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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>35.3. Client Interfaces</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="lo-implementation.html" title="35.2. Implementation Features" /><link rel="next" href="lo-funcs.html" title="35.4. Server-Side Functions" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">35.3. Client Interfaces</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="lo-implementation.html" title="35.2. Implementation Features">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="largeobjects.html" title="Chapter 35. Large Objects">Up</a></td><th width="60%" align="center">Chapter 35. Large Objects</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="lo-funcs.html" title="35.4. Server-Side Functions">Next</a></td></tr></table><hr /></div><div class="sect1" id="LO-INTERFACES"><div class="titlepage"><div><div><h2 class="title" style="clear: both">35.3. Client Interfaces</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="lo-interfaces.html#LO-CREATE">35.3.1. Creating a Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-IMPORT">35.3.2. Importing a Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-EXPORT">35.3.3. Exporting a Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-OPEN">35.3.4. Opening an Existing Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-WRITE">35.3.5. Writing Data to a Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-READ">35.3.6. Reading Data from a Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-SEEK">35.3.7. Seeking in a Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-TELL">35.3.8. Obtaining the Seek Position of a Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-TRUNCATE">35.3.9. Truncating a Large Object</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-CLOSE">35.3.10. Closing a Large Object Descriptor</a></span></dt><dt><span class="sect2"><a href="lo-interfaces.html#LO-UNLINK">35.3.11. Removing a Large Object</a></span></dt></dl></div><p>
+ This section describes the facilities that
+ <span class="productname">PostgreSQL</span>'s <span class="application">libpq</span>
+ client interface library provides for accessing large objects.
+ 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>
+ All large object manipulation using these functions
+ <span class="emphasis"><em>must</em></span> take place within an SQL transaction block,
+ since large object file descriptors are only valid for the duration of
+ a transaction.
+ </p><p>
+ 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 <a class="xref" href="libpq-status.html#LIBPQ-PQERRORMESSAGE"><code class="function">PQerrorMessage</code></a>.
+ </p><p>
+ Client applications that use these functions should include the header file
+ <code class="filename">libpq/libpq-fs.h</code> and link with the
+ <span class="application">libpq</span> library.
+ </p><p>
+ Client applications cannot use these functions while a libpq connection is in pipeline mode.
+ </p><div class="sect2" id="LO-CREATE"><div class="titlepage"><div><div><h3 class="title">35.3.1. Creating a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.7.2.1" class="indexterm"></a>
+ The function
+</p><pre class="synopsis">
+Oid lo_create(PGconn *conn, Oid lobjId);
+</pre><p>
+ creates a new large object. The OID to be assigned can be
+ specified by <em class="replaceable"><code>lobjId</code></em>;
+ if so, failure occurs if that OID is already in use for some large
+ object. If <em class="replaceable"><code>lobjId</code></em>
+ is <code class="symbol">InvalidOid</code> (zero) then <code class="function">lo_create</code>
+ assigns an unused OID.
+ The return value is the OID that was assigned to the new large object,
+ or <code class="symbol">InvalidOid</code> (zero) on failure.
+ </p><p>
+ An example:
+</p><pre class="programlisting">
+inv_oid = lo_create(conn, desired_oid);
+</pre><p>
+ </p><p>
+ <a id="id-1.7.4.8.7.4.1" class="indexterm"></a>
+ The older function
+</p><pre class="synopsis">
+Oid lo_creat(PGconn *conn, int mode);
+</pre><p>
+ also creates a new large object, always assigning an unused OID.
+ The return value is the OID that was assigned to the new large object,
+ or <code class="symbol">InvalidOid</code> (zero) on failure.
+ </p><p>
+ In <span class="productname">PostgreSQL</span> releases 8.1 and later,
+ the <em class="replaceable"><code>mode</code></em> is ignored,
+ so that <code class="function">lo_creat</code> is exactly equivalent to
+ <code class="function">lo_create</code> with a zero second argument.
+ However, there is little reason to use <code class="function">lo_creat</code>
+ unless you need to work with servers older than 8.1.
+ To work with such an old server, you must
+ use <code class="function">lo_creat</code> not <code class="function">lo_create</code>,
+ and you must set <em class="replaceable"><code>mode</code></em> to
+ one of <code class="symbol">INV_READ</code>, <code class="symbol">INV_WRITE</code>,
+ or <code class="symbol">INV_READ</code> <code class="literal">|</code> <code class="symbol">INV_WRITE</code>.
