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766 lines
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766 lines
25 KiB
Plaintext
<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/Attic/libpq++.sgml,v 1.17 2000/09/29 20:21:34 petere Exp $
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-->
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<chapter id="libpqplusplus">
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<title>libpq - C++ Binding Library</title>
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<para>
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<filename>libpq++</filename> is the C++ API to
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<productname>Postgres</productname>.
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<filename>libpq++</filename> is a set of classes which allow
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client programs to connect to the
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<productname>Postgres</productname> backend server. These connections
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come in two forms: a Database Class and a Large Object class.
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</para>
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<para>
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The Database Class is intended for manipulating a database. You can
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send all sorts of SQL queries to the <productname>Postgres</productname>
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backend server and retrieve the responses of the server.
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</para>
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<para>
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The Large Object Class is intended for manipulating a large object
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in a database. Although a Large Object instance can send normal
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queries to the <productname>Postgres</productname> backend server
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it is only intended for simple
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queries that do not return any data. A large object should be seen
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as a file stream. In the future it should behave much like the C++ file
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streams
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<literal>cin</literal>,
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<literal>cout</literal>
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and
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<literal>cerr</literal>.
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</para>
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<para>
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This chapter is based on the documentation
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for the <filename>libpq</filename> C library. Three
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short programs are listed at the end of this section as examples of
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<filename>libpq++</filename> programming
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(though not necessarily of good programming).
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There are several examples of <filename>libpq++</filename>
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applications in
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<filename>src/libpq++/examples</filename>, including the source
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code for the three examples in this chapter.
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</para>
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<sect1 id="libpqpp-init">
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<title>Control and Initialization</title>
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<sect2>
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<title>Environment Variables</title>
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<para>
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The following environment variables can be used to set up default
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values for an environment and to avoid hard-coding database names into
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an application program:
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<note>
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<para>
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Refer to the <xref endterm="libpq" linkend="libpq-envars"> for a complete
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list of available connection options.
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</para>
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</note>
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</para>
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<para>
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The following environment variables can be used to select default
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connection parameter values, which will be used by PQconnectdb or
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PQsetdbLogin if no value is directly specified by the calling code.
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These are useful to avoid hard-coding database names into simple
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application programs.
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<note>
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<para>
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<filename>libpq++</filename> uses only environment variables or PQconnectdb
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conninfo style strings.
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</para>
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</note>
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<itemizedlist>
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<listitem>
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<para>
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<envar>PGHOST</envar> sets the default server name.
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If a non-zero-length string is specified, TCP/IP communication is used.
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Without a host name, libpq will connect using a local Unix domain socket.
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</para>
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</listitem>
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<listitem>
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<para>
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<envar>PGPORT</envar> sets the default port or local Unix domain socket
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file extension for communicating with the <productname>Postgres</productname>
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backend.
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</para>
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</listitem>
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<listitem>
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<para>
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<envar>PGDATABASE</envar> sets the default
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<productname>Postgres</productname> database name.
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</para>
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</listitem>
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<listitem>
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<para>
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<envar>PGUSER</envar>
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sets the username used to connect to the database and for authentication.
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</para>
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</listitem>
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<listitem>
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<para>
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<envar>PGPASSWORD</envar>
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sets the password used if the backend demands password authentication.
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</para>
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</listitem>
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<listitem>
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<para>
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<envar>PGREALM</envar> sets the Kerberos realm to use with
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<productname>Postgres</productname>,
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if it is different from the local realm. If
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<envar>PGREALM</envar> is set, <productname>Postgres</productname>
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applications will attempt
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authentication with servers for this realm and use
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separate ticket files to avoid conflicts with local
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ticket files. This environment variable is only
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used if Kerberos authentication is selected by the backend.
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</para>
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</listitem>
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<listitem>
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<para>
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<envar>PGOPTIONS</envar> sets additional runtime options for
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the <productname>Postgres</productname> backend.
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</para>
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</listitem>
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<listitem>
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<para>
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<envar>PGTTY</envar> sets the file or tty on which debugging
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messages from the backend server are displayed.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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The following environment variables can be used to specify user-level default
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behavior for every Postgres session:
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<itemizedlist>
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<listitem>
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<para>
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<envar>PGDATESTYLE</envar>
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sets the default style of date/time representation.
