<!-- $Header: /usr/local/cvsroot/libpqpp/libpqpp/docs/libpq++.sgml,v 1.1 2002/11/27 04:27:27 bmomjian Exp $ --> <chapter id="libpqplusplus"> <title><application>libpq++</application> - C++ Binding Library</title> <indexterm zone="libpqplusplus"> <primary>libpq++</primary> </indexterm> <indexterm zone="libpqplusplus"> <primary>C++</primary> </indexterm> <sect1 id="libpqpp-introduction"> <title>Introduction</title> <para> <application>libpq++</application> is the C++ API to <productname>PostgreSQL</productname>. <application>libpq++</application> is a set of classes that allow client programs to connect to the <productname>PostgreSQL</productname> backend server. These connections come in two forms: a database class and a large object class. </para> <para> The database class is intended for manipulating a database. You can send all sorts of SQL queries and commands to the <productname>PostgreSQL</productname> backend server and retrieve the responses of the server. </para> <para> The large object class is intended for manipulating a large object in a database. Although a large object instance can send normal queries to the <productname>PostgreSQL</productname> backend server it is only intended for simple queries that do not return any data. A large object should be seen as a file stream. In the future it should behave much like the C++ file streams <literal>cin</literal>, <literal>cout</literal> and <literal>cerr</literal>. </para> <para> This chapter is based on the documentation for the <application>libpq</application> C library (see libpq). There are several examples of <application>libpq++</application> applications in <filename>src/interfaces/libpq++/examples</filename> in the source distribution. </para> </sect1> <sect1 id="libpqpp-init"> <title>Control and Initialization</title> <sect2> <title>Environment Variables</title> <para> The following environment variables can be used to set up default values for an environment and to avoid hard-coding database names into an application program: <note> <para> Refer to libpq for a complete list of available connection options. </para> </note> </para> <para> The following environment variables can be used to select default connection parameter values, which will be used by <function>PQconnectdb</> or <function>PQsetdbLogin</> if no value is directly specified by the calling code. These are useful to avoid hard-coding database names into simple application programs. <note> <para> <filename>libpq++</filename> uses only environment variables or <filename>libpq</>'s <function>PQconnectdb</> <parameter>conninfo</parameter> style strings. </para> </note> <itemizedlist> <listitem> <para> <envar>PGHOST</envar> sets the default server name. If this begins with a slash, it specifies Unix-domain communication rather than TCP/IP communication; the value is the name of the directory in which the socket file is stored (default <filename>/tmp</filename>). </para> </listitem> <listitem> <para> <envar>PGPORT</envar> sets the default TCP port number or Unix-domain socket file extension for communicating with the <productname>PostgreSQL</productname> backend. </para> </listitem> <listitem> <para> <envar>PGDATABASE</envar> sets the default <productname>PostgreSQL</productname> database name. </para> </listitem> <listitem> <para> <envar>PGUSER</envar> sets the user name used to connect to the database and for authentication. </para> </listitem> <listitem> <para> <envar>PGPASSWORD</envar> sets the password used if the backend demands password authentication. This is deprecated; use <envar>PGPASSWORDFILE</envar> instead. </para> </listitem> <listitem> <para> <envar>PGPASSWORDFILE</envar> sets the password file used if the backend demands password authentication. Refer to the libpq documentation for more details. </para> </listitem> <listitem> <para> <envar>PGREALM</envar> sets the Kerberos realm to use with <productname>PostgreSQL</productname>, if it is different from the local realm. If <envar>PGREALM</envar> is set, <productname>PostgreSQL</productname> applications will attempt authentication with servers for this realm and use separate ticket files to avoid conflicts with local ticket files. This environment variable is only used if Kerberos authentication is selected by the backend. </para> </listitem> <listitem> <para> <envar>PGOPTIONS</envar> sets additional run-time options for the <productname>PostgreSQL</productname> backend. </para> </listitem> <listitem> <para> <envar>PGTTY</envar> sets the file or <acronym>tty</acronym> on which debugging messages from the backend server are displayed. </para> </listitem> </itemizedlist> </para> <para> The following environment variables can be used to specify user-level default behavior for every <productname>PostgreSQL</productname> session: <itemizedlist> <listitem> <para> <envar>PGDATESTYLE</envar> sets the default style of date/time representation. </para> </listitem> <listitem> <para> <envar>PGTZ</envar> sets the default time zone. </para> </listitem> </itemizedlist> </para> <para> The following environment variables can be used to specify default internal behavior for every <productname>PostgreSQL</productname> session: <itemizedlist> <listitem> <para> <envar>PGGEQO</envar> sets the default mode for the genetic optimizer. </para> </listitem> </itemizedlist> </para> <para> Refer to the <command>SET</command> <acronym>SQL</acronym> command for information on correct values for these environment variables. </para> </sect2> </sect1> <sect1 id="libpqpp-classes"> <title><application>libpq++</application> Classes</title> <sect2> <title>Connection Class: <classname>PgConnection</classname></title> <para> The connection class makes the actual connection to the database and is inherited by all of the access classes. </para> </sect2> <sect2> <title>Database Class: <classname>PgDatabase</classname></title> <para> The database class provides C++ objects that have a connection to a backend server. To create such an object one first needs the appropriate environment for the backend to access. The following constructors deal with making a connection to a backend server from a C++ program. </para> </sect2> </sect1> <sect1 id="libpqpp-connect"> <title>Database Connection Functions</title> <para> <itemizedlist> <listitem> <para> <function>PgConnection</function> makes a new connection to a backend database server. <synopsis> PgConnection::PgConnection(const char *conninfo) </synopsis> The <parameter>conninfo</> string is the same as for the underlying <application>libpq</> <function>PQconnectdb</> function. </para> <para> Although typically called from one of the access classes, a connection to a backend server is possible by creating a <classname>PgConnection</> object. </para> </listitem> <listitem> <para> <function>ConnectionBad</function> returns whether or not the connection to the backend server succeeded or failed. <synopsis> bool PgConnection::ConnectionBad() const </synopsis> Returns true if the connection failed. </para> </listitem> <listitem> <para> <function>Status</function> returns the status of the connection to the backend server. <synopsis> ConnStatusType PgConnection::Status() </synopsis> Returns either <symbol>CONNECTION_OK</> or <symbol>CONNECTION_BAD</> depending on the state of the connection. </para> </listitem> <listitem> <para> <function>PgDatabase</function> makes a new connection to a backend database server. <synopsis> PgDatabase(const char *conninfo) </synopsis> After a <classname>PgDatabase</classname> has been created it should be checked to make sure the connection to the database succeeded before sending queries to the object. This can easily be done by retrieving the current status of the <classname>PgDatabase</classname> object with the <function>Status</function> or <function>ConnectionBad</function> methods. </para> </listitem> <listitem> <para> <function>DBName</function> returns the name of the current database. <synopsis> const char *PgConnection::DBName() </synopsis> </para> </listitem> <listitem> <para> <function>Notifies</function> returns the next notification from a list of unhandled notification messages received from the backend. <synopsis> PGnotify* PgConnection::Notifies() </synopsis> See <function>PQnotifies</function> in <application>libpq</> for details. </para> </listitem> </itemizedlist> </para> </sect1> <sect1 id="libpqpp-exec"> <title>Query Execution Functions</title> <sect2 id="libpqpp-exec-main"> <title>Main Routines</title> <para> <itemizedlist> <listitem> <para> <function>Exec</function> sends a command to the backend server. It's probably more desirable to use one of the next two functions. <synopsis> ExecStatusType PgConnection::Exec(const char* query) </synopsis> Returns the result status of the command. The following status results can be expected: <simplelist> <member> <symbol>PGRES_EMPTY_QUERY</symbol> </member> <member> <symbol>PGRES_COMMAND_OK</symbol>, if the command was not a query </member> <member> <symbol>PGRES_TUPLES_OK</symbol>, if the query successfully returned tuples </member> <member> <symbol>PGRES_COPY_OUT</symbol> </member> <member> <symbol>PGRES_COPY_IN</symbol> </member> <member> <symbol>PGRES_BAD_RESPONSE</symbol>, if an unexpected response was received </member> <member> <symbol>PGRES_NONFATAL_ERROR</symbol> </member> <member> <symbol>PGRES_FATAL_ERROR</symbol> </member> </simplelist> </para> </listitem> <listitem> <para> <function>ExecCommandOk</function> sends a non-query command (one