| PostgreSQL 9.1.14 Documentation | ||||
|---|---|---|---|---|
| Prev | Up | Chapter 31. libpq - C Library | Next | |
A client application can request cancellation of a command that is still being processed by the server, using the functions described in this section.
PQgetCancel
      Creates a data structure containing the information needed to cancel a command issued through a particular database connection.
PGcancel *PQgetCancel(PGconn *conn);
       PQgetCancel creates a
       PGcancel object
       given a PGconn connection object.  It will return
       NULL if the given conn is NULL or an invalid
       connection.  The PGcancel object is an opaque
       structure that is not meant to be accessed directly by the
       application; it can only be passed to PQcancel
       or PQfreeCancel.
      
PQfreeCancel
             Frees a data structure created by PQgetCancel.
void PQfreeCancel(PGcancel *cancel);
       PQfreeCancel frees a data object previously created
       by PQgetCancel.
      
PQcancel
      Requests that the server abandon processing of the current command.
int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);
The return value is 1 if the cancel request was successfully dispatched and 0 if not. If not, errbuf is filled with an explanatory error message. errbuf must be a char array of size errbufsize (the recommended size is 256 bytes).
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.
       PQcancel can safely be invoked from a signal
       handler, if the errbuf is a local variable in the
       signal handler.  The PGcancel object is read-only
       as far as PQcancel is concerned, so it can
       also be invoked from a thread that is separate from the one
       manipulating the PGconn object.
      
PQrequestCancel
             PQrequestCancel is a deprecated variant of
       PQcancel.
int PQrequestCancel(PGconn *conn);
       Requests that the server abandon processing of the current
       command.  It operates directly on the
       PGconn object, and in case of failure stores the
       error message in the PGconn object (whence it can
       be retrieved by PQerrorMessage).  Although
       the functionality is the same, this approach creates hazards for
       multiple-thread programs and signal handlers, since it is possible
       that overwriting the PGconn's error message will
       mess up the operation currently in progress on the connection.