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 error
message explaining why not. 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
Requests that the server abandon processing of the current command.
int PQrequestCancel(PGconn *conn);
PQrequestCancel
is a deprecated variant of
PQcancel
. 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.