></optional>)</function></literal>
      </entry>
      <entry><type>bytea</type></entry>
      <entry>
       Extract contents or a substring thereof.
      </entry>
      <entry><literal>lo_get(24528, 0, 3)</literal></entry>
      <entry><literal>\xffaaff</literal></entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <para>
   There are additional server-side functions corresponding to each of the
   client-side functions described earlier; indeed, for the most part the
   client-side functions are simply interfaces to the equivalent server-side
   functions.  The ones just as convenient to call via SQL commands are
   <function>lo_creat</function><indexterm><primary>lo_creat</></>,
   <function>lo_create</function>,
   <function>lo_unlink</function><indexterm><primary>lo_unlink</></>,
   <function>lo_import</function><indexterm><primary>lo_import</></>, and
   <function>lo_export</function><indexterm><primary>lo_export</></>.
   Here are examples of their use:

<programlisting>
CREATE TABLE image (
    name            text,
    raster          oid
);

SELECT lo_creat(-1);       -- returns OID of new, empty large object

SELECT lo_create(43213);   -- attempts to create large object with OID 43213

SELECT lo_unlink(173454);  -- deletes large object with OID 173454

INSERT INTO image (name, raster)
    VALUES ('beautiful image', lo_import('/etc/motd'));

INSERT INTO image (name, raster)  -- same as above, but specify OID to use
    VALUES ('beautiful image', lo_import('/etc/motd', 68583));

SELECT lo_export(image.raster, '/tmp/motd') FROM image
    WHERE name = 'beautiful image';
</programlisting>
  </para>

  <para>
    The server-side <function>lo_import</function> and
    <function>lo_export</function> functions behave considerably differently
    from their client-side analogs.  These two functions read and write files
    in the server's file system, using the permissions of the database's
    owning user.  Therefore, their use is restricted to superusers.  In
    contrast, the client-side import and export functions read and write files
    in the client's file system, using the permissions of the client program.
    The client-side functions do not require superuser privilege.
  </para>

  <para>
    The functionality of <function>lo_read</function> and
    <function>lo_write</function> is also available via server-side calls,
    but the names of the server-side functions differ from the client side
    interfaces in that they do not contain underscores.  You must call
    these functions as <function>loread</> and <function>lowrite</>.
  </para>

</sect1>

<sect1 id="lo-examplesect">
<title>Example Program</title>

<para>
     <xref linkend="lo-example"> is a sample program which shows how the large object
     interface
     in <application>libpq</> can be used.  Parts of the program are
     commented out but are left in the source for  the  reader's
     benefit.  This program can also be found in
     <filename>src/test/examples/testlo.c</filename> in the source distribution.
</para>

  <example id="lo-example">
   <title>Large Objects with <application>libpq</application> Example Program</title>
<programlisting><![CDATA[
/*-------------------------------------------------------------------------
 *
 * testlo.c
 *    test using large objects with libpq
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/test/examples/testlo.c
 *
 *-------------------------------------------------------------------------
 */
#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#include "libpq-fe.h"
#include "libpq/libpq-fs.h"

#define BUFSIZE         1024

/*
 * importFile -
 *    import file "in_filename" into database as large object "lobjOid"
 *
 */
static Oid
importFile(PGconn *conn, char *filename)
{
    Oid         lobjId;
    int         lobj_fd;
    char        buf[BUFSIZE];
    int         nbytes,
                tmp;
    int         fd;

    /*
     * open the file to be read in
     */
    fd = open(filename, O_RDONLY, 0666);
    if (fd < 0)
    {                           /* error */
        fprintf(stderr, "cannot open unix file\"%s\"\n", filename);
    }

    /*
     * create the large object
     */
    lobjId = lo_creat(conn, INV_READ | INV_WRITE);
    if (lobjId == 0)
        fprintf(stderr, "cannot create large object");

    lobj_fd = lo_open(conn, lobjId, INV_WRITE);

    /*
     * read in from the Unix file and write to the inversion file
     */
    while ((nbytes = read(fd, buf, BUFSIZE)) > 0)
    {
        tmp = lo_write(conn, lobj_fd, buf, nbytes);
        if (tmp < nbytes)
            fprintf(stderr, "error while reading \"%s\"", filename);
    }

    close(fd);
    lo_close(conn, lobj_fd);

    return lobjId;
}

static void
pickout(PGconn *conn, Oid lobjId, int start, int len)
{
    int         lobj_fd;
    char       *buf;
    int         nbytes;
    int         nread;