+ (These symbolic constants are defined
+ in the header file <code class="filename">libpq/libpq-fs.h</code>.)
+ </p><p>
+ An example:
+</p><pre class="programlisting">
+inv_oid = lo_creat(conn, INV_READ|INV_WRITE);
+</pre><p>
+ </p></div><div class="sect2" id="LO-IMPORT"><div class="titlepage"><div><div><h3 class="title">35.3.2. Importing a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.8.2.1" class="indexterm"></a>
+ 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>
+ <em class="replaceable"><code>filename</code></em>
+ 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 <code class="symbol">InvalidOid</code> (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><p>
+ <a id="id-1.7.4.8.8.3.1" class="indexterm"></a>
+ The function
+</p><pre class="synopsis">
+Oid lo_import_with_oid(PGconn *conn, const char *filename, Oid lobjId);
+</pre><p>
+ also imports a new large object. The OID to be assigned can be
+ specified by <em class="replaceable"><code>lobjId</code></em>;
+ if so, failure occurs if that OID is already in use for some large
+ object. If <em class="replaceable"><code>lobjId</code></em>
+ is <code class="symbol">InvalidOid</code> (zero) then <code class="function">lo_import_with_oid</code> assigns an unused
+ OID (this is the same behavior as <code class="function">lo_import</code>).
+ The return value is the OID that was assigned to the new large object,
+ or <code class="symbol">InvalidOid</code> (zero) on failure.
+ </p><p>
+ <code class="function">lo_import_with_oid</code> is new as of <span class="productname">PostgreSQL</span>
+ 8.4 and uses <code class="function">lo_create</code> internally which is new in 8.1; if this function is run against 8.0 or before, it will
+ fail and return <code class="symbol">InvalidOid</code>.
+ </p></div><div class="sect2" id="LO-EXPORT"><div class="titlepage"><div><div><h3 class="title">35.3.3. Exporting a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.9.2.1" class="indexterm"></a>
+ 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>
+ The <em class="parameter"><code>lobjId</code></em> argument specifies the OID of the large
+ object to export and the <em class="parameter"><code>filename</code></em> 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" id="LO-OPEN"><div class="titlepage"><div><div><h3 class="title">35.3.4. Opening an Existing Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.10.2.1" class="indexterm"></a>
+ 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>
+ The <em class="parameter"><code>lobjId</code></em> argument specifies the OID of the large
+ object to open. The <em class="parameter"><code>mode</code></em> bits control whether the
+ object is opened for reading (<code class="symbol">INV_READ</code>), writing
+ (<code class="symbol">INV_WRITE</code>), or both.
+ (These symbolic constants are defined
+ in the header file <code class="filename">libpq/libpq-fs.h</code>.)
+ <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_lseek64</code>, <code class="function">lo_tell</code>,
+ <code class="function">lo_tell64</code>, <code class="function">lo_truncate</code>,
+ <code class="function">lo_truncate64</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
+ <code class="symbol">INV_WRITE</code> and <code class="symbol">INV_READ</code> <code class="literal">|</code>
+ <code class="symbol">INV_WRITE</code>: you are allowed to read from the descriptor
+ in either case. However there is a significant difference between
+ these modes and <code class="symbol">INV_READ</code> alone: with <code class="symbol">INV_READ</code>
+ 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 <code class="symbol">INV_WRITE</code> 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 <code class="literal">REPEATABLE READ</code> versus <code class="literal">READ COMMITTED</code> transaction
+ modes for ordinary SQL <code class="command">SELECT</code> commands.
+ </p><p>
+ <code class="function">lo_open</code> will fail if <code class="literal">SELECT</code>
+ privilege is not available for the large object, or
+ if <code class="symbol">INV_WRITE</code> is specified and <code class="literal">UPDATE</code>
+ privilege is not available.
+ (Prior to <span class="productname">PostgreSQL</span> 11, these privilege
+ checks were instead performed at the first actual read or write call
+ using the descriptor.)
