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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>33.4. Asynchronous Command Processing</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="libpq-exec.html" title="33.3. Command Execution Functions" /><link rel="next" href="libpq-single-row-mode.html" title="33.5. Retrieving Query Results Row-by-Row" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">33.4. Asynchronous Command Processing</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-exec.html" title="33.3. Command Execution Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 33. libpq - C Library">Up</a></td><th width="60%" align="center">Chapter 33. <span xmlns="http://www.w3.org/1999/xhtml" class="application">libpq</span> - C Library</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-single-row-mode.html" title="33.5. Retrieving Query Results Row-by-Row">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LIBPQ-ASYNC"><div class="titlepage"><div><div><h2 class="title" style="clear: both">33.4. Asynchronous Command Processing</h2></div></div></div><a id="id-1.7.3.11.2" class="indexterm"></a><p>
   The <code class="function">PQexec</code> function is adequate for submitting
   commands in normal, synchronous applications.  It has a few
   deficiencies, however, that can be of importance to some users:

   </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
      <code class="function">PQexec</code> waits for the command to be completed.
      The application might have other work to do (such as maintaining a
      user interface), in which case it won't want to block waiting for
      the response.
     </p></li><li class="listitem"><p>
      Since the execution of the client application is suspended while it
      waits for the result, it is hard for the application to decide that
      it would like to try to cancel the ongoing command.  (It can be done
      from a signal handler, but not otherwise.)
     </p></li><li class="listitem"><p>
      <code class="function">PQexec</code> can return only one
      <code class="structname">PGresult</code> structure.  If the submitted command
      string contains multiple <acronym class="acronym">SQL</acronym> commands, all but
      the last <code class="structname">PGresult</code> are discarded by
      <code class="function">PQexec</code>.
     </p></li><li class="listitem"><p>
      <code class="function">PQexec</code> always collects the command's entire result,
      buffering it in a single <code class="structname">PGresult</code>.  While
      this simplifies error-handling logic for the application, it can be
      impractical for results containing many rows.
     </p></li></ul></div><p>
  </p><p>
   Applications that do not like these limitations can instead use the
   underlying functions that <code class="function">PQexec</code> is built from:
   <code class="function">PQsendQuery</code> and <code class="function">PQgetResult</code>.
   There are also
   <code class="function">PQsendQueryParams</code>,
   <code class="function">PQsendPrepare</code>,
   <code class="function">PQsendQueryPrepared</code>,
   <code class="function">PQsendDescribePrepared</code>, and
   <code class="function">PQsendDescribePortal</code>,
   which can be used with <code class="function">PQgetResult</code> to duplicate
   the functionality of
   <code class="function">PQexecParams</code>,
   <code class="function">PQprepare</code>,
   <code class="function">PQexecPrepared</code>,
   <code class="function">PQdescribePrepared</code>, and
   <code class="function">PQdescribePortal</code>
   respectively.

   </p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQSENDQUERY"><span class="term">
      <code class="function">PQsendQuery</code>
      <a id="id-1.7.3.11.4.15.1.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Submits a command to the server without waiting for the result(s).
       1 is returned if the command was successfully dispatched and 0 if
       not (in which case, use <code class="function">PQerrorMessage</code> to get more
       information about the failure).
</p><pre class="synopsis">
int PQsendQuery(PGconn *conn, const char *command);
</pre><p>

       After successfully calling <code class="function">PQsendQuery</code>, call
       <code class="function">PQgetResult</code> one or more times to obtain the
       results.  <code class="function">PQsendQuery</code> cannot be called again
       (on the same connection) until <code class="function">PQgetResult</code>
       has returned a null pointer, indicating that the command is done.
      </p></dd><dt id="LIBPQ-PQSENDQUERYPARAMS"><span class="term">
      <code class="function">PQsendQueryParams</code>
      <a id="id-1.7.3.11.4.15.2.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Submits a command and separate parameters to the server without
       waiting for the result(s).
</p><pre class="synopsis">
int PQsendQueryParams(PGconn *conn,
                      const char *command,
                      int nParams,
                      const Oid *paramTypes,
                      const char * const *paramValues,
                      const int *paramLengths,
                      const int *paramFormats,
                      int resultFormat);
</pre><p>