    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd < 0)
        fprintf(stderr, "cannot open large object %u", lobjId);

    lo_lseek(conn, lobj_fd, start, SEEK_SET);
    buf = malloc(len + 1);

    nread = 0;
    while (len - nread > 0)
    {
        nbytes = lo_read(conn, lobj_fd, buf, len - nread);
        buf[nbytes] = '\0';
        fprintf(stderr, ">>> %s", buf);
        nread += nbytes;
        if (nbytes <= 0)
            break;              /* no more data? */
    }
    free(buf);
    fprintf(stderr, "\n");
    lo_close(conn, lobj_fd);
}

static void
overwrite(PGconn *conn, Oid lobjId, int start, int len)
{
    int         lobj_fd;
    char       *buf;
    int         nbytes;
    int         nwritten;
    int         i;

    lobj_fd = lo_open(conn, lobjId, INV_WRITE);
    if (lobj_fd < 0)
        fprintf(stderr, "cannot open large object %u", lobjId);

    lo_lseek(conn, lobj_fd, start, SEEK_SET);
    buf = malloc(len + 1);

    for (i = 0; i < len; i++)
        buf[i] = 'X';
    buf[i] = '\0';

    nwritten = 0;
    while (len - nwritten > 0)
    {
        nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
        nwritten += nbytes;
        if (nbytes <= 0)
        {
            fprintf(stderr, "\nWRITE FAILED!\n");
            break;
        }
    }
    free(buf);
    fprintf(stderr, "\n");
    lo_close(conn, lobj_fd);
}


/*
 * exportFile -
 *    export large object "lobjOid" to file "out_filename"
 *
 */
static void
exportFile(PGconn *conn, Oid lobjId, char *filename)
{
    int         lobj_fd;
    char        buf[BUFSIZE];
    int         nbytes,
                tmp;
    int         fd;

    /*
     * open the large object
     */
    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd < 0)
        fprintf(stderr, "cannot open large object %u", lobjId);

    /*
     * open the file to be written to
     */
    fd = open(filename, O_CREAT | O_WRONLY | O_TRUNC, 0666);
    if (fd < 0)
    {                           /* error */
        fprintf(stderr, "cannot open unix file\"%s\"",
                filename);
    }

    /*
     * read in from the inversion file and write to the Unix file
     */
    while ((nbytes = lo_read(conn, lobj_fd, buf, BUFSIZE)) > 0)
    {
        tmp = write(fd, buf, nbytes);
        if (tmp < nbytes)
        {
            fprintf(stderr, "error while writing \"%s\"",
                    filename);
        }
    }

    lo_close(conn, lobj_fd);
    close(fd);

    return;
}

static void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}

int
main(int argc, char **argv)
{
    char       *in_filename,
               *out_filename;
    char       *database;
    Oid         lobjOid;
    PGconn     *conn;
    PGresult   *res;

    if (argc != 4)
    {
        fprintf(stderr, "Usage: %s database_name in_filename out_filename\n",
                argv[0]);
        exit(1);
    }

    database = argv[1];
    in_filename = argv[2];
    out_filename = argv[3];

    /*
     * set up the connection
     */
    conn = PQsetdb(NULL, NULL, NULL, NULL, database);

    /* check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "Connection to database failed: %s",
                PQerrorMessage(conn));
        exit_nicely(conn);
    }

    res = PQexec(conn, "begin");
    PQclear(res);
    printf("importing file \"%s\" ...\n", in_filename);
/*  lobjOid = importFile(conn, in_filename); */
    lobjOid = lo_import(conn, in_filename);
    if (lobjOid == 0)
        fprintf(stderr, "%s\n", PQerrorMessage(conn));
    else
    {
        printf("\tas large object %u.\n", lobjOid);

        printf("picking out bytes 1000-2000 of the large object\n");
        pickout(conn, lobjOid, 1000, 1000);

        printf("overwriting bytes 1000-2000 of the large object with X's\n");
        overwrite(conn, lobjOid, 1000, 1000);

        printf("exporting large object to file \"%s\" ...\n", out_filename);
/*      exportFile(conn, lobjOid, out_filename); */
        if (lo_export(conn, lobjOid, out_filename) < 0)
            fprintf(stderr, "%s\n", PQerrorMessage(conn));
    }

    res = PQexec(conn, "end");
    PQclear(res);
    PQfinish(conn);
    return 0;
}
]]>
</programlisting>
</example>

</sect1>
</chapter>