+ These privilege checks can be disabled with the
+ <a class="xref" href="runtime-config-compatible.html#GUC-LO-COMPAT-PRIVILEGES">lo_compat_privileges</a> run-time parameter.
+ </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" id="LO-WRITE"><div class="titlepage"><div><div><h3 class="title">35.3.5. Writing Data to a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.11.2.1" class="indexterm"></a>
+ The function
+</p><pre class="synopsis">
+int lo_write(PGconn *conn, int fd, const char *buf, size_t len);
+</pre><p>
+ writes <em class="parameter"><code>len</code></em> bytes from <em class="parameter"><code>buf</code></em>
+ (which must be of size <em class="parameter"><code>len</code></em>) to large object
+ descriptor <em class="parameter"><code>fd</code></em>. The <em class="parameter"><code>fd</code></em> argument must
+ have been returned by a previous <code class="function">lo_open</code>. The
+ number of bytes actually written is returned (in the current
+ implementation, this will always equal <em class="parameter"><code>len</code></em> unless
+ there is an error). In the event of an error, the return value is -1.
+</p><p>
+ Although the <em class="parameter"><code>len</code></em> parameter is declared as
+ <code class="type">size_t</code>, this function will reject length values larger than
+ <code class="literal">INT_MAX</code>. In practice, it's best to transfer data in chunks
+ of at most a few megabytes anyway.
+</p></div><div class="sect2" id="LO-READ"><div class="titlepage"><div><div><h3 class="title">35.3.6. Reading Data from a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.12.2.1" class="indexterm"></a>
+ The function
+</p><pre class="synopsis">
+int lo_read(PGconn *conn, int fd, char *buf, size_t len);
+</pre><p>
+ reads up to <em class="parameter"><code>len</code></em> bytes from large object descriptor
+ <em class="parameter"><code>fd</code></em> into <em class="parameter"><code>buf</code></em> (which must be
+ of size <em class="parameter"><code>len</code></em>). The <em class="parameter"><code>fd</code></em>
+ argument must have been returned by a previous
+ <code class="function">lo_open</code>. The number of bytes actually read is
+ returned; this will be less than <em class="parameter"><code>len</code></em> if the end of
+ the large object is reached first. In the event of an error, the return
+ value is -1.
+</p><p>
+ Although the <em class="parameter"><code>len</code></em> parameter is declared as
+ <code class="type">size_t</code>, this function will reject length values larger than
+ <code class="literal">INT_MAX</code>. In practice, it's best to transfer data in chunks
+ of at most a few megabytes anyway.
+</p></div><div class="sect2" id="LO-SEEK"><div class="titlepage"><div><div><h3 class="title">35.3.7. Seeking in a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.13.2.1" class="indexterm"></a>
+ 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>
+ This function moves the
+ current location pointer for the large object descriptor identified by
+ <em class="parameter"><code>fd</code></em> to the new location specified by
+ <em class="parameter"><code>offset</code></em>. The valid values for <em class="parameter"><code>whence</code></em>
+ are <code class="symbol">SEEK_SET</code> (seek from object start),
+ <code class="symbol">SEEK_CUR</code> (seek from current position), and
+ <code class="symbol">SEEK_END</code> (seek from object end). The return value is
+ the new location pointer, or -1 on error.
+</p><p>
+ <a id="id-1.7.4.8.13.3.1" class="indexterm"></a>
+ When dealing with large objects that might exceed 2GB in size,
+ instead use
+</p><pre class="synopsis">
+pg_int64 lo_lseek64(PGconn *conn, int fd, pg_int64 offset, int whence);
+</pre><p>
+ This function has the same behavior
+ as <code class="function">lo_lseek</code>, but it can accept an
+ <em class="parameter"><code>offset</code></em> larger than 2GB and/or deliver a result larger
+ than 2GB.
+ Note that <code class="function">lo_lseek</code> will fail if the new location
+ pointer would be greater than 2GB.
+</p><p>
+ <code class="function">lo_lseek64</code> is new as of <span class="productname">PostgreSQL</span>
+ 9.3. If this function is run against an older server version, it will
+ fail and return -1.
+</p></div><div class="sect2" id="LO-TELL"><div class="titlepage"><div><div><h3 class="title">35.3.8. Obtaining the Seek Position of a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.14.2.1" class="indexterm"></a>
+ 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>
+ If there is an error, the return value is -1.
