libpq.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/libpq.sgml,v 1.108 2003/02/13 05:37:43 momjian Exp $
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 <chapter id="libpq">
  <title><application>libpq</application> - C Library</title>

  <indexterm zone="libpq">
   <primary>libpq</primary>
  </indexterm>

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

  <para>
   <application>libpq</application> is the <acronym>C</acronym>
   application programmer's interface to
   <productname>PostgreSQL</productname>.  <application>libpq</application> is a set
   of library routines that allow client programs to pass queries to the
   <productname>PostgreSQL</productname> backend server and to receive the
   results of these queries.  <application>libpq</application> is also the
   underlying engine for several other <productname>PostgreSQL</productname>
   application interfaces, including <application>libpq++</application> (C++),
   <filename>libpgtcl</filename> (Tcl), <productname>Perl</productname>, and
   <filename>ecpg</filename>.  So some aspects of <application>libpq</>'s behavior will be
   important to you if you use one of those packages.
  </para>

  <para>
   Three short programs are included at the end of this section to show how
   to write programs that use <filename>libpq</filename>.  There are several
   complete examples of <filename>libpq</filename> applications in the
   following directories:

   <simplelist>
    <member><filename>src/test/examples</filename></member>
    <member><filename>src/bin/psql</filename></member>
   </simplelist>
  </para>

  <para>
   Frontend programs that use <filename>libpq</filename> must include the
   header file <filename>libpq-fe.h</filename> and must link with the
   <filename>libpq</filename> library.
  </para>
 </sect1>

 <sect1 id="libpq-connect">
  <title>Database Connection Functions</title>

  <para>
   The following routines deal with making a connection to a
   <productname>PostgreSQL</productname> backend server.  The
   application program can have several backend connections open at
   one time.  (One reason to do that is to access more than one
   database.)  Each connection is represented by a
   <structname>PGconn</> object which is obtained from
   <function>PQconnectdb</> or <function>PQsetdbLogin</>.  Note that
   these functions will always return a non-null object pointer,
   unless perhaps there is too little memory even to allocate the
   <structname>PGconn</> object.  The <function>PQstatus</> function
   should be called to check whether a connection was successfully
   made before queries are sent via the connection object.

    <itemizedlist>
     <listitem>
      <para>
       <function>PQconnectdb</function> 
       Makes a new connection to the database server.
       <synopsis>
PGconn *PQconnectdb(const char *conninfo)
       </synopsis>

   This routine opens a new database connection using the parameters taken
   from the string <literal>conninfo</literal>.  Unlike <function>PQsetdbLogin</> below,
   the parameter set can be extended without changing the function signature,
   so use either of this routine or the nonblocking analogues <function>PQconnectStart</>
   and <function>PQconnectPoll</function> is preferred for application programming.  The passed string
   can be empty to use all default parameters, or it can contain one or more
   parameter settings separated by whitespace.
   </para>

   <para>
   Each parameter setting is in the form <literal>keyword = value</literal>.
   (To write an empty value or a value containing
   spaces, surround it with single quotes, e.g.,
   <literal>keyword = 'a value'</literal>.
   Single quotes and backslashes within the value must be escaped with a
   backslash, e.g., <literal>\'</literal> or <literal>\\</literal>.)
   Spaces around the equal sign are optional.  The currently recognized
   parameter keywords are:

   <variablelist>
    <varlistentry>
     <term><literal>host</literal></term>
     <listitem>
     <para>
      Name of host to connect to. 
      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.
      The default is to connect to a Unix-domain socket in
      <filename>/tmp</filename>.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>hostaddr</literal></term>
     <listitem>
     <para>
      IP address of host to connect to. This should be in standard
      IPv4 address format, e.g. <literal>172.28.40.9</>.  If your machine 
      supports IPv6, you can also use those addresses. If a nonzero-length 
      string is specified, TCP/IP communication is used.
     </para>
     <para>
      Using <literal>hostaddr</> instead of host allows the application to avoid a host
      name look-up, which may be important in applications with time
      constraints. However, Kerberos authentication requires the host
      name. The following therefore applies: If host is specified without
      <literal>hostaddr</>, a host name lookup is forced. If <literal>hostaddr</> is specified without
      host, the value for <literal>hostaddr</> gives the remote address; if Kerberos is
      used, this causes a reverse name query. If both host and <literal>hostaddr</> are
      specified, the value for <literal>hostaddr</> gives the remote address; the value
      for host is ignored, unless Kerberos is used, in which case that value
      is used for Kerberos authentication. Note that authentication is likely
      to fail if <application>libpq</application> is passed a host name that is not the name of the
      machine at <literal>hostaddr</>.
     </para>
     <para>
      Without either a host name or host address, <application>libpq</application> will connect using a
      local Unix domain socket.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>port</literal></term>
     <listitem>
     <para>
      Port number to connect to at the server host,
      or socket file name extension for Unix-domain connections.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>dbname</literal></term>
     <listitem>
     <para>
      The database name.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>user</literal></term> 
     <listitem>
     <para>
      User name to connect as.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>password</literal></term>
     <listitem>
     <para>
      Password to be used if the server demands password authentication.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>connect_timeout</literal></term>
     <listitem>
     <para>
      Time space in seconds given to connect routine. Zero or not set means infinite.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>options</literal></term>
     <listitem>
      <para>
       Trace/debug options to be sent to the server.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>tty</literal></term>
     <listitem>
     <para>
      A file or <acronym>tty</acronym> for optional debug output from the backend.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>requiressl</literal></term>
     <listitem>
     <para>
      Set to 1 to require <acronym>SSL</acronym> connection to the server.
      <application>Libpq</> will then refuse to connect if the server does not
      accept an <acronym>SSL</acronym> connection.
      Set to 0 (default) to negotiate with server.
      This option is only available if
      <productname>PostgreSQL</> is compiled with SSL support.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>service</literal></term>
     <listitem>
     <para>
      Service name to use for additional parameters.  It specifies a service
      name in pg_service.conf that holds additional connection parameters.
      This allows applications to specify only a service name so connection parameters 
      can be centrally maintained.  See <literal>pg_service.conf.sample</> for
      information on how to set up the file.
     </para>
     </listitem>
    </varlistentry>
   </variablelist>

   If  any  parameter is unspecified, then the corresponding
   environment variable (see <xref linkend="libpq-envars">)
   is checked. If the  environment  variable is not set either,
   then hardwired defaults are used.
   The return value is a pointer to an abstract <type>struct</type>
   representing the connection to the backend.
   </para>
  </listitem>

  <listitem>
   <para>
   <function>PQsetdbLogin</function>
       Makes a new connection to the database server.
<synopsis>
PGconn *PQsetdbLogin(const char *pghost,
                     const char *pgport,
                     const char *pgoptions,
                     const char *pgtty,
                     const char *dbName,
                     const char *login,
                     const char *pwd)
</synopsis>

   This is the predecessor of <function>PQconnectdb</function> with a fixed number
   of parameters but the same functionality.   
   </para>
  </listitem>

  <listitem>
   <para>
   <function>PQsetdb</function> Makes a new connection to the database server.
<synopsis>
PGconn *PQsetdb(char *pghost,
                char *pgport,
                char *pgoptions,
                char *pgtty,
                char *dbName)
</synopsis>
   This is a macro that calls <function>PQsetdbLogin</function> with null pointers
   for the <parameter>login</> and <parameter>pwd</> parameters.  It is provided primarily
   for backward compatibility with old programs.
   </para>
  </listitem>

 <listitem>
  <para>
   <function>PQconnectStart</function>,
   <function>PQconnectPoll</function>
   <indexterm><primary>nonblocking connection</primary></indexterm>
   Make a connection to the database server in a nonblocking manner.
<synopsis>
PGconn *PQconnectStart(const char *conninfo)
</synopsis>
<synopsis>
PostgresPollingStatusType PQconnectPoll(PGconn *conn)
</synopsis>
   These two routines are used to open a connection to a database server such
   that your application's thread of execution is not blocked on remote I/O
   whilst doing so.
  </para>
  <para>
   The database connection is made using the parameters taken from the string
   <literal>conninfo</literal>, passed to <function>PQconnectStart</function>. This string is in
   the same format as described above for <function>PQconnectdb</function>.
  </para>
  <para>
   Neither <function>PQconnectStart</function> nor <function>PQconnectPoll</function> will block, as long as a number of
   restrictions are met:
   <itemizedlist>
    <listitem>
     <para>
      The <literal>hostaddr</> and <literal>host</> parameters are used appropriately to ensure that
      name and reverse name queries are not made. See the documentation of
      these parameters under <function>PQconnectdb</function> above for details.
     </para>
    </listitem>

    <listitem>
     <para>
      If you call <function>PQtrace</function>, ensure that the stream object into which you trace
      will not block.
     </para>
    </listitem>

    <listitem>
     <para>
      You ensure for yourself that the socket is in the appropriate state
      before calling <function>PQconnectPoll</function>, as described below.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   To begin, call <literal>conn=PQconnectStart("<replaceable>connection_info_string</>")</literal>.
   If <varname>conn</varname> is NULL, then <application>libpq</> has been unable to allocate a new <structname>PGconn</>
   structure. Otherwise, a valid <structname>PGconn</> pointer is returned (though not yet
   representing a valid connection to the database). On return from
   <function>PQconnectStart</function>, call <literal>status=PQstatus(conn)</literal>. If status equals
   <symbol>CONNECTION_BAD</symbol>, <function>PQconnectStart</function> has failed.
  </para>
  <para>
   If <function>PQconnectStart</> succeeds, the next stage is to poll <application>libpq</> so that it may
   proceed with the connection sequence.  Loop thus: Consider a connection
   <quote>inactive</quote> by default. If <function>PQconnectPoll</function> last returned <symbol>PGRES_POLLING_ACTIVE</>,
   consider it <quote>active</quote> instead. If <function>PQconnectPoll(conn)</function> last returned
   <symbol>PGRES_POLLING_READING</symbol>, perform a <function>select()</> for reading on <function>PQsocket(conn)</function>. If
   it last returned <symbol>PGRES_POLLING_WRITING</symbol>, perform a <function>select()</> for writing on
   <function>PQsocket(conn)</function>. If you have yet to call <function>PQconnectPoll</function>, i.e. after the call
   to <function>PQconnectStart</function>, behave as if it last returned <symbol>PGRES_POLLING_WRITING</symbol>.  If
   the <function>select()</> shows that the socket is ready, consider it <quote>active</quote>. If it has
   been decided that this connection is <quote>active</quote>, call <function>PQconnectPoll(conn)</function>
   again. If this call returns <symbol>PGRES_POLLING_FAILED</symbol>, the connection procedure
   has failed.  If this call returns <symbol>PGRES_POLLING_OK</symbol>, the connection has been
   successfully made.
  </para>