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</para>
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</listitem>
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<listitem>
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<para>
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<envar>PGTZ</envar>
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sets the default time zone.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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The following environment variables can be used to specify default internal
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behavior for every Postgres session:
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<itemizedlist>
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<listitem>
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<para>
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<envar>PGGEQO</envar>
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sets the default mode for the genetic optimizer.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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Refer to the <command>SET</command> <acronym>SQL</acronym> command
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for information on correct values for these environment variables.
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</para>
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</sect2>
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</sect1>
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<sect1 id="libpqpp-classes">
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<title>libpq++ Classes</title>
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<sect2>
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<title>Connection Class: <classname>PgConnection</classname></title>
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<para>
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The connection class makes the actual connection to the database and is inherited
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by all of the access classes.
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</para>
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</sect2>
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<sect2>
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<title>Database Class: <classname>PgDatabase</classname></title>
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<para>
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The database class provides C++ objects that have a connection
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to a backend server. To create such an object one first needs
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the apropriate environment for the backend to access.
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The following constructors deal with making a connection to a backend
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server from a C++ program.
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</para>
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</sect2>
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</sect1>
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<sect1 id="libpqpp-connect">
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<title>Database Connection Functions</title>
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<para>
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<itemizedlist>
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<listitem>
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<para>
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<function>PgConnection</function>
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makes a new connection to a backend database server.
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<synopsis>
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PgConnection::PgConnection(const char *conninfo)
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</synopsis>
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Although typically called from one of the access classes, a connection to
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a backend server is possible by creating a PgConnection object.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>ConnectionBad</function>
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returns whether or not the connection to the backend server succeeded or
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failed.
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<synopsis>
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int PgConnection::ConnectionBad()
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</synopsis>
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Returns TRUE if the connection failed.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>Status</function>
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returns the status of the connection to the backend server.
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<synopsis>
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ConnStatusType PgConnection::Status()
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</synopsis>
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Returns either CONNECTION_OK or CONNECTION_BAD depending on the state
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of the connection.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>PgDatabase</function>
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makes a new connection to a backend database server.
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<synopsis>
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PgDatabase(const char *conninfo)
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</synopsis>
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After a PgDatabase has been created it should be checked to make sure
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the connection to the database succeded before sending
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queries to the object. This can easily be done by
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retrieving the current status of the PgDatabase object with the
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<function>Status</function> or <function>ConnectionBad</function> methods.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>DBName</function>
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Returns the name of the current database.
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<synopsis>
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const char *PgConnection::DBName()
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</synopsis>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>Notifies</function>
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Returns the next notification from a list of unhandled notification messages
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received from the backend.
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<synopsis>
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PGnotify* PgConnection::Notifies()
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</synopsis>
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See PQnotifies() for details.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</sect1>
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<sect1 id="libpqpp-exec">
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<title>Query Execution Functions</title>
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<para>
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<itemizedlist>
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<listitem>
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<para>
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<function>Exec</function>
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Sends a query to the backend server. It's probably more desirable to
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use one of the next two functions.
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<synopsis>
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ExecStatusType PgConnection::Exec(const char* query)
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</synopsis>
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Returns the result of the query. The following status results can be expected:
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<simplelist>
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<member>
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PGRES_EMPTY_QUERY
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</member>
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<member>
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PGRES_COMMAND_OK, if the query was a command
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</member>
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<member>
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PGRES_TUPLES_OK, if the query successfully returned tuples
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</member>
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<member>
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PGRES_COPY_OUT
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</member>
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<member>
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PGRES_COPY_IN
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</member>
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<member>
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PGRES_BAD_RESPONSE, if an unexpected response was received
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</member>
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<member>
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PGRES_NONFATAL_ERROR
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</member>
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<member>
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PGRES_FATAL_ERROR
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</member>
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</simplelist>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>ExecCommandOk</function>
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Sends a command query to the backend server.
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<synopsis>
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int PgConnection::ExecCommandOk(const char *query)
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</synopsis>
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Returns TRUE if the command query succeeds.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>ExecTuplesOk</function>
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Sends a command query to the backend server.