       This is equivalent to <code class="function">PQsendQuery</code> except that
       query parameters can be specified separately from the query string.
       The function's parameters are handled identically to
       <code class="function">PQexecParams</code>.  Like
       <code class="function">PQexecParams</code>, it will not work on 2.0-protocol
       connections, and it allows only one command in the query string.
      </p></dd><dt id="LIBPQ-PQSENDPREPARE"><span class="term">
      <code class="function">PQsendPrepare</code>
      <a id="id-1.7.3.11.4.15.3.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Sends a request to create a prepared statement with the given
       parameters, without waiting for completion.
</p><pre class="synopsis">
int PQsendPrepare(PGconn *conn,
                  const char *stmtName,
                  const char *query,
                  int nParams,
                  const Oid *paramTypes);
</pre><p>

       This is an asynchronous version of <code class="function">PQprepare</code>: it
       returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <code class="function">PQgetResult</code> to
       determine whether the server successfully created the prepared
       statement.  The function's parameters are handled identically to
       <code class="function">PQprepare</code>.  Like
       <code class="function">PQprepare</code>, it will not work on 2.0-protocol
       connections.
      </p></dd><dt id="LIBPQ-PQSENDQUERYPREPARED"><span class="term">
      <code class="function">PQsendQueryPrepared</code>
      <a id="id-1.7.3.11.4.15.4.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Sends a request to execute a prepared statement with given
       parameters, without waiting for the result(s).
</p><pre class="synopsis">
int PQsendQueryPrepared(PGconn *conn,
                        const char *stmtName,
                        int nParams,
                        const char * const *paramValues,
                        const int *paramLengths,
                        const int *paramFormats,
                        int resultFormat);
</pre><p>

       This is similar to <code class="function">PQsendQueryParams</code>, but
       the command to be executed is specified by naming a
       previously-prepared statement, instead of giving a query string.
       The function's parameters are handled identically to
       <code class="function">PQexecPrepared</code>.  Like
       <code class="function">PQexecPrepared</code>, it will not work on
       2.0-protocol connections.
      </p></dd><dt id="LIBPQ-PQSENDDESCRIBEPREPARED"><span class="term">
      <code class="function">PQsendDescribePrepared</code>
      <a id="id-1.7.3.11.4.15.5.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Submits a request to obtain information about the specified
       prepared statement, without waiting for completion.
</p><pre class="synopsis">
int PQsendDescribePrepared(PGconn *conn, const char *stmtName);
</pre><p>

       This is an asynchronous version of <code class="function">PQdescribePrepared</code>:
       it returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <code class="function">PQgetResult</code> to
       obtain the results.  The function's parameters are handled
       identically to <code class="function">PQdescribePrepared</code>.  Like
       <code class="function">PQdescribePrepared</code>, it will not work on
       2.0-protocol connections.
      </p></dd><dt id="LIBPQ-PQSENDDESCRIBEPORTAL"><span class="term">
      <code class="function">PQsendDescribePortal</code>
      <a id="id-1.7.3.11.4.15.6.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Submits a request to obtain information about the specified
       portal, without waiting for completion.
</p><pre class="synopsis">
int PQsendDescribePortal(PGconn *conn, const char *portalName);
</pre><p>

