34.6. 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.

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

```synopsis
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.

```synopsis
void PQfreeCancel(PGcancel *cancel);
```

`PQfreeCancel` frees a data object previously created
by `PQgetCancel`.

PQcancel : Requests that the server abandon processing of the current command.

```synopsis
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.

```synopsis
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.