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<synopsis>
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int PgConnection::ExecTuplesOk(const char *query)
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</synopsis>
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Returns TRUE if the command query succeeds.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>ErrorMessage</function>
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Returns the last error message text.
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<synopsis>
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const char *PgConnection::ErrorMessage()
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</synopsis>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>Tuples</function>
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Returns the number of tuples (instances) in the query result.
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<synopsis>
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int PgDatabase::Tuples()
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</synopsis>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>CmdTuples</function>
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Returns the number of rows affected after an INSERT, UPDATE or DELETE.
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If the command was anything else, it returns -1.
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<synopsis>
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int PgDatabase::CmdTuples()
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</synopsis>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>Fields</function>
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Returns the number of fields (attributes) in each tuple of the query result.
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<synopsis>
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int PgDatabase::Fields()
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</synopsis>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>FieldName</function>
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Returns the field (attribute) name associated with the given field index.
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Field indices start at 0.
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<synopsis>
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const char *PgDatabase::FieldName(int field_num)
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</synopsis>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>FieldNum</function>
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PQfnumber Returns the field (attribute) index associated with
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the given field name.
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<synopsis>
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int PgDatabase::FieldNum(const char* field_name)
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</synopsis>
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-1 is returned if the given name does not match any field.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>FieldType</function>
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Returns the field type associated with the given field index. The
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integer returned is an internal coding of the type. Field indices
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start at 0.
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<synopsis>
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Oid PgDatabase::FieldType(int field_num)
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</synopsis>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>FieldType</function>
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Returns the field type associated with the given field name. The
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integer returned is an internal coding of the type. Field indices
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start at 0.
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<synopsis>
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Oid PgDatabase::FieldType(const char* field_name)
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</synopsis>
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</para>
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</listitem>
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<listitem>
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<para>
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<function>FieldSize</function>
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Returns the size in bytes of the field associated with the given
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field index. Field indices start at 0.
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<synopsis>
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short PgDatabase::FieldSize(int field_num)
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</synopsis>
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Returns the space allocated for this field in a database tuple given
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the field number. In other words the size of the server's binary
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representation of the data type. -1 is returned if the field is
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variable size.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>FieldSize</function>
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Returns the size in bytes of the field associated with the given
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field index. Field indices start at 0.
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<synopsis>
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short PgDatabase::FieldSize(const char *field_name)
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</synopsis>
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Returns the space allocated for this field in a database tuple given
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the field name. In other words the size of the server's binary
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representation of the data type. -1 is returned if the field is
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variable size.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>GetValue</function>
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Returns a single field (attribute) value of one tuple of a PGresult.
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Tuple and field indices start at 0.
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<synopsis>
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const char *PgDatabase::GetValue(int tup_num, int field_num)
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</synopsis>
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For most queries, the value returned by GetValue is a null-terminated
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ASCII string representation of the attribute value. But if BinaryTuples()
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is TRUE, the value returned by GetValue is the binary representation
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of the type in the internal format of the backend server (but not including
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the size word, if the field is variable-length). It is then the programmer's
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|
responsibility to cast and convert the data to the correct C type. The
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pointer returned by GetValue points to storage that is part of the
|
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PGresult structure. One should not modify it, and one must explicitly
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|
copy the value into other storage if it is to be used past the lifetime
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|
of the PGresult structure itself. BinaryTuples() is not yet implemented.
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</para>
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</listitem>
|
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<listitem>
|
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<para>
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<function>GetValue</function>
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Returns a single field (attribute) value of one tuple of a PGresult.
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Tuple and field indices start at 0.
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<synopsis>
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const char *PgDatabase::GetValue(int tup_num, const char *field_name)
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</synopsis>
|
|
For most queries, the value returned by GetValue is a null-terminated
|
|
ASCII string representation of the attribute value. But if BinaryTuples()
|
|
is TRUE, the value returned by GetValue is the binary representation
|
|
of the type in the internal format of the backend server (but not including
|
|
the size word, if the field is variable-length). It is then the programmer's
|
|
responsibility to cast and convert the data to the correct C type. The
|
|
pointer returned by GetValue points to storage that is part of the
|
|
PGresult structure. One should not modify it, and one must explicitly
|
|
copy the value into other storage if it is to be used past the lifetime
|
|
of the PGresult structure itself. BinaryTuples() is not yet implemented.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>GetLength</function>
|
|
Returns the length of a field (attribute) in bytes. Tuple and field
|
|
indices start at 0.
