<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Cancelling Queries in Progress</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 8.0.11 Documentation" HREF="index.html"><LINK REL="UP" TITLE="libpq - C Library" HREF="libpq.html"><LINK REL="PREVIOUS" TITLE="Asynchronous Command Processing" HREF="libpq-async.html"><LINK REL="NEXT" TITLE="The Fast-Path Interface" HREF="libpq-fastpath.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2007-02-02T03:57:22"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" >PostgreSQL 8.0.11 Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="libpq-async.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="libpq.html" >Fast Backward</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 27. <SPAN CLASS="APPLICATION" >libpq</SPAN > - C Library</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="libpq.html" >Fast Forward</A ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="libpq-fastpath.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="LIBPQ-CANCEL" >27.5. Cancelling Queries in Progress</A ></H1 ><A NAME="AEN24096" ></A ><P >A client application can request cancellation of a command that is still being processed by the server, using the functions described in this section. <P ></P ></P><DIV CLASS="VARIABLELIST" ><DL ><DT ><CODE CLASS="FUNCTION" >PQgetCancel</CODE ><A NAME="AEN24104" ></A ></DT ><DD ><P > Creates a data structure containing the information needed to cancel a command issued through a particular database connection. </P><PRE CLASS="SYNOPSIS" >PGcancel *PQgetCancel(PGconn *conn);</PRE ><P></P ><P ><CODE CLASS="FUNCTION" >PQgetCancel</CODE > creates a <TT CLASS="STRUCTNAME" >PGcancel</TT ><A NAME="AEN24112" ></A > object given a <TT CLASS="STRUCTNAME" >PGconn</TT > connection object. It will return NULL if the given <TT CLASS="PARAMETER" >conn</TT > is NULL or an invalid connection. The <TT CLASS="STRUCTNAME" >PGcancel</TT > object is an opaque structure that is not meant to be accessed directly by the application; it can only be passed to <CODE CLASS="FUNCTION" >PQcancel</CODE > or <CODE CLASS="FUNCTION" >PQfreeCancel</CODE >.</P ></DD ><DT ><CODE CLASS="FUNCTION" >PQfreeCancel</CODE ><A NAME="AEN24122" ></A ></DT ><DD ><P > Frees a data structure created by <CODE CLASS="FUNCTION" >PQgetCancel</CODE >. </P><PRE CLASS="SYNOPSIS" >void PQfreeCancel(PGcancel *cancel);</PRE ><P></P ><P ><CODE CLASS="FUNCTION" >PQfreeCancel</CODE > frees a data object previously created by <CODE CLASS="FUNCTION" >PQgetCancel</CODE >.</P ></DD ><DT ><CODE CLASS="FUNCTION" >PQcancel</CODE ><A NAME="AEN24134" ></A ></DT ><DD ><P > Requests that the server abandon processing of the current command. </P><PRE CLASS="SYNOPSIS" >int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);</PRE ><P></P ><P >The return value is 1 if the cancel request was successfully dispatched and 0 if not. If not, <TT CLASS="PARAMETER" >errbuf</TT > is filled with an error message explaining why not. <TT CLASS="PARAMETER" >errbuf</TT > must be a char array of size <TT CLASS="PARAMETER" >errbufsize</TT > (the recommended size is 256 bytes).</P ><P >Successful dispatch is no guarantee that the request will have any effect, however. If the cancellation is effective, the current command will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all.</P ><P ><CODE CLASS="FUNCTION" >PQcancel</CODE > can safely be invoked from a signal handler, if the <TT CLASS="PARAMETER" >errbuf</TT > is a local variable in the signal handler. The <TT CLASS="STRUCTNAME" >PGcancel</TT > object is read-only as far as <CODE CLASS="FUNCTION" >PQcancel</CODE > is concerned, so it can also be invoked from a thread that is separate from the one manipulating the <TT CLASS="STRUCTNAME" >PGconn</TT > object.</P ></DD ></DL ></DIV ><P> <P ></P ></P><DIV CLASS="VARIABLELIST" ><DL ><DT ><CODE CLASS="FUNCTION" >PQrequestCancel</CODE ><A NAME="AEN24154" ></A ></DT ><DD ><P > Requests that the server abandon processing of the current command. </P><PRE CLASS="SYNOPSIS" >int PQrequestCancel(PGconn *conn);</PRE ><P></P ><P ><CODE CLASS="FUNCTION" >PQrequestCancel</CODE > is a deprecated variant of <CODE CLASS="FUNCTION" >PQcancel</CODE >. It operates directly on the <TT CLASS="STRUCTNAME" >PGconn</TT > object, and in case of failure stores the error message in the <TT CLASS="STRUCTNAME" >PGconn</TT > object (whence it can be retrieved by <CODE CLASS="FUNCTION" >PQerrorMessage</CODE >). Although the functionality is the same, this approach creates hazards for multiple-thread programs and signal handlers, since it is possible that overwriting the <TT CLASS="STRUCTNAME" >PGconn</TT >'s error message will mess up the operation currently in progress on the connection.</P ></DD ></DL ></DIV ><P></P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="libpq-async.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="libpq-fastpath.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Asynchronous Command Processing</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="libpq.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >The Fast-Path Interface</TD ></TR ></TABLE ></DIV ></BODY ></HTML >