that does not return rows) to the backend server. <synopsis> int PgConnection::ExecCommandOk(const char *query) </synopsis> Returns true (1) if the command succeeds. </para> </listitem> <listitem> <para> <function>ExecTuplesOk</function> Sends a query command (one that returns rows) to the backend server. <synopsis> int PgConnection::ExecTuplesOk(const char *query) </synopsis> Returns true (1) if the query succeeds. </para> </listitem> <listitem> <para> <function>ErrorMessage</function> returns the last error message text. <synopsis> const char *PgConnection::ErrorMessage() </synopsis> </para> </listitem> </itemizedlist> </para> </sect2> <sect2 id="libpqpp-exec-select-info"> <title>Retrieving SELECT Result Information</title> <itemizedlist> <listitem> <para> <function>Tuples</function> returns the number of tuples (rows) in the query result. <synopsis> int PgDatabase::Tuples() const </synopsis> </para> </listitem> <listitem> <para> <function>Fields</function> returns the number of fields (rows) in each tuple of the query result. <synopsis> int PgDatabase::Fields() </synopsis> </para> </listitem> <listitem> <para> <function>FieldName</function> returns the field (column) name associated with the given field index. Field indices start at 0. <synopsis> const char *PgDatabase::FieldName(int field_num) const </synopsis> </para> </listitem> <listitem> <para> <function>FieldNum</function> returns the field (column) index associated with the given field name. <synopsis> int PgDatabase::FieldNum(const char* field_name) const </synopsis> -1 is returned if the given name does not match any field. </para> </listitem> <listitem> <para> <function>FieldType</function> returns the field type associated with the given field index. The integer returned is an internal coding of the type. Field indices start at 0. <synopsis> Oid PgDatabase::FieldType(int field_num) const </synopsis> </para> </listitem> <listitem> <para> <function>FieldType</function> returns the field type associated with the given field name. The integer returned is an internal coding of the type. Field indices start at 0. <synopsis> Oid PgDatabase::FieldType(const char* field_name) const </synopsis> </para> </listitem> <listitem> <para> <function>FieldSize</function> returns the size in bytes of the field associated with the given field index. Field indices start at 0. <synopsis> int PgDatabase::FieldSize(int field_num) const </synopsis> Returns the space allocated for this field in a database tuple given the field number. In other words the size of the server's binary representation of the data type. -1 is returned if the field is variable size. </para> </listitem> <listitem> <para> <function>FieldSize</function> returns the size in bytes of the field associated with the given field index. Field indices start at 0. <synopsis> int PgDatabase::FieldSize(const char *field_name) const </synopsis> Returns the space allocated for this field in a database tuple given the field name. In other words the size of the server's binary representation of the data type. -1 is returned if the field is variable size. </para> </listitem> </itemizedlist> </sect2> <sect2 id="libpqpp-exec-select-values"> <title>Retrieving SELECT Result Values</title> <itemizedlist> <listitem> <para> <function>GetValue</function> returns a single field (column) value of one tuple of a <structname>PGresult</structname>. Tuple and field indices start at 0. <synopsis> const char *PgDatabase::GetValue(int tup_num, int field_num) const </synopsis> For most queries, the value returned by <function>GetValue</function> is a null-terminated string representation of the attribute value. But if <function>BinaryTuples</function> is true, the value returned by <function>GetValue</function> 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 <function>GetValue</function> points to storage that is part of the <structname>PGresult</structname> 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 <structname>PGresult</structname> structure itself. <function>BinaryTuples</function> is not yet implemented. </para> </listitem> <listitem> <para> <function>GetValue</function> returns a single field (column) value of one tuple of a <structname>PGresult</structname>. Tuple and field indices start at 0. <synopsis> const char *PgDatabase::GetValue(int tup_num, const char *field_name) const </synopsis> For most queries, the value returned by <function>GetValue</function> is a null-terminated string representation of the attribute value. But if <function>BinaryTuples</function> is true, the value returned by <function>GetValue</function> 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 <function>GetValue</function> points to storage that is part of the <structname>PGresult</structname> 