|
|
<synopsis>
|
|
int PgDatabase::GetLength(int tup_num, int field_num)
|
|
</synopsis>
|
|
This is the actual data length for the particular data value, that
|
|
is the size of the object pointed to by GetValue. Note that for
|
|
ASCII-represented values, this size has little to do with the binary
|
|
size reported by PQfsize.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>GetLength</function>
|
|
Returns the length of a field (attribute) in bytes. Tuple and field
|
|
indices start at 0.
|
|
<synopsis>
|
|
int PgDatabase::GetLength(int tup_num, const char* field_name)
|
|
</synopsis>
|
|
This is the actual data length for the particular data value, that
|
|
is the size of the object pointed to by GetValue. Note that for
|
|
ASCII-represented values, this size has little to do with the binary
|
|
size reported by PQfsize.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>DisplayTuples</function>
|
|
Prints out all the tuples and, optionally, the attribute names to the
|
|
specified output stream.
|
|
<synopsis>
|
|
void PgDatabase::DisplayTuples(FILE *out = 0, int fillAlign = 1,
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|
const char* fieldSep = "|",int printHeader = 1, int quiet = 0)
|
|
</synopsis>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>PrintTuples</function>
|
|
Prints out all the tuples and, optionally, the attribute names to the
|
|
specified output stream.
|
|
<synopsis>
|
|
void PgDatabase::PrintTuples(FILE *out = 0, int printAttName = 1,
|
|
int terseOutput = 0, int width = 0)
|
|
</synopsis>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>GetLine</function>
|
|
<synopsis>
|
|
int PgDatabase::GetLine(char* string, int length)
|
|
</synopsis>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>PutLine</function>
|
|
<synopsis>
|
|
void PgDatabase::PutLine(const char* string)
|
|
</synopsis>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>OidStatus</function>
|
|
<synopsis>
|
|
const char *PgDatabase::OidStatus()
|
|
</synopsis>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>EndCopy</function>
|
|
<synopsis>
|
|
int PgDatabase::EndCopy()
|
|
</synopsis>
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="libpqpp-notify">
|
|
<title>Asynchronous Notification</title>
|
|
|
|
<para>
|
|
<productname>Postgres</productname> supports asynchronous notification
|
|
via the <command>LISTEN</command> and <command>NOTIFY</command>
|
|
commands. A backend registers its interest in a particular semaphore
|
|
with the <command>LISTEN</command> command.
|
|
All backends that are listening on a
|
|
particular named semaphore will be notified asynchronously when
|
|
a <command>NOTIFY</command> of
|
|
that name is executed by another backend. No additional
|
|
information is passed from the notifier to the listener. Thus,
|
|
typically, any actual data that needs to be communicated is transferred
|
|
through the relation.
|
|
|
|
<note>
|
|
<para>
|
|
In the past, the documentation has associated the names used for asyncronous
|
|
notification with relations or classes. However, there is in fact no
|
|
direct linkage of the two concepts in the implementation, and the
|
|
named semaphore in fact does not need to have a corresponding relation
|
|
previously defined.
|
|
</para>
|
|
</note>
|
|
</para>
|
|
<para>
|
|
<filename>libpq++</filename> applications are notified whenever a
|
|
connected backend has
|
|
received an asynchronous notification. However, the communication from
|
|
the backend to the frontend is not asynchronous.
|
|
The <filename>libpq++</filename> application
|
|
must poll the backend to see if there is any pending notification
|
|
information. After the execution of a query, a frontend may call
|
|
<function>PgDatabase::Notifies</function>
|
|
to see if any notification data is currently available from the backend.
|
|
<function>PgDatabase::Notifies</function>
|
|
returns the notification from a list of unhandled notifications from the
|
|
backend. The function eturns NULL if there is no pending notifications from the
|
|
backend.
|
|
<function>PgDatabase::Notifies</function>
|
|
behaves like the popping of a stack. Once a notification is returned
|
|
from <function>PgDatabase::Notifies</function>,
|
|
it is considered handled and will be removed from the list of
|
|
notifications.