+</p><p>
+ <a id="id-1.7.4.8.14.3.1" class="indexterm"></a>
+ When dealing with large objects that might exceed 2GB in size,
+ instead use
+</p><pre class="synopsis">
+pg_int64 lo_tell64(PGconn *conn, int fd);
+</pre><p>
+ This function has the same behavior
+ as <code class="function">lo_tell</code>, but it can deliver a result larger
+ than 2GB.
+ Note that <code class="function">lo_tell</code> will fail if the current
+ read/write location is greater than 2GB.
+</p><p>
+ <code class="function">lo_tell64</code> is new as of <span class="productname">PostgreSQL</span>
+ 9.3. If this function is run against an older server version, it will
+ fail and return -1.
+</p></div><div class="sect2" id="LO-TRUNCATE"><div class="titlepage"><div><div><h3 class="title">35.3.9. Truncating a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.15.2.1" class="indexterm"></a>
+ To truncate a large object to a given length, call
+</p><pre class="synopsis">
+int lo_truncate(PGconn *conn, int fd, size_t len);
+</pre><p>
+ This function truncates the large object
+ descriptor <em class="parameter"><code>fd</code></em> to length <em class="parameter"><code>len</code></em>. The
+ <em class="parameter"><code>fd</code></em> argument must have been returned by a
+ previous <code class="function">lo_open</code>. If <em class="parameter"><code>len</code></em> is
+ greater than the large object's current length, the large object
+ is extended to the specified length with null bytes ('\0').
+ On success, <code class="function">lo_truncate</code> returns
+ zero. On error, the return value is -1.
+</p><p>
+ The read/write location associated with the descriptor
+ <em class="parameter"><code>fd</code></em> is not changed.
+</p><p>
+ Although the <em class="parameter"><code>len</code></em> parameter is declared as
+ <code class="type">size_t</code>, <code class="function">lo_truncate</code> will reject length
+ values larger than <code class="literal">INT_MAX</code>.
+</p><p>
+ <a id="id-1.7.4.8.15.5.1" class="indexterm"></a>
+ When dealing with large objects that might exceed 2GB in size,
+ instead use
+</p><pre class="synopsis">
+int lo_truncate64(PGconn *conn, int fd, pg_int64 len);
+</pre><p>
+ This function has the same
+ behavior as <code class="function">lo_truncate</code>, but it can accept a
+ <em class="parameter"><code>len</code></em> value exceeding 2GB.
+</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 -1.
+</p><p>
+ <code class="function">lo_truncate64</code> is new as of <span class="productname">PostgreSQL</span>
+ 9.3; if this function is run against an older server version, it will
+ fail and return -1.
+</p></div><div class="sect2" id="LO-CLOSE"><div class="titlepage"><div><div><h3 class="title">35.3.10. Closing a Large Object Descriptor</h3></div></div></div><p>
+ <a id="id-1.7.4.8.16.2.1" class="indexterm"></a>
+ A large object descriptor can be closed by calling
+</p><pre class="synopsis">
+int lo_close(PGconn *conn, int fd);
+</pre><p>
+ where <em class="parameter"><code>fd</code></em> 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 -1.
+</p><p>
+ Any large object descriptors that remain open at the end of a
+ transaction will be closed automatically.
+</p></div><div class="sect2" id="LO-UNLINK"><div class="titlepage"><div><div><h3 class="title">35.3.11. Removing a Large Object</h3></div></div></div><p>
+ <a id="id-1.7.4.8.17.2.1" class="indexterm"></a>
+ To remove a large object from the database, call
+</p><pre class="synopsis">
+int lo_unlink(PGconn *conn, Oid lobjId);
+</pre><p>
+ The <em class="parameter"><code>lobjId</code></em> argument specifies the OID of the
+ large object to remove. Returns 1 if successful, -1 on failure.
+ </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="lo-implementation.html" title="35.2. Implementation Features">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="largeobjects.html" title="Chapter 35. Large Objects">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="lo-funcs.html" title="35.4. Server-Side Functions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">35.2. Implementation Features </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 35.4. Server-Side Functions</td></tr></table></div></body></html> \ No newline at end of file
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