  <para>
    Note that the use of <function>select()</function> to ensure that the socket is ready is merely
    a (likely) example; those with other facilities available, such as a
    <function>poll()</function> call, may of course use that instead.
  </para>

  <para>
    At any time during connection, the status of the connection may be
    checked, by calling <function>PQstatus</>. If this is <symbol>CONNECTION_BAD</>, then the
    connection procedure has failed; if this is <function>CONNECTION_OK</>, then the
    connection is ready.  Either of these states should be equally detectable
    from the return value of <function>PQconnectPoll</>, as above. Other states may be
    shown during (and only during) an asynchronous connection procedure. These
    indicate the current stage of the connection procedure, and may be useful
    to provide feedback to the user for example. These statuses may include:

    <variablelist>
     <varlistentry>
      <term><symbol>CONNECTION_STARTED</symbol></term>
      <listitem>
       <para>
        Waiting for connection to be made.
       </para>
      </listitem>
     </varlistentry> 

     <varlistentry>
      <term><symbol>CONNECTION_MADE</symbol></term>
      <listitem>
       <para>
        Connection OK; waiting to send.
       </para>
      </listitem>
     </varlistentry>  

     <varlistentry>
      <term><symbol>CONNECTION_AWAITING_RESPONSE</symbol></term>
      <listitem>
       <para>
        Waiting for a response from the server.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><symbol>CONNECTION_AUTH_OK</symbol></term>
      <listitem>
       <para>
        Received authentication; waiting for connection start-up to continue.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><symbol>CONNECTION_SETENV</symbol></term>

      <listitem>
       <para>
        Negotiating environment (part of the connection start-up).
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    Note that, although these constants will remain (in order to maintain
    compatibility), an application should never rely upon these appearing in a
    particular order, or at all, or on the status always being one of these
    documented values. An application may do something like this:
<programlisting>
switch(PQstatus(conn))
{
    case CONNECTION_STARTED:
        feedback = "Connecting...";
        break;

    case CONNECTION_MADE:
        feedback = "Connected to server...";
        break;
.
.
.
    default:
        feedback = "Connecting...";
}
</programlisting>
  </para>

  <para>
   Note that if <function>PQconnectStart</function> returns a non-NULL pointer, you must call
   <function>PQfinish</function> when you are finished with it, in order to dispose of
   the structure and any associated memory blocks. This must be done even if a
   call to <function>PQconnectStart</function> or <function>PQconnectPoll</function> failed.
  </para>

  <para>
   <function>PQconnectPoll</function> will currently block if
   <application>libpq</> is compiled with <symbol>USE_SSL</symbol>
   defined. This restriction may be removed in the future.
  </para>

  <para>
   These functions leave the socket in a nonblocking state as if 
   <function>PQsetnonblocking</function> had been called.
  </para>
 </listitem>

  <listitem>
   <para>
   <function>PQconndefaults</function> Returns the default connection options.
<synopsis>
PQconninfoOption *PQconndefaults(void)

struct PQconninfoOption
{
    char   *keyword;   /* The keyword of the option */
    char   *envvar;    /* Fallback environment variable name */
    char   *compiled;  /* Fallback compiled in default value */
    char   *val;       /* Option's current value, or NULL */
    char   *label;     /* Label for field in connect dialog */
    char   *dispchar;  /* Character to display for this field
                          in a connect dialog. Values are:
                          ""        Display entered value as is
                          "*"       Password field - hide value
                          "D"       Debug option - don't show by default */
    int     dispsize;  /* Field size in characters for dialog */
}
</synopsis>
   Returns a connection options array.  This may
   be used to determine all possible <function>PQconnectdb</function> options and their
   current default values.  The return value points to an array of
   <structname>PQconninfoOption</structname> <type>struct</>s, which ends with an entry having a NULL
   keyword pointer.  Note that the default values (<structfield>val</structfield> fields)
   will depend on environment variables and other context.
   Callers must treat the connection options data as read-only.
   </para>

   <para>
    After processing the options array, free it by passing it to
    <function>PQconninfoFree</function>.  If this is not done, a small amount of memory
    is leaked for each call to <function>PQconndefaults</function>.
   </para>

   <para>
    In <productname>PostgreSQL</productname> versions before 7.0, <function>PQconndefaults</function> returned a pointer
    to a static array, rather than a dynamically allocated array.  That
    was not thread-safe, so the behavior has been changed.
   </para>
  </listitem>

  <listitem>
   <para>
   <function>PQfinish</function>
   Close  the  connection to the backend.  Also frees
   memory used by the <structname>PGconn</structname> object.
<synopsis>
void PQfinish(PGconn *conn)
</synopsis>
   Note that even if the backend connection attempt fails (as
   indicated by <function>PQstatus</function>), the application should call <function>PQfinish</function>
   to free the memory used by the <structname>PGconn</structname> object.
   The <structname>PGconn</> pointer should not be used after <function>PQfinish</function> has been called.
   </para>
  </listitem>

  <listitem>
   <para>
   <function>PQreset</function>
   Reset the communication  port  with  the  backend.
<synopsis>
void PQreset(PGconn *conn)
</synopsis>
   This function will close the connection
   to the backend and attempt to  reestablish  a  new
   connection to the same server, using all the same
   parameters previously used.  This may be useful for
   error recovery if a working connection is lost.
   </para>
  </listitem>

  <listitem>
   <para>
   <function>PQresetStart</function>
   <function>PQresetPoll</function>
   Reset the communication  port  with  the  backend, in a nonblocking manner.
<synopsis>
int PQresetStart(PGconn *conn);
</synopsis>
<synopsis>
PostgresPollingStatusType PQresetPoll(PGconn *conn);
</synopsis>
    These functions will close the connection to the backend and attempt to
    reestablish a new connection to the same server, using all the same
    parameters previously used. This may be useful for error recovery if a
    working connection is lost. They differ from <function>PQreset</function> (above) in that they
    act in a nonblocking manner. These functions suffer from the same
    restrictions as <function>PQconnectStart</> and <function>PQconnectPoll</>.
   </para>
   <para>
    Call <function>PQresetStart</function>. If it returns 0, the reset has failed. If it returns 1,
    poll the reset using <function>PQresetPoll</function> in exactly the same way as you would
    create the connection using <function>PQconnectPoll</function>.
   </para>
  </listitem>

 </itemizedlist>
</para>

<para>
<indexterm><primary>libpq-fe.h</></>
<indexterm><primary>libpq-int.h</></>
<application>libpq</application> application programmers should be careful to
maintain the <structname>PGconn</structname> abstraction.  Use the accessor functions below to get
at the contents of <structname>PGconn</structname>.  Avoid directly referencing the fields of the
<structname>PGconn</> structure because they are subject to change in the future.
(Beginning in <productname>PostgreSQL</productname> release 6.4, the
definition of <type>struct PGconn</> is not even provided in <filename>libpq-fe.h</filename>.
If you have old code that accesses <structname>PGconn</structname> fields directly, you can keep using it
by including <filename>libpq-int.h</filename> too, but you are encouraged to fix the code
soon.)

<itemizedlist>
<listitem>
<para>
<function>PQdb</function>  
         Returns the database name of the connection.
<synopsis>
char *PQdb(const PGconn *conn)
</synopsis>
<function>PQdb</> and the next several functions return the values established
at connection.  These values are fixed for the life of the <structname>PGconn</>
object.
</para>
</listitem>

<listitem>
<para>
<function>PQuser</function>
         Returns the user name of the connection.
<synopsis>
char *PQuser(const PGconn *conn)
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQpass</function>
         Returns the password of the connection.
<synopsis>
char *PQpass(const PGconn *conn)
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQhost</function>
         Returns the server host name of the connection.
<synopsis>
char *PQhost(const PGconn *conn)
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQport</function>
         Returns the port of the connection.
<synopsis>
char *PQport(const PGconn *conn)
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQtty</function>
         Returns the debug <acronym>tty</acronym> of the connection.
<synopsis>
char *PQtty(const PGconn *conn)
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQoptions</function>
       Returns the backend options used in  the  connection.
<synopsis>
char *PQoptions(const PGconn *conn)
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQstatus</function>
         Returns the status of the connection. 
<synopsis>
ConnStatusType PQstatus(const PGconn *conn)
</synopsis>
</para>

      <para>
       The status can be one of a number of values.
       However, only two of these are
       seen outside of an asynchronous connection procedure -
       <literal>CONNECTION_OK</literal> or
       <literal>CONNECTION_BAD</literal>. A good
       connection to the database has the status <literal>CONNECTION_OK</literal>.
       A failed connection
       attempt is signaled by status
       <literal>CONNECTION_BAD</literal>.
       Ordinarily, an OK status will remain so until
       <function>PQfinish</function>, but a
       communications failure might result in the status changing to
       <literal>CONNECTION_BAD</literal> prematurely.
       In that case the application
       could try to recover by calling <function>PQreset</function>.
      </para>

      <para>
       See the entry for <function>PQconnectStart</> and <function>PQconnectPoll</> with regards
       to other status codes
       that might be seen.
      </para>
     </listitem>

     <listitem>
      <para>
       <function>PQerrorMessage</function>
       <indexterm><primary>error message</></>
       Returns the error message most recently generated by
       an operation on the connection.
       <synopsis>
char *PQerrorMessage(const PGconn* conn);
       </synopsis>
      </para>