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
<function>PgDatabase::Notifies</function>
|
|
retrieves pending notifications from the server.
|
|
|
|
<synopsis>
|
|
PGnotify* PgDatabase::Notifies()
|
|
</synopsis>
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
<para>
|
|
The second sample program gives an example of the use of asynchronous
|
|
notification.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="libpqpp-copy">
|
|
<title>Functions Associated with the COPY Command</title>
|
|
|
|
<para>
|
|
The <command>copy</command> command in <productname>Postgres</productname>
|
|
has options to read from or write to the network
|
|
connection used by <filename>libpq++</filename>.
|
|
Therefore, functions are necessary to
|
|
access this network connection directly so applications may take full
|
|
advantage of this capability.
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
<function>PgDatabase::GetLine</function>
|
|
reads a newline-terminated line of characters (transmitted by the
|
|
backend server) into a buffer
|
|
<replaceable class="parameter">string</replaceable>
|
|
of size <replaceable class="parameter">length</replaceable>.
|
|
<synopsis>
|
|
int PgDatabase::GetLine(char* string, int length)
|
|
</synopsis>
|
|
</para>
|
|
<para>
|
|
Like the Unix system routine
|
|
<function>fgets (3)</function>,
|
|
this routine copies up to
|
|
<literal><replaceable class="parameter">length</replaceable>-1</literal>
|
|
characters into
|
|
<replaceable class="parameter">string</replaceable>.
|
|
It is like
|
|
<function>gets (3)</function>,
|
|
however, in that it converts the terminating newline into a null
|
|
character.
|
|
</para>
|
|
<para>
|
|
<function>PgDatabase::GetLine</function>
|
|
returns EOF at end of file, 0 if the entire line has been read, and 1 if the
|
|
buffer is full but the terminating newline has not yet been read.
|
|
</para>
|
|
<para>
|
|
Notice that the application must check to see if a new line consists
|
|
of a single period ("."), which indicates that the backend
|
|
server has finished sending the results of the
|
|
<command>copy</command>.
|
|
Therefore, if the application ever expects to receive lines
|
|
that are more than
|
|
<literal><replaceable class="parameter">length</replaceable>-1</literal>
|
|
characters long, the application must be sure to check the return
|
|
value of <function>PgDatabase::GetLine</function> very carefully.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>PgDatabase::PutLine</function>
|
|
Sends a null-terminated <replaceable class="parameter">string</replaceable>
|
|
to the backend server.
|
|
<synopsis>
|
|
void PgDatabase::PutLine(char* string)
|
|
</synopsis>
|
|
</para>
|
|
<para>
|
|
The application must explicitly send a single period character (".")
|
|
to indicate to the backend that it has finished sending its data.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<function>PgDatabase::EndCopy</function>
|
|
syncs with the backend.
|
|
<synopsis>
|
|
int PgDatabase::EndCopy()
|
|
</synopsis>
|
|
This function waits until the backend has
|
|
finished processing the <command>copy</command>.
|
|
It should either be issued when the
|
|
last string has been sent to the backend using
|
|
<function>PgDatabase::PutLine</function>
|
|
or when the last string has been received from the backend using
|
|
<function>PgDatabase::GetLine</function>.
|
|
It must be issued or the backend may get "out of sync" with
|
|
the frontend. Upon return from this function, the backend is ready to
|
|
receive the next query.
|
|
</para>
|
|
<para>
|
|
The return value is 0 on successful completion, nonzero otherwise.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
<para>
|
|
As an example:
|
|
|
|
<programlisting>
|
|
PgDatabase data;
|
|
data.Exec("create table foo (a int4, b char(16), d float8)");
|
|
data.Exec("copy foo from stdin");
|
|
data.PutLine("3\tHello World\t4.5\n");
|
|
data.PutLine("4\tGoodbye World\t7.11\n");
|
|
&...
|
|
data.PutLine("\\.\n");
|
|
data.EndCopy();
|
|
</programlisting>
|
|
</para>
|
|
</sect1>
|
|
|
|
</chapter>
|
|
|
|
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|
|
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|
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|
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|
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|
|
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|
|
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|
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|