Canceling Queries in Progress
A client application can request cancellation of a command that is still being processed by the server, using the functions described in this section.
Creates a data structure containing the information needed to cancel a command issued through a particular database connection.
PGcancel *PQgetCancel(PGconn *conn);
PQgetCancelcreates 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
Frees a data structure created by
void PQfreeCancel(PGcancel *cancel);
PQfreeCancelfrees a data object previously created by
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.
PQcancelcan 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
PQcancelis concerned, so it can also be invoked from a thread that is separate from the one manipulating the PGconn object.
PQrequestCancelis a deprecated variant of
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.