      <para>
       Nearly all <application>libpq</> functions will set
       <function>PQerrorMessage</function> if they fail.
       Note that by <application>libpq</application> convention, a non-empty
       <function>PQerrorMessage</function> will
       include a trailing newline.
      </para>
     </listitem>

     <listitem>
      <para>
       <function>PQbackendPID</function>
       Returns the process <acronym>ID</acronym> of the backend server 
      handling this connection.
       <synopsis>
int PQbackendPID(const PGconn *conn);
       </synopsis>
       The backend <acronym>PID</acronym> is useful for debugging
       purposes and for comparison
       to NOTIFY messages (which include the <acronym>PID</acronym> of
       the notifying backend).
       Note that the <acronym>PID</acronym> belongs to a process
       executing on the database
       server host, not the local host!
      </para>
     </listitem>

     <listitem>
      <para>
       <function>PQgetssl</function>
       <indexterm><primary>SSL</></>
       Returns the SSL structure used in the connection, or NULL
       if SSL is not in use. 
       <synopsis>
SSL *PQgetssl(const PGconn *conn);
       </synopsis>
       This structure can be used to verify encryption levels, check
       server certificate and more. Refer to the SSL documentation
       for information about this structure.
      </para>
      <para>
       You must define <literal>USE_SSL</literal> in order to get the
       prototype for this function. Doing this will also 
       automatically include <filename>ssl.h</filename> from <productname>OpenSSL</productname>.
      </para>
     </listitem>

    </itemizedlist>
   </para>
  </sect1>

<sect1 id="libpq-exec">
<title>Command Execution Functions</title>

<para>
Once a connection to a database server has been successfully
established, the functions described here are used to perform
SQL queries and commands.
</para>

<sect2 id="libpq-exec-main">
  <title>Main Routines</title>
<itemizedlist>
<listitem>
<para>
<function>PQexec</function>
          Submit a command to the server
          and wait for the result.
<synopsis>
PGresult *PQexec(PGconn *conn,
                 const char *query);
</synopsis>
          Returns a <structname>PGresult</structname> pointer or possibly a NULL pointer.
          A non-NULL pointer will generally be returned except in
          out-of-memory conditions or serious errors such as inability
          to send the command to the backend.
          If a NULL is returned, it
	  should be treated like a <symbol>PGRES_FATAL_ERROR</symbol> result.  Use
	  <function>PQerrorMessage</function> to get more information about the error.
</para>
</listitem>
</itemizedlist>

<para>
The <function>PGresult</function> structure encapsulates the result
returned by the backend.
<filename>libpq</filename> application programmers should be careful to
maintain the <structname>PGresult</structname> abstraction.  Use the accessor functions below to get
at the contents of <structname>PGresult</structname>.  Avoid directly referencing the fields of the
<structname>PGresult</structname> structure because they are subject to change in the future.
(Beginning in <productname>PostgreSQL</productname> 6.4, the
definition of <type>struct PGresult</> is not even provided in <filename>libpq-fe.h</>.  If you
have old code that accesses <structname>PGresult</structname> fields directly, you can keep using it
by including <filename>libpq-int.h</filename> too, but you are encouraged to fix the code
soon.)
</para>

<itemizedlist>
<listitem>
<para>
<function>PQresultStatus</function>
          Returns the result status of the command.
<synopsis>
ExecStatusType PQresultStatus(const PGresult *res)
</synopsis>
<function>PQresultStatus</function> can return one of the following values:

<itemizedlist>
 <listitem>
  <para><literal>PGRES_EMPTY_QUERY</literal> -- The string sent to the backend was empty.</para>
 </listitem>
 <listitem>
  <para><literal>PGRES_COMMAND_OK</literal> -- Successful completion of a command returning no data</para>
 </listitem>
 <listitem>
  <para><literal>PGRES_TUPLES_OK</literal> -- The query successfully executed</para>
 </listitem>
 <listitem>
  <para><literal>PGRES_COPY_OUT</literal> -- Copy Out (from server) data transfer started</para>
 </listitem>
 <listitem>
  <para><literal>PGRES_COPY_IN</literal> -- Copy In (to server) data transfer started</para>
 </listitem>
 <listitem>
  <para><literal>PGRES_BAD_RESPONSE</literal> -- The server's response was not understood</para>
 </listitem>
 <listitem>
  <para><literal>PGRES_NONFATAL_ERROR</literal></para>
 </listitem>
 <listitem>
  <para><literal>PGRES_FATAL_ERROR</literal></para>
 </listitem>
</itemizedlist>

If  the result status is <literal>PGRES_TUPLES_OK</literal>, then the
routines described below can be  used  to  retrieve  the
rows returned by the query.  Note that a SELECT command that
happens to retrieve zero rows still shows <literal>PGRES_TUPLES_OK</literal>.
<literal>PGRES_COMMAND_OK</literal> is for commands that can never return rows
(INSERT, UPDATE, etc.). A response of <literal>PGRES_EMPTY_QUERY</literal> often
exposes a bug in the client software.
</para>
</listitem>

<listitem>
<para>
<function>PQresStatus</function>
	Converts the enumerated type returned by <function>PQresultStatus</> into
	a string constant describing the status code.
<synopsis>
char *PQresStatus(ExecStatusType status);
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQresultErrorMessage</function>
returns the error message associated with the query, or an empty string
if there was no error.
<synopsis>
char *PQresultErrorMessage(const PGresult *res);
</synopsis>
Immediately following a <function>PQexec</function> or <function>PQgetResult</function>
call, <function>PQerrorMessage</function> (on the connection) will return the same
string as <function>PQresultErrorMessage</function> (on the result).  However, a
<structname>PGresult</structname> will retain its error message
until destroyed, whereas the connection's error message will change when
subsequent operations are done.  Use <function>PQresultErrorMessage</function> when you want to
know the status associated with a particular <structname>PGresult</structname>; use <function>PQerrorMessage</function>
when you want to know the status from the latest operation on the connection.
</para>
</listitem>

<listitem>
<para>
<function>PQclear</function>
          Frees  the  storage  associated with the <structname>PGresult</structname>.
          Every query result should be freed via <function>PQclear</function> when
          it  is  no  longer needed.
<synopsis>
void PQclear(PQresult *res);
</synopsis>
          You can keep a <structname>PGresult</structname> object around for as long as you
          need it; it does not go away when you issue a new query,
          nor even if you close the connection.  To get rid of it,
          you must call <function>PQclear</function>.  Failure to do this will
          result in memory leaks in  the  frontend  application.
</para>
</listitem>

<listitem>
<para>
<function>PQmakeEmptyPGresult</function>
          Constructs an empty <structname>PGresult</structname> object with the given status.
<synopsis>
PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
</synopsis>
This is <application>libpq</>'s internal routine to allocate and initialize an empty
<structname>PGresult</structname> object.  It is exported because some applications find it
useful to generate result objects (particularly objects with error
status) themselves.  If <parameter>conn</parameter> is not NULL and status indicates an error,
the connection's current error message is copied into the <structname>PGresult.</structname>
Note that <function>PQclear</function> should eventually be called on the object, just
as with a <structname>PGresult</structname> returned by <application>libpq</application> itself.
</para>
</listitem>
</itemizedlist>
</sect2>

<sect2 id="libpq-exec-escape-string">
  <title>Escaping strings for inclusion in SQL queries</title>

   <indexterm zone="libpq-exec-escape-string"><primary>escaping strings</></>

<para>
<function>PQescapeString</function>
          Escapes a string for use within an SQL query.
<synopsis>
size_t PQescapeString (char *to, const char *from, size_t length);
</synopsis>
If you want to include strings that have been received
from a source that is not trustworthy (for example, because a random user
entered them), you cannot directly include them in SQL
queries for security reasons.  Instead, you have to quote special
characters that are otherwise interpreted by the SQL parser.
</para>
<para>
<function>PQescapeString</> performs this operation.  The
<parameter>from</> points to the first character of the string that
is to be escaped, and the <parameter>length</> parameter counts the
number of characters in this string (a terminating zero byte is
neither necessary nor counted).  <parameter>to</> shall point to a
buffer that is able to hold at least one more character than twice
the value of <parameter>length</>, otherwise the behavior is
undefined.  A call to <function>PQescapeString</> writes an escaped
version of the <parameter>from</> string to the <parameter>to</>
buffer, replacing special characters so that they cannot cause any
harm, and adding a terminating zero byte.  The single quotes that
must surround <productname>PostgreSQL</> string literals are not part of the result
string.
</para>
<para>
<function>PQescapeString</> returns the number of characters written
to <parameter>to</>, not including the terminating zero byte.
Behavior is undefined when the <parameter>to</> and <parameter>from</>
strings overlap.
</para>
</sect2>


 <sect2 id="libpq-exec-escape-bytea">
  <title>Escaping binary strings for inclusion in SQL queries</title>
  <indexterm zone="libpq-exec-escape-bytea">
   <primary>escaping binary strings</primary>
  </indexterm>
  <para>
   <function>PQescapeBytea</function>
   Escapes a binary string (<type>bytea</type> type) for use within an SQL query.
   <synopsis>
    unsigned char *PQescapeBytea(const unsigned char *from,
                                         size_t from_length,
                                         size_t *to_length);
   </synopsis>

   Certain <acronym>ASCII</acronym> characters <emphasis>must</emphasis>
   be escaped (but all characters <emphasis>may</emphasis> be escaped)
   when used as part of a <type>bytea</type>
   string literal in an <acronym>SQL</acronym> statement. In general, to
   escape a character, it is converted into the three digit octal number
   equal to the decimal <acronym>ASCII</acronym> value, and preceded by
   two backslashes. The single quote (') and backslash (\) characters have
   special alternate escape sequences. See the &cite-user;
   for more information. <function>PQescapeBytea
   </function> performs this operation, escaping only the minimally
   required characters.
  </para>

