Creates a data structure containing the information needed to cancel a command issued through a particular database connection.

PGcancel *PQgetCancel(PGconn *conn);

PQgetCancel creates aPGcancel object given aPGconn connection object. It will returnNULL if the givenconn isNULL or an invalid connection. ThePGcancel object is an opaque structure that is not meant to be accessed directly by the application; it can only be passed toPQcancel orPQfreeCancel.

Frees a data structure created byPQgetCancel.

void PQfreeCancel(PGcancel *cancel);

PQfreeCancel frees a data object previously created byPQgetCancel.

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 sizeerrbufsize (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 theerrbuf is a local variable in the signal handler. ThePGcancel object is read-only as far asPQcancel is concerned, so it can also be invoked from a thread that is separate from the one manipulating thePGconn object.

PQrequestCancel is a deprecated variant ofPQcancel.

int PQrequestCancel(PGconn *conn);

Requests that the server abandon processing of the current command. It operates directly on thePGconn object, and in case of failure stores the error message in thePGconn object (whence it can be retrieved byPQerrorMessage). Although the functionality is the same, this approach creates hazards for multiple-thread programs and signal handlers, since it is possible that overwriting thePGconn's error message will mess up the operation currently in progress on the connection.