       This is an asynchronous version of <code class="function">PQdescribePortal</code>:
       it returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <code class="function">PQgetResult</code> to
       obtain the results.  The function's parameters are handled
       identically to <code class="function">PQdescribePortal</code>.  Like
       <code class="function">PQdescribePortal</code>, it will not work on
       2.0-protocol connections.
      </p></dd><dt id="LIBPQ-PQGETRESULT"><span class="term">
      <code class="function">PQgetResult</code>
      <a id="id-1.7.3.11.4.15.7.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Waits for the next result from a prior
       <code class="function">PQsendQuery</code>,
       <code class="function">PQsendQueryParams</code>,
       <code class="function">PQsendPrepare</code>,
       <code class="function">PQsendQueryPrepared</code>,
       <code class="function">PQsendDescribePrepared</code>, or
       <code class="function">PQsendDescribePortal</code>
       call, and returns it.
       A null pointer is returned when the command is complete and there
       will be no more results.
</p><pre class="synopsis">
PGresult *PQgetResult(PGconn *conn);
</pre><p>
      </p><p>
       <code class="function">PQgetResult</code> must be called repeatedly until
       it returns a null pointer, indicating that the command is done.
       (If called when no command is active,
       <code class="function">PQgetResult</code> will just return a null pointer
       at once.) Each non-null result from
       <code class="function">PQgetResult</code> should be processed using the
       same <code class="structname">PGresult</code> accessor functions previously
       described.  Don't forget to free each result object with
       <code class="function">PQclear</code> when done with it.  Note that
       <code class="function">PQgetResult</code> will block only if a command is
       active and the necessary response data has not yet been read by
       <code class="function">PQconsumeInput</code>.
      </p><div class="note"><h3 class="title">Note</h3><p>
        Even when <code class="function">PQresultStatus</code> indicates a fatal
        error, <code class="function">PQgetResult</code> should be called until it
        returns a null pointer, to allow <span class="application">libpq</span> to
        process the error information completely.
       </p></div></dd></dl></div><p>
  </p><p>
   Using <code class="function">PQsendQuery</code> and
   <code class="function">PQgetResult</code> solves one of
   <code class="function">PQexec</code>'s problems:  If a command string contains
   multiple <acronym class="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 client can be handling the results of one
   command while the server is still working on later queries in the same
   command string.)
  </p><p>
   Another frequently-desired feature that can be obtained with
   <code class="function">PQsendQuery</code> and <code class="function">PQgetResult</code>
   is retrieving large query results a row at a time.  This is discussed
   in <a class="xref" href="libpq-single-row-mode.html" title="33.5. Retrieving Query Results Row-by-Row">Section 33.5</a>.
  </p><p>
   By itself, calling <code class="function">PQgetResult</code>
   will still cause the client to block until the server completes the
   next <acronym class="acronym">SQL</acronym> command.  This can be avoided by proper
   use of two more functions:

   </p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQCONSUMEINPUT"><span class="term">
      <code class="function">PQconsumeInput</code>
      <a id="id-1.7.3.11.7.3.1.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       If input is available from the server, consume it.
</p><pre class="synopsis">
int PQconsumeInput(PGconn *conn);
</pre><p>
      </p><p>
       <code class="function">PQconsumeInput</code> normally returns 1 indicating
       <span class="quote">“<span class="quote">no error</span>”</span>, but returns 0 if there was some kind of
       trouble (in which case <code class="function">PQerrorMessage</code> can be
       consulted).  Note that the result does not say whether any input
       data was actually collected. After calling
       <code class="function">PQconsumeInput</code>, the application can check
       <code class="function">PQisBusy</code> and/or
       <code class="function">PQnotifies</code> to see if their state has changed.
      </p><p>
       <code class="function">PQconsumeInput</code> can be called even if the
       application is not prepared to deal with a result or notification
       just yet.  The function will read available data and save it in
       a buffer, thereby causing a <code class="function">select()</code>
       read-ready indication to go away.  The application can thus use
       <code class="function">PQconsumeInput</code> to clear the
       <code class="function">select()</code> condition immediately, and then
       examine the results at leisure.
      </p></dd><dt id="LIBPQ-PQISBUSY"><span class="term">
      <code class="function">PQisBusy</code>
      <a id="id-1.7.3.11.7.3.2.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Returns 1 if a command is busy, that is,
       <code class="function">PQgetResult</code> would block waiting for input.
       A 0 return indicates that <code class="function">PQgetResult</code> can be
       called with assurance of not blocking.
</p><pre class="synopsis">
int PQisBusy(PGconn *conn);
</pre><p>
      </p><p>
       <code class="function">PQisBusy</code> will not itself attempt to read data
       from the server; therefore <code class="function">PQconsumeInput</code>
       must be invoked first, or the busy state will never end.
      </p></dd></dl></div><p>
  </p><p>
   A typical application using these functions will have a main loop that
   uses <code class="function">select()</code> or <code class="function">poll()</code> to wait for
   all the conditions that it must respond to.  One of the conditions
   will be input available from the server, which in terms of
   <code class="function">select()</code> means readable data on the file
   descriptor identified by <code class="function">PQsocket</code>.  When the main
   loop detects input ready, it should call
   <code class="function">PQconsumeInput</code> to read the input.  It can then
   call <code class="function">PQisBusy</code>, followed by
   <code class="function">PQgetResult</code> if <code class="function">PQisBusy</code>
   returns false (0).  It can also call <code class="function">PQnotifies</code>
   to detect <code class="command">NOTIFY</code> messages (see <a class="xref" href="libpq-notify.html" title="33.8. Asynchronous Notification">Section 33.8</a>).
  </p><p>
   A client that uses
   <code class="function">PQsendQuery</code>/<code class="function">PQgetResult</code>
   can also attempt to cancel a command that is still being processed
   by the server; see <a class="xref" href="libpq-cancel.html" title="33.6. Canceling Queries in Progress">Section 33.6</a>.  But regardless of
   the return value of <code class="function">PQcancel</code>, the application
   must continue with the normal result-reading sequence using
   <code class="function">PQgetResult</code>.  A successful cancellation will
   simply cause the command to terminate sooner than it would have
   otherwise.
  </p><p>
   By using the functions described above, it is possible to avoid
   blocking while waiting for input from the database server.  However,
   it is still possible that the application will block waiting to send
   output to the server.  This is relatively uncommon but can happen if
   very long SQL commands or data values are sent.  (It is much more
   probable if the application sends data via <code class="command">COPY IN</code>,
   however.)  To prevent this possibility and achieve completely
   nonblocking database operation, the following additional functions
   can be used.