  <para>
   The <parameter>from</parameter> parameter points to the first
   character of the string that is to be escaped, and the
   <parameter>from_length</parameter> parameter reflects the number of
   characters in this binary string (a terminating zero byte is
   neither necessary nor counted).  The <parameter>to_length</parameter>
   parameter shall point to a buffer suitable to hold the resultant
   escaped string length. The result string length includes the terminating
   zero byte of the result.
  </para>

  <para>
   <function>PQescapeBytea</> returns an escaped version of the
   <parameter>from</parameter> parameter binary string, to a
   caller-provided buffer. The return string has all special
   characters replaced so that they can be properly processed by the
   <productname>PostgreSQL</> string literal parser, and the
   <type>bytea</type> input function. A terminating zero byte is also
   added.  The single quotes that must surround
   <productname>PostgreSQL</> string literals are not part of the
   result string.
  </para>

  <para>
   <function>PQunescapeBytea</function>
   Converts an escaped string representation of binary data into binary
   data - the reverse of <function>PQescapeBytea</function>.
   <synopsis>
    unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
   </synopsis>

   The <parameter>from</parameter> parameter points to an escaped string
   such as might be returned by <function>PQgetvalue</function> of a
   <type>BYTEA</type> column. <function>PQunescapeBytea</function> converts
   this string representation into its binary representation, filling the supplied buffer.
   It returns a pointer to the buffer which is NULL on error, and the size
   of the buffer in <parameter>to_length</parameter>. The pointer may
   subsequently be used as an argument to the function
   <function>free(3)</function>.
  </para>
   
 </sect2>


<sect2 id="libpq-exec-select-info">
  <title>Retrieving SELECT Result Information</title>

<itemizedlist>
<listitem>
<para>
<function>PQntuples</function>
          Returns the number of tuples (rows)
          in the query result.
<synopsis>
int PQntuples(const PGresult *res);
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQnfields</function>
          Returns   the   number    of    fields
          (columns) in each row of the query result.
<synopsis>
int PQnfields(const PGresult *res);
</synopsis>
</para>
</listitem>


<listitem>
<para>
<function>PQfname</function>
 Returns the field (column) name associated with the given field index.
 Field  indices start at 0.
<synopsis>
char *PQfname(const PGresult *res,
                    int field_index);
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQfnumber</function>
            Returns  the  field  (column)  index
          associated with the given field name.
<synopsis>
int PQfnumber(const PGresult *res,
              const char *field_name);
</synopsis>
</para>

<para>
        -1 is returned if the given name does not match any field.
</para>
</listitem>

<listitem>
<para>
<function>PQftype</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 PQftype(const PGresult *res,
            int field_index);
</synopsis>
You can query the system table <literal>pg_type</literal> to obtain
the name and properties of the various data types. The <acronym>OID</acronym>s
of the built-in data types are defined in <filename>src/include/catalog/pg_type.h</filename>
in the source tree.
</para>
</listitem>

<listitem>
<para>
<function>PQfmod</function>
          Returns  the type-specific modification data of the field
          associated with the given field index.
          Field indices start at 0.
<synopsis>
int PQfmod(const PGresult *res,
           int field_index);
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQfsize</function>
          Returns  the  size  in bytes of the field
          associated with the given field index.
          Field indices start at 0.
<synopsis>
int PQfsize(const PGresult *res,
            int field_index);
</synopsis>
	<function>PQfsize</> returns the space allocated for this field in a database
	tuple, 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>PQbinaryTuples</function>
          Returns 1 if the <structname>PGresult</> contains binary tuple data,
	  0 if it contains ASCII data.
<synopsis>
int PQbinaryTuples(const PGresult *res);
</synopsis>
Currently, binary tuple data can only be returned by a query that
extracts data from a binary cursor.
</para>
</listitem>
</itemizedlist>
</sect2>

<sect2 id="libpq-exec-select-values">
  <title>Retrieving SELECT Result Values</title>

<itemizedlist>
<listitem>
<para>
<function>PQgetvalue</function>
            Returns a single field  (column)  value of one tuple (row)
	    of a <structname>PGresult</structname>.
	    Tuple and field indices start at 0.
<synopsis>
char* PQgetvalue(const PGresult *res,
                 int tup_num,
                 int field_num);
</synopsis>
For most queries, the value returned by <function>PQgetvalue</function>
is a null-terminated character string  representation
of the attribute value.  But if <function>PQbinaryTuples()</function> is 1,
the  value  returned  by <function>PQgetvalue</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>PQgetvalue</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.
</para>
</listitem>

<listitem>
<para>
<function>PQgetisnull</function>
           Tests a field for a NULL entry.
           Tuple and field indices start at 0.
<synopsis>
int PQgetisnull(const PGresult *res,
                int tup_num,
                int field_num);
</synopsis>
            This function returns  1 if the field contains a NULL, 0 if
            it contains a non-null value.  (Note that <function>PQgetvalue</function>
            will return an empty string, not a null pointer, for a NULL
            field.)
</para>
</listitem>

<listitem>
<para>
<function>PQgetlength</function>
          Returns   the   length  of  a  field (attribute) value in bytes.
          Tuple and field indices start at 0.
<synopsis>
int PQgetlength(const PGresult *res,
                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 <function>PQgetvalue</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>PQprint</function>
          Prints out all the  tuples  and,  optionally,  the
          attribute  names  to  the specified output stream.
       <synopsis>
void PQprint(FILE* fout,      /* output stream */
             const PGresult *res,
             const PQprintOpt *po);

struct {
    pqbool  header;      /* print output field headings and row count */
    pqbool  align;       /* fill align the fields */
    pqbool  standard;    /* old brain dead format */
    pqbool  html3;       /* output html tables */
    pqbool  expanded;    /* expand tables */
    pqbool  pager;       /* use pager for output if needed */
    char    *fieldSep;   /* field separator */
    char    *tableOpt;   /* insert to HTML <replaceable>table ...</replaceable> */
    char    *caption;    /* HTML <replaceable>caption</replaceable> */
    char    **fieldName; /* null terminated array of replacement field names */
} PQprintOpt;
       </synopsis>
This function was formerly used by <application>psql</application>
to print query results, but this is no longer the case and this
function is no longer actively supported.
</para>
</listitem>
</itemizedlist>
</sect2>

<sect2 id="libpq-exec-nonselect">
  <title>Retrieving Non-SELECT Result Information</title>

<itemizedlist>
<listitem>
<para>
<function>PQcmdStatus</function>
          Returns the command status string from the SQL command that
	  generated the <structname>PGresult</structname>.
<synopsis>
char * PQcmdStatus(PGresult *res);
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQcmdTuples</function>
	  Returns the number of rows affected by the SQL command.
<synopsis>
char * PQcmdTuples(PGresult *res);
</synopsis>
          If the <acronym>SQL</acronym> command that generated the
	  <structname>PGresult</structname> was INSERT, UPDATE or DELETE, this returns a
	  string containing the number of rows affected.  If the
          command was anything else, it returns the empty string.
</para>
</listitem>

<listitem>
<para>
<function>PQoidValue</function>
          Returns the object ID of the inserted row, if the
	  <acronym>SQL</acronym> command was an INSERT
	  that inserted exactly one row into a table that has OIDs.
          Otherwise, returns <literal>InvalidOid</literal>.
<synopsis>
Oid PQoidValue(const PGresult *res);
</synopsis>
          The type <type>Oid</type> and the constant <literal>InvalidOid</literal>
          will be defined if you include the <application>libpq</application>
          header file. They will both be some integer type.
</para>
</listitem>

<listitem>
<para>
<function>PQoidStatus</function>
          Returns a string with the object ID of the inserted row, if the
	  <acronym>SQL</acronym> command was an INSERT.
	  (The string will be <literal>0</> if the INSERT did not insert exactly one
	  row, or if the target table does not have OIDs.)  If the command
	  was not an INSERT, returns an empty string.
<synopsis>
char * PQoidStatus(const PGresult *res);
</synopsis>
This function is deprecated in favor of <function>PQoidValue</function>
and is not thread-safe.
</para>
</listitem>
</itemizedlist>

</sect2>
</sect1>

<sect1 id="libpq-async">
<title>Asynchronous Query Processing</title>

  <indexterm zone="libpq-async"><primary>nonblocking connection</></>

<para>
The <function>PQexec</function> function is adequate for submitting commands in
simple synchronous
applications.  It has a couple of major deficiencies however:

<itemizedlist>
<listitem>
<para>
<function>PQexec</function> waits for the command to be completed.  The application may have other
work to do (such as maintaining a user interface), in which case it won't
want to block waiting for the response.
</para>
</listitem>
<listitem>
<para>
Since control is buried inside <function>PQexec</function>, it is hard for the frontend
to decide it would like to try to cancel the ongoing command.  (It can be
done from a signal handler, but not otherwise.)
</para>
</listitem>
<listitem>
<para>
<function>PQexec</function> can return only one <structname>PGresult</structname> structure.  If the submitted command
string contains multiple <acronym>SQL</acronym> commands, all but the last <structname>PGresult</structname> are
discarded by <function>PQexec</function>.
</para>
</listitem>
</itemizedlist>
</para>

<para>
Applications that do not like these limitations can instead use the
underlying functions that <function>PQexec</function> is built from:
<function>PQsendQuery</function> and <function>PQgetResult</function>.
</para>
<para>
Older programs that used this functionality as well as 
<function>PQputline</function> and <function>PQputnbytes</function>
could block waiting to send data to the backend. To
address that issue, the function <function>PQsetnonblocking</function>
was added.
</para>
<para>
Old applications can neglect to use <function>PQsetnonblocking</function>
and get the older potentially blocking behavior.  Newer programs can use 
<function>PQsetnonblocking</function> to achieve a completely nonblocking
connection to the backend.

