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>libpq</SPAN
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><A
NAME="LIBPQ-CANCEL"
>30.5. Cancelling Queries in Progress</A
></H1
><A
NAME="AEN31977"
></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="AEN31985"
></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="AEN31993"
></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="AEN32003"
></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="AEN32015"
></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="AEN32035"
></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
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