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 <structname>PGresult</structname> structure itself. <function>BinaryTuples</function> is not yet implemented. </para> </listitem> <listitem> <para> <function>GetLength</function> returns the length of a field (column) in bytes. Tuple and field indices start at 0. <synopsis> int PgDatabase::GetLength(int tup_num, int field_num) const </synopsis> This is the actual data length for the particular data value, that is the size of the object pointed to by <function>GetValue</function>. Note that for character-represented values, this size has little to do with the binary size reported by <function>PQfsize</function>. </para> </listitem> <listitem> <para> <function>GetLength</function> returns the length of a field (column) in bytes. Tuple and field indices start at 0. <synopsis> int PgDatabase::GetLength(int tup_num, const char* field_name) const </synopsis> This is the actual data length for the particular data value, that is the size of the object pointed to by <function>GetValue</function>. Note that for character-represented values, this size has little to do with the binary size reported by <function>PQfsize</function>. </para> </listitem> <listitem> <para> <function>GetIsNull</function> returns whether a field has the null value. <synopsis> bool GetIsNull(int tup_num, int field_num) const </synopsis> Note that <function>GetValue</function> will return the empty string for null fields, not the NULL pointer. </para> </listitem> <listitem> <para> <function>GetIsNull</function> returns whether a field has the null value. <synopsis> bool GetIsNull(int tup_num, const char *field_name) const </synopsis> Note that <function>GetValue</function> will return the empty string for null fields, not the NULL pointer. </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, bool fillAlign = true, const char* fieldSep = "|", bool printHeader = true, bool quiet = false) const </synopsis> This function is obsolescent. </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, bool printAttName = true, bool terseOutput = false, bool fillAlign = false) const </synopsis> This function is obsolescent. </para> </listitem> </itemizedlist> </sect2> <sect2 id="libpqpp-exec-nonselect"> <title>Retrieving Non-SELECT Result Information</title> <itemizedlist> <listitem> <para> <function>CmdTuples</function> returns the number of rows affected after an <command>INSERT</command>, <command>UPDATE</command>, or <command>DELETE</command>. If the command was anything else, it returns -1. <synopsis> int PgDatabase::CmdTuples() const </synopsis> </para> </listitem> <listitem> <para> <function>OidStatus</function> <synopsis> const char *PgDatabase::OidStatus() const </synopsis> </para> </listitem> </itemizedlist> </sect2> </sect1> <sect1 id="libpqpp-notify"> <title>Asynchronous Notification</title> <para> <productname>PostgreSQL</productname> supports asynchronous notification via the <command>LISTEN</command> and <command>NOTIFY</command> commands. A backend registers its interest in a particular notification condition with the <command>LISTEN</command> command. All backends that are listening on a particular condition 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 a relation. </para> <para> <application>libpq++</application> 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 <application>libpq++</application> application must poll the backend to see if there is any pending notification information. After the execution of a command, 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 returns <symbol>NULL</symbol> if there are 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>PostgreSQL</productname> has options to read from or write to the network connection used by <application>libpq++</application>. 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()</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()</function>, however, in that it converts the terminating newline into a zero byte. </para> <para> <function>PgDatabase::GetLine</function> returns <symbol>EOF</symbol> 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 backslash followed by a period (<literal>\.</>), 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 the characters <literal>\.</literal> to indicate to the backend that it has finished sending its data. </para> </listitem> <listitem> <para> <function>PgDatabase::EndCopy</function> synchronizes 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 <quote>out of sync</quote> with the frontend. Upon return from this function, the backend is ready to receive the next command. </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 double precision)"); 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>