<itemizedlist>
 <listitem>
   <para>
    <function>PQsetnonblocking</function> Sets the nonblocking status of the
    connection.
<synopsis>
int PQsetnonblocking(PGconn *conn, int arg)
</synopsis>
    Sets the state of the connection to nonblocking if <parameter>arg</parameter> is 1,
    blocking if <parameter>arg</parameter> is 0.  Returns 0 if OK, -1 if error.
   </para>
   <para>
    In the nonblocking state, calls to
    <function>PQputline</function>, <function>PQputnbytes</function>,
    <function>PQsendQuery</function> and <function>PQendcopy</function>
    will not block but instead return an error if they need to be called
    again.
   </para>
   <para>
    When a database connection has been set to nonblocking mode and
    <function>PQexec</function> is called, it will temporarily set the state
    of the connection to blocking until the <function>PQexec</function> 
    completes. 
   </para>
   <para>
    More of <application>libpq</application> is expected to be made safe for 
    <function>PQsetnonblocking</function> functionality in the near future.
  </para>
 </listitem>

<listitem>
<para>
<function>PQisnonblocking</function>
       Returns the blocking status of the database connection.
<synopsis>
int PQisnonblocking(const PGconn *conn)
</synopsis>
       Returns 1 if the connection is set to nonblocking mode,
       0 if blocking.
</para>
</listitem>

<listitem>
<para>
<function>PQsendQuery</function>
          Submit a command to the server without
	  waiting for the result(s).  1 is returned if the command was
	  successfully dispatched, 0 if not (in which case, use
	  <function>PQerrorMessage</> to get more information about the failure).
<synopsis>
int PQsendQuery(PGconn *conn,
                const char *query);
</synopsis>
	  After successfully calling <function>PQsendQuery</function>, call
          <function>PQgetResult</function> one or more
	  times to obtain the results.  <function>PQsendQuery</function> may not be called
	  again (on the same connection) until <function>PQgetResult</function> has returned NULL,
	  indicating that the command is done.
</para>
</listitem>

<listitem>
<para>
<function>PQgetResult</function>
          Wait for the next result from a prior <function>PQsendQuery</function>,
	  and return it.  NULL is returned when the query is complete
	  and there will be no more results.
<synopsis>
PGresult *PQgetResult(PGconn *conn);
</synopsis>
	  <function>PQgetResult</function> must be called repeatedly until it returns NULL,
	  indicating that the command is done.  (If called when no command is
	  active, <function>PQgetResult</function> will just return NULL at once.)
	  Each non-NULL result from <function>PQgetResult</function> should be processed using
	  the same <structname>PGresult</> accessor functions previously described.
	  Don't forget to free each result object with <function>PQclear</function> when done with it.
	  Note that <function>PQgetResult</function> will block only if a query is active and the
	  necessary response data has not yet been read by <function>PQconsumeInput</function>.
</para>
</listitem>

</itemizedlist>
</para>

<para>
Using <function>PQsendQuery</function> and <function>PQgetResult</function>
solves one of <function>PQexec</function>'s problems:
If a command string contains multiple <acronym>SQL</acronym> commands, the results of those
commands can be obtained individually.  (This allows a simple form of
overlapped processing, by the way: the frontend can be handling the
results of one query while the backend is still working on later
queries in the same command string.)  However, calling <function>PQgetResult</function> will
still cause the frontend to block until the backend completes the
next <acronym>SQL</acronym> command.  This can be avoided by proper use of three more
functions:

<itemizedlist>
<listitem>
<para>
<function>PQconsumeInput</function>
	  If input is available from the backend, consume it.
<synopsis>
int PQconsumeInput(PGconn *conn);
</synopsis>
<function>PQconsumeInput</function> normally returns 1 indicating <quote>no error</quote>,
but returns 0 if there was some kind of trouble (in which case
<function>PQerrorMessage</function> is set).  Note that the result does not say
whether any input data was actually collected. After calling
<function>PQconsumeInput</function>, the application may check
<function>PQisBusy</function> and/or <function>PQnotifies</function> to see if
their state has changed.
</para>
<para>
<function>PQconsumeInput</function> may be called even if the application is not
prepared to deal with a result or notification just yet.  The
routine will read available data and save it in a buffer, thereby
causing a <function>select()</function> read-ready indication to go away.  The
application can thus use <function>PQconsumeInput</function> to clear the
<function>select()</function> condition immediately, and then examine the results at leisure.
</para>
</listitem>

<listitem>
<para>
<function>PQisBusy</function>
Returns 1 if a query is busy, that is, <function>PQgetResult</function> would block
waiting for input.  A 0 return indicates that <function>PQgetResult</function> can
be called with assurance of not blocking.
<synopsis>
int PQisBusy(PGconn *conn);
</synopsis>
<function>PQisBusy</function> will not itself attempt to read data from the backend;
therefore <function>PQconsumeInput</function> must be invoked first, or the busy
state will never end.
</para>
</listitem>

<listitem>
<para>
<function>PQflush</function> Attempt to flush any data queued to the backend,
returns 0 if successful (or if the send queue is empty) or <symbol>EOF</symbol> if it failed for
some reason.
<synopsis>
int PQflush(PGconn *conn);
</synopsis>
<function>PQflush</function> needs to be called on a nonblocking connection 
before calling <function>select()</function> to determine if a response has
arrived.  If 0 is returned it ensures that there is no data queued to the 
backend that has not actually been sent.  Only applications that have used
<function>PQsetnonblocking</function> have a need for this.
</para>
</listitem>

<listitem>
<para>
<function>PQsocket</function>
	  Obtain the file descriptor number for the backend connection socket.
	  A valid descriptor will be &gt;= 0; a result of -1 indicates that
	  no backend connection is currently open.
<synopsis>
int PQsocket(const PGconn *conn);
</synopsis>
<function>PQsocket</function> should be used to obtain the backend socket descriptor
in preparation for executing <function>select()</function>.  This allows an
application using a blocking connection to wait for either backend responses or
other conditions.
If the result of <function>select()</function> indicates that data can be read from
the backend socket, then <function>PQconsumeInput</function> should be called to read the
data; after which, <function>PQisBusy</function>, <function>PQgetResult</function>,
and/or <function>PQnotifies</function> can be used to process the response.
</para>
<para>
Nonblocking connections (that have used <function>PQsetnonblocking</function>)
should not use <function>select()</function> until <function>PQflush</function>
has returned 0 indicating that there is no buffered data waiting to be sent
to the backend.
</para>
</listitem>

</itemizedlist>
</para>

<para>
A typical frontend using these functions will have a main loop that uses
<function>select</function> to wait for all the conditions that it must
respond to.  One of the conditions will be input available from the backend,
which in <function>select</function>'s terms is readable data on the file
descriptor identified by <function>PQsocket</function>.
When the main loop detects input ready, it should call
<function>PQconsumeInput</function> to read the input.  It can then call
<function>PQisBusy</function>, followed by <function>PQgetResult</function>
if <function>PQisBusy</function> returns false (0).  It can also call
<function>PQnotifies</function> to detect NOTIFY messages (see <xref linkend="libpq-notify">).
</para>

<para>
A frontend that uses <function>PQsendQuery</function>/<function>PQgetResult</function>
can also attempt to cancel a command that is still being processed by the backend.
</para>

<para>
<itemizedlist>
<listitem>
<para>
<function>PQrequestCancel</function>
	  Request that <productname>PostgreSQL</productname> abandon
	  processing of the current command.
<synopsis>
int PQrequestCancel(PGconn *conn);
</synopsis>
The return value is 1 if the cancel request was successfully
dispatched, 0 if not.  (If not, <function>PQerrorMessage</function> tells why not.)
Successful dispatch is no guarantee that the request will have any
effect, however.  Regardless of the return value of <function>PQrequestCancel</function>,
the application must continue with the normal result-reading
sequence using <function>PQgetResult</function>.  If the cancellation
is effective, the current command will terminate early and return
an error result.  If the cancellation fails (say, because the
backend was already done processing the command), then there will
be no visible result at all.
</para>
</listitem>
</itemizedlist>
</para>

<para>
Note that if the current command is part of a transaction, cancellation
will abort the whole transaction.
</para>

<para>
<function>PQrequestCancel</function> can safely be invoked from a signal handler.
So, it is also possible to use it in conjunction with plain
<function>PQexec</function>, if the decision to cancel can be made in a signal
handler.  For example, <application>psql</application> invokes
<function>PQrequestCancel</function> from a <systemitem>SIGINT</> signal handler, thus allowing
interactive cancellation of queries that it issues through <function>PQexec</function>.
Note that <function>PQrequestCancel</function> will have no effect if the connection
is not currently open or the backend is not currently processing a command.
</para>

</sect1>

<sect1 id="libpq-fastpath">
<title>The Fast-Path Interface</title>

<para>
<productname>PostgreSQL</productname> provides a fast-path interface to send
function calls to the backend.  This is a trapdoor into system internals and
can be a potential security hole.  Most users will not need this feature.

<itemizedlist>
<listitem>
<para>
<function>PQfn</function>
	Request execution of a backend function via the fast-path interface.
<synopsis>
PGresult* PQfn(PGconn* conn,
               int fnid,
               int *result_buf,
               int *result_len,
               int result_is_int,
               const PQArgBlock *args,
               int nargs);
</synopsis>
     The <parameter>fnid</> argument is the object identifier of the function to be
     executed.
     <parameter>result_buf</parameter> is the buffer in which
     to place the return value.  The caller must  have  allocated
     sufficient space to store the return value (there is no check!).
     The actual result length will be returned in the integer pointed
     to  by  <parameter>result_len</parameter>.   If a 4-byte integer result is expected, set
     <parameter>result_is_int</parameter> to 1; otherwise set it to 0.  (Setting <parameter>result_is_int</parameter> to 1
     tells <application>libpq</> to byte-swap the value if necessary, so that it is
     delivered as a proper int value for the client machine.  When
     <parameter>result_is_int</> is 0, the byte string sent by the backend is returned
     unmodified.)
     <parameter>args</> and <parameter>nargs</> specify the arguments to be passed to the function.
<synopsis>
typedef struct {
    int len;
    int isint;
    union {
        int *ptr;
        int integer;
    } u;
} PQArgBlock;
</synopsis>
     <function>PQfn</function> always returns a valid <structname>PGresult*</structname>. The result status
     should be checked before the result is used.   The
     caller is responsible for  freeing  the  <structname>PGresult</structname>  with
     <function>PQclear</function> when it is no longer needed.
</para>
</listitem>
</itemizedlist>
</para>

</sect1>

<sect1 id="libpq-notify">
<title>Asynchronous Notification</title>

  <indexterm zone="libpq-notify"><primary>NOTIFY</primary></indexterm>

<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 (and can stop listening
with the <command>UNLISTEN</command> command).  All backends listening on a
particular condition will be notified asynchronously when a <command>NOTIFY</command> of that
condition name is executed by any 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 database relation.
Commonly the condition name is the same as the associated relation, but it is
not necessary for there to be any associated relation.
</para>

<para>
<filename>libpq</filename> applications submit <command>LISTEN</command> and <command>UNLISTEN</command>
commands as ordinary SQL command.  Subsequently, arrival of <command>NOTIFY</command>
messages can be detected by calling <function>PQnotifies</function>.