   </p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQSETNONBLOCKING"><span class="term">
      <code class="function">PQsetnonblocking</code>
      <a id="id-1.7.3.11.10.2.1.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Sets the nonblocking status of the connection.
</p><pre class="synopsis">
int PQsetnonblocking(PGconn *conn, int arg);
</pre><p>
      </p><p>
       Sets the state of the connection to nonblocking if
       <em class="parameter"><code>arg</code></em> is 1, or blocking if
       <em class="parameter"><code>arg</code></em> is 0.  Returns 0 if OK, -1 if error.
      </p><p>
       In the nonblocking state, calls to
       <code class="function">PQsendQuery</code>, <code class="function">PQputline</code>,
       <code class="function">PQputnbytes</code>, <code class="function">PQputCopyData</code>,
       and <code class="function">PQendcopy</code> will not block but instead return
       an error if they need to be called again.
      </p><p>
       Note that <code class="function">PQexec</code> does not honor nonblocking
       mode; if it is called, it will act in blocking fashion anyway.
      </p></dd><dt id="LIBPQ-PQISNONBLOCKING"><span class="term">
      <code class="function">PQisnonblocking</code>
      <a id="id-1.7.3.11.10.2.2.1.2" class="indexterm"></a>
     </span></dt><dd><p>
       Returns the blocking status of the database connection.
</p><pre class="synopsis">
int PQisnonblocking(const PGconn *conn);
</pre><p>
      </p><p>
       Returns 1 if the connection is set to nonblocking mode and 0 if
       blocking.
      </p></dd><dt id="LIBPQ-PQFLUSH"><span class="term">
      <code class="function">PQflush</code>
       <a id="id-1.7.3.11.10.2.3.1.2" class="indexterm"></a>
      </span></dt><dd><p>
       Attempts to flush any queued output data to the server.  Returns
       0 if successful (or if the send queue is empty), -1 if it failed
       for some reason, or 1 if it was unable to send all the data in
       the send queue yet (this case can only occur if the connection
       is nonblocking).
</p><pre class="synopsis">
int PQflush(PGconn *conn);
</pre><p>
      </p></dd></dl></div><p>
  </p><p>
   After sending any command or data on a nonblocking connection, call
   <code class="function">PQflush</code>.  If it returns 1, wait for the socket
   to become read- or write-ready.  If it becomes write-ready, call
   <code class="function">PQflush</code> again.  If it becomes read-ready, call
   <code class="function">PQconsumeInput</code>, then call
   <code class="function">PQflush</code> again.  Repeat until
   <code class="function">PQflush</code> returns 0.  (It is necessary to check for
   read-ready and drain the input with <code class="function">PQconsumeInput</code>,
   because the server can block trying to send us data, e.g. NOTICE
   messages, and won't read our data until we read its.)  Once
   <code class="function">PQflush</code> returns 0, wait for the socket to be
   read-ready and then read the response as described above.
  </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="libpq-exec.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="libpq.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="libpq-single-row-mode.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">33.3. Command Execution Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 33.5. Retrieving Query Results Row-by-Row</td></tr></table></div></body></html>