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>Chapter 29. <SPAN
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>libpq</SPAN
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><A
NAME="LIBPQ-CANCEL"
>29.5. Cancelling Queries in Progress</A
></H1
><A
NAME="AEN28178"
></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="AEN28186"
></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="AEN28194"
></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="AEN28204"
></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="AEN28216"
></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="AEN28236"
></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
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