<itemizedlist>
<listitem>
<para>
<function>PQnotifies</function>
          Returns  the next notification from a list of unhandled
          notification messages received from the backend.  Returns NULL if
          there are no pending notifications.  Once a notification is
	  returned from <function>PQnotifies</>, it is considered handled and will be
	  removed from the list of notifications.
<synopsis>
PGnotify* PQnotifies(PGconn *conn);

typedef struct pgNotify {
    char *relname;              /* name of relation containing data */
    int  be_pid;                /* process id of backend */
} PGnotify;
</synopsis>
After processing a <structname>PGnotify</structname> object returned by <function>PQnotifies</function>,
be sure to free it with <function>free()</function> to avoid a memory leak.
</para>
<note>
<para>
 In <productname>PostgreSQL</productname> 6.4 and later,
 the <literal>be_pid</literal> is that of the notifying backend,
 whereas in earlier versions it was always the <acronym>PID</acronym> of your own backend.
</para>
</note>
</listitem>
</itemizedlist>
</para>

<para>
The  second  sample program gives an example of the use
of asynchronous notification.
</para>

<para>
<function>PQnotifies()</function> does not actually read backend data; it just
returns messages previously absorbed by another <application>libpq</application>
function.  In prior releases of <application>libpq</application>, the only way
to ensure timely receipt of NOTIFY messages was to constantly submit queries,
even empty ones, and then check <function>PQnotifies()</function> after each
<function>PQexec()</function>.  While this still works, it is
deprecated as a waste of processing power.
</para>
<para>
A better way to check for NOTIFY
messages when you have no useful queries to make is to call
<function>PQconsumeInput()</function>, then check
<function>PQnotifies()</function>.
You can use <function>select()</function> to wait for backend data to
arrive, thereby using no <acronym>CPU</acronym> power unless there is something
to do.  (See <function>PQsocket()</function> to obtain the file descriptor
number to use with <function>select()</function>.)
Note that this will work OK whether you submit queries with
<function>PQsendQuery</function>/<function>PQgetResult</function> or simply
use <function>PQexec</function>.  You should, however, remember to
check <function>PQnotifies()</function> after each
<function>PQgetResult</function> or <function>PQexec</function>, to see
if any notifications came in during the processing of the query.
</para>

</sect1>

<sect1 id="libpq-copy">
<title>Functions Associated with the COPY Command</title>

<indexterm zone="libpq-copy">
 <primary>COPY</primary>
 <secondary>with libpq</secondary>
</indexterm>

<para>
 The COPY command in <productname>PostgreSQL</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 advantage of this capability.
</para>

<para>
 These functions should be executed only after obtaining a <literal>PGRES_COPY_OUT</literal>
 or <literal>PGRES_COPY_IN</literal> result object from <function>PQexec</function>
 or <function>PQgetResult</function>.
</para>

<para>
<itemizedlist>
<listitem>
<para>
<function>PQgetline</function>
          Reads  a  newline-terminated  line  of  characters
          (transmitted  by the backend server) into a buffer
          string of size length.
<synopsis>
int PQgetline(PGconn *conn,
              char *string,
              int length)
</synopsis>
Like <function>fgets</function>,  this  routine copies up to length-1 characters
into string. It is like <function>gets</function>, however, in that it converts
the terminating newline into a zero byte.
<function>PQgetline</function> returns <symbol>EOF</symbol> at the end of input, 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  the  two characters  <literal>\.</literal>,
which  indicates  that the backend server has finished sending
the results  of  the  copy  command.
If  the  application might
receive lines that are more than length-1  characters  long,
care is needed to be sure one recognizes the <literal>\.</literal> line correctly
(and does not, for example, mistake the end of a long data line
for a terminator line).
The code in
<filename>
src/bin/psql/copy.c
</filename>
contains example routines that correctly handle the  copy protocol.
</para>
</listitem>

<listitem>
<para>
<function>PQgetlineAsync</function>
          Reads  a  newline-terminated  line  of  characters
          (transmitted  by the backend server) into a buffer
          without blocking.
<synopsis>
int PQgetlineAsync(PGconn *conn,
                   char *buffer,
                   int bufsize)
</synopsis>
This routine is similar to <function>PQgetline</function>, but it can be used
by applications
that must read COPY data asynchronously, that is without blocking.
Having issued the COPY command and gotten a <literal>PGRES_COPY_OUT</literal>
response, the
application should call <function>PQconsumeInput</function> and
<function>PQgetlineAsync</function> until the
end-of-data signal is detected.  Unlike <function>PQgetline</function>, this routine takes
responsibility for detecting end-of-data.
On each call, <function>PQgetlineAsync</function> will return data if a complete newline-
terminated data line is available in <application>libpq</>'s input buffer, or if the
incoming data line is too long to fit in the buffer offered by the caller.
Otherwise, no data is returned until the rest of the line arrives.
</para>
<para>
The routine returns -1 if the end-of-copy-data marker has been recognized,
or 0 if no data is available, or a positive number giving the number of
bytes of data returned.  If -1 is returned, the caller must next call
<function>PQendcopy</function>, and then return to normal processing.
The data returned will not extend beyond a newline character.  If possible
a whole line will be returned at one time.  But if the buffer offered by
the caller is too small to hold a line sent by the backend, then a partial
data line will be returned.  This can be detected by testing whether the
last returned byte is <literal>\n</literal> or not.
The returned string is not null-terminated.  (If you want to add a
terminating null, be sure to pass a <parameter>bufsize</parameter> one smaller than the room
actually available.)
</para>
</listitem>

<listitem>
<para>
<function>PQputline</function>
Sends  a  null-terminated  string  to  the backend server.
Returns 0 if OK, <symbol>EOF</symbol> if unable to send the string.
<synopsis>
int PQputline(PGconn *conn,
              const char *string);
</synopsis>
Note the application must explicitly  send  the  two
characters  <literal>\.</literal> on a final line  to indicate to
the backend that it has finished sending its data.
</para>
</listitem>

<listitem>
<para>
<function>PQputnbytes</function>
Sends  a  non-null-terminated  string  to  the backend server.
Returns 0 if OK, <symbol>EOF</symbol> if unable to send the string.
<synopsis>
int PQputnbytes(PGconn *conn,
                const char *buffer,
                int nbytes);
</synopsis>
This is exactly like <function>PQputline</function>, except that the data buffer need
not be null-terminated since the number of bytes to send is
specified directly.
</para>
</listitem>

<listitem>
<para>
<function>PQendcopy</function>
 Synchronizes with the backend.  This function waits until
 the  backend  has  finished  the  copy.  It should
 either be issued when the  last  string  has  been
 sent  to  the  backend using <function>PQputline</function> or when the
 last string has been  received  from  the  backend
 using <function>PGgetline</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 SQL command.
 The return value is 0  on  successful  completion,
 nonzero otherwise.
<synopsis>
int PQendcopy(PGconn *conn);
</synopsis>
</para>

<para>
As an example:

<programlisting>
PQexec(conn, "CREATE TABLE foo (a int4, b char(16), d double precision)");
PQexec(conn, "COPY foo FROM STDIN");
PQputline(conn, "3\thello world\t4.5\n");
PQputline(conn,"4\tgoodbye world\t7.11\n");
...
PQputline(conn,"\\.\n");
PQendcopy(conn);
</programlisting>
</para>
</listitem>
</itemizedlist>
</para>

<para>
When using <function>PQgetResult</function>, the application should respond to
a <literal>PGRES_COPY_OUT</literal> result by executing <function>PQgetline</function>
repeatedly, followed by <function>PQendcopy</function> after the terminator line is seen.
It should then return to the <function>PQgetResult</function> loop until
<function>PQgetResult</function> returns NULL. Similarly a <literal>PGRES_COPY_IN</literal>
result is processed by a series of <function>PQputline</function> calls followed by
<function>PQendcopy</function>, then return to the <function>PQgetResult</function> loop.
This arrangement will ensure that
a copy in or copy out command embedded in a series of <acronym>SQL</acronym> commands
will be executed correctly.
</para>
<para>
Older applications are likely to submit a copy in or copy out
via <function>PQexec</function> and assume that the transaction is done after
<function>PQendcopy</function>.
This will work correctly only if the copy in/out is the only
<acronym>SQL</acronym> command in the command string.
</para>

</sect1>

<sect1 id="libpq-trace">
<title><application>libpq</application> Tracing Functions</title>

<para>
<itemizedlist>
<listitem>
<para>
<function>PQtrace</function>
          Enable  tracing of the frontend/backend communication to a debugging file stream.
<synopsis>
void PQtrace(PGconn *conn
             FILE *debug_port)
</synopsis>
</para>
</listitem>

<listitem>
<para>
<function>PQuntrace</function>
          Disable tracing started by <function>PQtrace</function>.
<synopsis>
void PQuntrace(PGconn *conn)
</synopsis>
</para>
</listitem>
</itemizedlist>
</para>

</sect1>

<sect1 id="libpq-control">
<title><application>libpq</application> Control Functions</title>

<para>
<itemizedlist>
<listitem>
<para>
<function>PQsetNoticeProcessor</function>
<indexterm><primary>notice processor</></>
Control reporting of notice and warning messages generated by <application>libpq</>.
<synopsis>
typedef void (*PQnoticeProcessor) (void *arg, const char *message);

PQnoticeProcessor
PQsetNoticeProcessor(PGconn *conn,
                     PQnoticeProcessor proc,
                     void *arg);
</synopsis>
</para>
</listitem>
</itemizedlist>
</para>

<para>
By default, <application>libpq</application> prints notice
messages from the backend on <filename>stderr</filename>,
as well as a few error messages that it generates by itself.
This behavior can be overridden by supplying a callback function that
does something else with the messages.  The callback function is passed
the text of the error message (which includes a trailing newline), plus
a void pointer that is the same one passed to
<function>PQsetNoticeProcessor</function>.
(This pointer can be used to access application-specific state if needed.)
The default notice processor is simply
<programlisting>
static void
defaultNoticeProcessor(void * arg, const char * message)
{
    fprintf(stderr, "%s", message);
}
</programlisting>
To use a special notice processor, call
<function>PQsetNoticeProcessor</function> just after
creation of a new <structname>PGconn</> object.
</para>

<para>
The return value is the pointer to the previous notice processor.
If you supply a callback function pointer of NULL, no action is taken,
but the current pointer is returned.
</para>

<para>
Once you have set a notice processor, you should expect that that function
could be called as long as either the <structname>PGconn</> object or <structname>PGresult</> objects
made from it exist.  At creation of a <structname>PGresult</>, the <structname>PGconn</>'s current
notice processor pointer is copied into the <structname>PGresult</> for possible use by
routines like <function>PQgetvalue</function>.
</para>

</sect1>

<sect1 id="libpq-envars">
<title>Environment Variables</title>

<indexterm zone="libpq-envars">
 <primary>environment variables</primary>
</indexterm>

<para>
The following environment variables can be used to select default
connection parameter values, which will be used by
<function>PQconnectdb</>, <function>PQsetdbLogin</> and
<function>PQsetdb</> if no value is directly specified by the calling
code.  These are useful to avoid hard-coding database connection
information into simple client applications.

<itemizedlist>
<listitem>
<para>
<indexterm>
 <primary><envar>PGHOST</envar></primary>
</indexterm>
<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>
<indexterm>
 <primary><envar>PGPORT</envar></primary>
</indexterm>
<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>
<indexterm>
 <primary><envar>PGDATABASE</envar></primary>
</indexterm>
<envar>PGDATABASE</envar>  sets the default 
<productname>PostgreSQL</productname> database name.
</para>
</listitem>
<listitem>
<para>
<indexterm>
 <primary><envar>PGUSER</envar></primary>
</indexterm>
<envar>PGUSER</envar>
sets the user name used to connect to the database and for authentication.
</para>
</listitem>
<listitem>
<para>
<indexterm>
 <primary><envar>PGPASSWORD</envar></primary>
</indexterm>
<envar>PGPASSWORD</envar>
sets the password used if the backend demands password
authentication.  This functionality is deprecated for security
reasons; consider migrating to use the <link linkend='pgpassfile'>
<filename>$HOME/.pgpass</></link>
file.
</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 tty on which  debugging  
messages from the backend server are displayed.
</para>
</listitem>
<listitem>
<para>
<envar>PGREQUIRESSL</envar> sets whether or not the connection must be
made over <acronym>SSL</acronym>. If set to
<quote>1</quote>, <application>libpq</>
will refuse to connect if the server does not accept
an <acronym>SSL</acronym> connection.
This option is only available if
<productname>PostgreSQL</> is compiled with SSL support.
</para>
</listitem>
<listitem>
<para>
<envar>PGCONNECT_TIMEOUT</envar> sets the maximum number of seconds
that <application>libpq</application> will wait when attempting to
connect to the <productname>PostgreSQL</productname> server. This
option should be set to at least 2 seconds.
</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>
<listitem>
<para>
<envar>PGCLIENTENCODING</envar>
sets the default client encoding.
</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>

</sect1>


<sect1 id="libpq-files">
<title>Files</title>

<indexterm zone="libpq-files">
 <primary>files</primary>
</indexterm>
<para>
<anchor id="pgpassfile">
<indexterm>
 <primary>password</primary>
 <secondary>.pgpass</secondary>
</indexterm>
The <filename>.pgpass</filename> file in a user's home directory is a
file that can contain passwords to be used if the connection requires
a password. This file should have the format:
<synopsis>
<replaceable>hostname</replaceable>:<replaceable>port</replaceable>:<replaceable>database</replaceable>:<replaceable>username</replaceable>:<replaceable>password</replaceable>
</synopsis>
Any of these may be a literal name, or <literal>*</literal>, which
matches anything.  The first matching entry will be used, so put more-specific
entries first.  When an entry contains <literal>:</literal> or
<literal>\</literal>, it must be escaped with <literal>\</literal>.
</para>
<para>
The permissions on <filename>.pgpass</filename> must disallow any
access to world or group; achieve this by the command
<command>chmod 0600 ~/.pgpass</command>.
If the permissions are less strict than this, the file will be ignored.
</para>
</sect1>

<sect1 id="libpq-threading">
<title>Threading Behavior</title>

<indexterm zone="libpq-threading">
 <primary>threads</primary>
 <secondary>with libpq</secondary>
</indexterm>

<para>
<filename>libpq</filename> is thread-safe as of
<productname>PostgreSQL</productname> 7.0, so long as no two threads
attempt to manipulate the same <structname>PGconn</> object at the same
time. In particular, you cannot issue concurrent queries from different
threads through the same connection object. (If you need to run
concurrent queries, start up multiple connections.)
</para>

<para>
<structname>PGresult</> objects are read-only after creation, and so can be passed around
freely between threads.
</para>

<para>
The deprecated functions <function>PQoidStatus</function> and
<function>fe_setauthsvc</function> are not thread-safe and should not be
used in multithread programs.  <function>PQoidStatus</function> can be
replaced by <function>PQoidValue</function>.  There is no good reason to
call <function>fe_setauthsvc</function> at all.
</para>

<para>
<filename>Libpq</filename> clients using the <literal>crypt</literal>
encryption method rely on the <literal>crypt()</literal> operating
system function, which is often not thread-safe. It is better to use
<literal>MD5</literal> encryption, which is thread-safe on all
platforms.
</para>
</sect1>


 <sect1 id="libpq-build">
  <title>Building <application>Libpq</application> Programs</title>

  <para>
   To build (i.e., compile and link) your <application>libpq</application> programs you need to
   do all of the following things:

   <itemizedlist>
    <listitem>
     <para>
      Include the <filename>libpq-fe.h</filename> header file:
<programlisting>
#include &lt;libpq-fe.h&gt;
</programlisting>
      If you failed to do that then you will normally get error
      messages from your compiler similar to
<screen>
foo.c: In function `main':
foo.c:34: `PGconn' undeclared (first use in this function)
foo.c:35: `PGresult' undeclared (first use in this function)
foo.c:54: `CONNECTION_BAD' undeclared (first use in this function)
foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function)
foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function)
</screen>
     </para>
    </listitem>

    <listitem>
     <para>
      Point your compiler to the directory where the <productname>PostgreSQL</> header
      files were installed, by supplying the
      <literal>-I<replaceable>directory</replaceable></literal> option
      to your compiler.  (In some cases the compiler will look into
      the directory in question by default, so you can omit this
      option.)  For instance, your compile command line could look
      like:
<programlisting>
cc -c -I/usr/local/pgsql/include testprog.c
</programlisting>
      If you are using makefiles then add the option to the
      <varname>CPPFLAGS</varname> variable:
<programlisting>
CPPFLAGS += -I/usr/local/pgsql/include
</programlisting>
     </para>

     <para>
      <indexterm><primary>pg_config</></>
      If there is any chance that your program might be compiled by
      other users then you should not hardcode the directory location
      like that.  Instead, you can run the utility
      <command>pg_config</command> to find out where the header files
      are on the local system:
<screen>
<prompt>$</prompt> pg_config --includedir
<computeroutput>/usr/local/include</computeroutput>
</screen>
     </para>

     <para>
      Failure to specify the correct option to the compiler will
      result in an error message such as
<screen>
testlibpq.c:8:22: libpq-fe.h: No such file or directory
</screen>
     </para>
    </listitem>

    <listitem>
     <para>
      When linking the final program, specify the option
      <literal>-lpq</literal> so that the <application>libpq</application> library gets pulled
      in, as well as the option
      <literal>-L<replaceable>directory</replaceable></literal> to
      point it to the directory where the <application>libpq</application> library resides.  (Again, the
      compiler will search some directories by default.)  For maximum
      portability, put the <option>-L</option> option before the
      <option>-lpq</option> option.  For example:
<programlisting>
cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq
</programlisting>
     </para>

     <para>
      You can find out the library directory using
      <command>pg_config</command> as well:
<screen>
<prompt>$</prompt> pg_config --libdir
<computeroutput>/usr/local/pgsql/lib</computeroutput>
</screen>
     </para>

     <para>
      Error messages that point to problems in this area could look
      like the following.
<screen>
testlibpq.o: In function `main':
testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
testlibpq.o(.text+0x71): undefined reference to `PQstatus'
testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'
</screen>
      This means you forgot <option>-lpq</option>.
<screen>
/usr/bin/ld: cannot find -lpq
</screen>
      This means you forgot the <option>-L</option> or did not specify
      the right path.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   <indexterm><primary>libpq-int.h</></>
   If your codes references the header file
   <filename>libpq-int.h</filename> and you refuse to fix your code to
   not use it, starting in <productname>PostgreSQL</> 7.2, this file will be found in
   <filename><replaceable>includedir</replaceable>/postgresql/internal/libpq-int.h</filename>,
   so you need to add the appropriate <option>-I</option> option to
   your compiler command line.
  </para>

 </sect1>


 <sect1 id="libpq-example">
  <title>Example Programs</title>

  <example id="libpq-example-1">
   <title><application>libpq</application> Example Program 1</title>

<programlisting>
/*
 * testlibpq.c
 *
 * Test the C version of libpq, the <productname>PostgreSQL</> frontend
 * library.
 */
#include &lt;stdio.h&gt;
#include &lt;libpq-fe.h&gt;

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

main()
{
    char       *pghost,
               *pgport,
               *pgoptions,
               *pgtty;
    char       *dbName;
    int         nFields;
    int         i,
                j;

    /* FILE *debug; */

    PGconn     *conn;
    PGresult   *res;

    /*
     * begin, by setting the parameters for a backend connection if the
     * parameters are null, then the system will try to use reasonable
     * defaults by looking up environment variables or, failing that,
     * using hardwired constants
     */
    pghost = NULL;              /* host name of the backend server */
    pgport = NULL;              /* port of the backend server */
    pgoptions = NULL;           /* special options to start up the backend
                                 * server */
    pgtty = NULL;               /* debugging tty for the backend server */
    dbName = "template1";

    /* make a connection to the database */
    conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);

    /*
     * check to see that the backend connection was successfully made
     */
    if (PQstatus(conn) == CONNECTION_BAD)
    {
        fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }

    /* debug = fopen("/tmp/trace.out","w"); */
    /* PQtrace(conn, debug);  */

    /* start a transaction block */
    res = PQexec(conn, "BEGIN");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "BEGIN command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }

    /*
     * should PQclear PGresult whenever it is no longer needed to avoid
     * memory leaks
     */
    PQclear(res);

    /*
     * fetch rows from the pg_database, the system catalog of
     * databases
     */
    res = PQexec(conn, "DECLARE mycursor CURSOR FOR SELECT * FROM pg_database");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "DECLARE CURSOR command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);
    res = PQexec(conn, "FETCH ALL in mycursor");
    if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
        PQclear(res);
        exit_nicely(conn);
    }

    /* first, print out the attribute names */
    nFields = PQnfields(res);
    for (i = 0; i &lt; nFields; i++)
        printf("%-15s", PQfname(res, i));
    printf("\n\n");

    /* next, print out the rows */
    for (i = 0; i &lt; PQntuples(res); i++)
    {
        for (j = 0; j &lt; nFields; j++)
            printf("%-15s", PQgetvalue(res, i, j));
        printf("\n");
    }
    PQclear(res);

    /* close the cursor */
    res = PQexec(conn, "CLOSE mycursor");
    PQclear(res);

    /* commit the transaction */
    res = PQexec(conn, "COMMIT");
    PQclear(res);

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    /* fclose(debug); */
    return 0;

}
</programlisting>
  </example>

  <example id="libpq-example-2">
   <title><application>libpq</application> Example Program 2</title>

<programlisting>
/*
 * testlibpq2.c
 *  Test of the asynchronous notification interface
 *
 * Start this program, then from psql in another window do
 *   NOTIFY TBL2;
 *
 * Or, if you want to get fancy, try this:
 * Populate a database with the following:
 *
 *   CREATE TABLE TBL1 (i int4);
 *
 *   CREATE TABLE TBL2 (i int4);
 *
 *   CREATE RULE r1 AS ON INSERT TO TBL1 DO
 *     (INSERT INTO TBL2 values (new.i); NOTIFY TBL2);
 *
 * and do
 *
 *   INSERT INTO TBL1 values (10);
 *
 */
#include &lt;stdio.h&gt;
#include "libpq-fe.h"

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

main()
{
    char       *pghost,
               *pgport,
               *pgoptions,
               *pgtty;
    char       *dbName;
    int         nFields;
    int         i,
                j;

    PGconn     *conn;
    PGresult   *res;
    PGnotify   *notify;

    /*
     * begin, by setting the parameters for a backend connection if the
     * parameters are null, then the system will try to use reasonable
     * defaults by looking up environment variables or, failing that,
     * using hardwired constants
     */
    pghost = NULL;              /* host name of the backend server */
    pgport = NULL;              /* port of the backend server */
    pgoptions = NULL;           /* special options to start up the backend
                                 * server */
    pgtty = NULL;               /* debugging tty for the backend server */
    dbName = getenv("USER");    /* change this to the name of your test
                                 * database */

    /* make a connection to the database */
    conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);

    /*
     * check to see that the backend connection was successfully made
     */
    if (PQstatus(conn) == CONNECTION_BAD)
    {
        fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }

    res = PQexec(conn, "LISTEN TBL2");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "LISTEN command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }

    /*
     * should PQclear PGresult whenever it is no longer needed to avoid
     * memory leaks
     */
    PQclear(res);

    while (1)
    {

        /*
         * wait a little bit between checks; waiting with select()
         * would be more efficient.
         */
        sleep(1);
        /* collect any asynchronous backend messages */
        PQconsumeInput(conn);
        /* check for asynchronous notify messages */
        while ((notify = PQnotifies(conn)) != NULL)
        {
            fprintf(stderr,
                 "ASYNC NOTIFY of '%s' from backend pid '%d' received\n",
                    notify-&gt;relname, notify-&gt;be_pid);
            free(notify);
        }
    }

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    return 0;
}
</programlisting>
  </example>

  <example id="libpq-example-3">
   <title><application>libpq</application> Example Program 3</>

<programlisting>
/*
 * testlibpq3.c Test the C version of Libpq, the <productname>PostgreSQL</> frontend
 * library. tests the binary cursor interface
 *
 *
 *
 * populate a database by doing the following:
 *
 * CREATE TABLE test1 (i int4, d real, p polygon);
 *
 * INSERT INTO test1 values (1, 3.567, polygon '(3.0, 4.0, 1.0, 2.0)');
 *
 * INSERT INTO test1 values (2, 89.05, polygon '(4.0, 3.0, 2.0, 1.0)');
 *
 * the expected output is:
 *
 * tuple 0: got i = (4 bytes) 1, d = (4 bytes) 3.567000, p = (4
 * bytes) 2 points   boundbox = (hi=3.000000/4.000000, lo =
 * 1.000000,2.000000) tuple 1: got i = (4 bytes) 2, d = (4 bytes)
 * 89.050003, p = (4 bytes) 2 points   boundbox =
 * (hi=4.000000/3.000000, lo = 2.000000,1.000000)
 *
 *
 */
#include &lt;stdio.h&gt;
#include "libpq-fe.h"
#include "utils/geo_decls.h"    /* for the POLYGON type */

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

main()
{
    char       *pghost,
               *pgport,
               *pgoptions,
               *pgtty;
    char       *dbName;
    int         nFields;
    int         i,
                j;
    int         i_fnum,
                d_fnum,
                p_fnum;
    PGconn     *conn;
    PGresult   *res;

    /*
     * begin, by setting the parameters for a backend connection if the
     * parameters are null, then the system will try to use reasonable
     * defaults by looking up environment variables or, failing that,
     * using hardwired constants
     */
    pghost = NULL;              /* host name of the backend server */
    pgport = NULL;              /* port of the backend server */
    pgoptions = NULL;           /* special options to start up the backend
                                 * server */
    pgtty = NULL;               /* debugging tty for the backend server */

    dbName = getenv("USER");    /* change this to the name of your test
                                 * database */

    /* make a connection to the database */
    conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);

    /*
     * check to see that the backend connection was successfully made
     */
    if (PQstatus(conn) == CONNECTION_BAD)
    {
        fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }

    /* start a transaction block */
    res = PQexec(conn, "BEGIN");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "BEGIN command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }

    /*
     * should PQclear PGresult whenever it is no longer needed to avoid
     * memory leaks
     */
    PQclear(res);

    /*
     * fetch rows from the pg_database, the system catalog of
     * databases
     */
    res = PQexec(conn, "DECLARE mycursor BINARY CURSOR FOR SELECT * FROM test1");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "DECLARE CURSOR command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);

    res = PQexec(conn, "FETCH ALL in mycursor");
    if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
        PQclear(res);
        exit_nicely(conn);
    }

    i_fnum = PQfnumber(res, "i");
    d_fnum = PQfnumber(res, "d");
    p_fnum = PQfnumber(res, "p");

    for (i = 0; i &lt; 3; i++)
    {
        printf("type[%d] = %d, size[%d] = %d\n",
               i, PQftype(res, i),
               i, PQfsize(res, i));
    }
    for (i = 0; i &lt; PQntuples(res); i++)
    {
        int        *ival;
        float      *dval;
        int         plen;
        POLYGON    *pval;

        /* we hard-wire this to the 3 fields we know about */
        ival = (int *) PQgetvalue(res, i, i_fnum);
        dval = (float *) PQgetvalue(res, i, d_fnum);
        plen = PQgetlength(res, i, p_fnum);

        /*
         * plen doesn't include the length field so need to
         * increment by VARHDSZ
         */
        pval = (POLYGON *) malloc(plen + VARHDRSZ);
        pval-&gt;size = plen;
        memmove((char *) &amp;pval-&gt;npts, PQgetvalue(res, i, p_fnum), plen);
        printf("tuple %d: got\n", i);
        printf(" i = (%d bytes) %d,\n",
               PQgetlength(res, i, i_fnum), *ival);
        printf(" d = (%d bytes) %f,\n",
               PQgetlength(res, i, d_fnum), *dval);
        printf(" p = (%d bytes) %d points \tboundbox = (hi=%f/%f, lo = %f,%f)\n",
               PQgetlength(res, i, d_fnum),
               pval-&gt;npts,
               pval-&gt;boundbox.xh,
               pval-&gt;boundbox.yh,
               pval-&gt;boundbox.xl,
               pval-&gt;boundbox.yl);
    }
    PQclear(res);

    /* close the cursor */
    res = PQexec(conn, "CLOSE mycursor");
    PQclear(res);

    /* commit the transaction */
    res = PQexec(conn, "COMMIT");
    PQclear(res);

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    return 0;
}
</programlisting>
  </example>

 </sect1>
</chapter>

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