These functions may be used to interrogate the status of an existing database connection object.
libpq application programmers should be careful to
maintain the PGconn
abstraction. Use the accessor
functions described below to get at the contents of PGconn
.
Reference to internal PGconn
fields using
libpq-int.h
is not recommended because they are subject to change
in the future.
The following functions return parameter values established at connection.
These values are fixed for the life of the PGconn
object.
PQdb
Returns the database name of the connection.
char *PQdb(const PGconn *conn);
PQuser
Returns the user name of the connection.
char *PQuser(const PGconn *conn);
PQpass
Returns the password of the connection.
char *PQpass(const PGconn *conn);
PQhost
Returns the server host name of the connection.
char *PQhost(const PGconn *conn);
PQport
Returns the port of the connection.
char *PQport(const PGconn *conn);
PQtty
Returns the debug TTY of the connection. (This is obsolete, since the server no longer pays attention to the TTY setting, but the function remains for backwards compatibility.)
char *PQtty(const PGconn *conn);
PQoptions
Returns the command-line options passed in the connection request.
char *PQoptions(const PGconn *conn);
The following functions return status data that can change as operations
are executed on the PGconn
object.
PQstatus
Returns the status of the connection.
ConnStatusType PQstatus(const PGconn *conn);
The status can be one of a number of values.
However, only two of these are
seen outside of an asynchronous connection procedure:
CONNECTION_OK
and
CONNECTION_BAD
. A good
connection to the database has the status CONNECTION_OK
.
A failed connection
attempt is signaled by status
CONNECTION_BAD
.
Ordinarily, an OK status will remain so until
PQfinish
, but a
communications failure might result in the status changing to
CONNECTION_BAD
prematurely.
In that case the application
could try to recover by calling PQreset
.
See the entry for PQconnectStart
and PQconnectPoll
with regards
to other status codes
that might be seen.
PQtransactionStatus
Returns the current in-transaction status of the server.
PGTransactionStatusType PQtransactionStatus(const PGconn *conn);
The status can be PQTRANS_IDLE
(currently idle),
PQTRANS_ACTIVE
(a command is in progress),
PQTRANS_INTRANS
(idle, in a valid transaction block),
or PQTRANS_INERROR
(idle, in a failed transaction block).
PQTRANS_UNKNOWN
is reported if the connection is bad.
PQTRANS_ACTIVE
is reported only when a query
has been sent to the server and not yet completed.
PQtransactionStatus
will give incorrect results when using
a PostgreSQL 7.3 server that has the parameter autocommit
set to off. The server-side autocommit feature has been
deprecated and does not exist in later server versions.
PQparameterStatus
Looks up a current parameter setting of the server.
const char *PQparameterStatus(const PGconn *conn, const char *paramName);
Certain parameter values are reported by the server automatically at
connection startup or whenever their values change.
PQparameterStatus
can be used to interrogate these settings.
It returns the current value of a parameter if known, or NULL
if the parameter is not known.
Parameters reported as of the current release include
server_version
,
server_encoding
,
client_encoding
,
is_superuser
,
session_authorization
,
DateStyle
,
TimeZone
,
integer_datetimes
, and
standard_conforming_strings
.
(server_encoding
, TimeZone
, and
integer_datetimes
were not reported by releases before 8.0;
standard_conforming_strings
was not reported by releases
before 8.1.)
Note that
server_version
,
server_encoding
and
integer_datetimes
cannot change after startup.
Pre-3.0-protocol servers do not report parameter settings, but
libpq includes logic to obtain values for
server_version
and client_encoding
anyway.
Applications are encouraged to use PQparameterStatus
rather than ad hoc code to determine these values.
(Beware however
that on a pre-3.0 connection, changing client_encoding
via
SET
after connection startup will not be reflected by
PQparameterStatus
.) For server_version
,
see also PQserverVersion
, which returns the information
in a numeric form that is much easier to compare against.
If no value for standard_conforming_strings
is reported,
applications may assume it is off
, that is, backslashes
are treated as escapes in string literals. Also, the presence of this
parameter may be taken as an indication that the escape string syntax
(E'...'
) is accepted.
Although the returned pointer is declared const
, it in fact
points to mutable storage associated with the PGconn
structure.
It is unwise to assume the pointer will remain valid across queries.
PQprotocolVersion
Interrogates the frontend/backend protocol being used.
int PQprotocolVersion(const PGconn *conn);
Applications may wish to use this to determine whether certain features are supported. Currently, the possible values are 2 (2.0 protocol), 3 (3.0 protocol), or zero (connection bad). This will not change after connection startup is complete, but it could theoretically change during a connection reset. The 3.0 protocol will normally be used when communicating with PostgreSQL 7.4 or later servers; pre-7.4 servers support only protocol 2.0. (Protocol 1.0 is obsolete and not supported by libpq.)
PQserverVersion
Returns an integer representing the backend version.
int PQserverVersion(const PGconn *conn);
Applications may use this to determine the version of the database server they are connected to. The number is formed by converting the major, minor, and revision numbers into two-decimal-digit numbers and appending them together. For example, version 8.1.5 will be returned as 80105, and version 8.2 will be returned as 80200 (leading zeroes are not shown). Zero is returned if the connection is bad.
PQerrorMessage
Returns the error message most recently generated by an operation on the connection.
char *PQerrorMessage(const PGconn *conn);
Nearly all libpq functions will set a message for
PQerrorMessage
if they fail.
Note that by libpq convention, a nonempty
PQerrorMessage
result will
include a trailing newline. The caller should not free the result
directly. It will be freed when the associated PGconn
handle is passed to PQfinish
. The result string
should not be expected to remain the same across operations on the
PGconn
structure.
PQsocket
Obtains the file descriptor number of the connection socket to the server. A valid descriptor will be greater than or equal to 0; a result of -1 indicates that no server connection is currently open. (This will not change during normal operation, but could change during connection setup or reset.)
int PQsocket(const PGconn *conn);
PQbackendPID
Returns the process ID (PID) of the backend server process handling this connection.
int PQbackendPID(const PGconn *conn);
The backend PID is useful for debugging
purposes and for comparison to NOTIFY
messages (which include the PID of the
notifying backend process). Note that the
PID belongs to a process executing on the
database server host, not the local host!
PQgetssl
Returns the SSL structure used in the connection, or null if SSL is not in use.
SSL *PQgetssl(const PGconn *conn);
This structure can be used to verify encryption levels, check server certificates, and more. Refer to the OpenSSL documentation for information about this structure.
You must define USE_SSL
in order to get the
correct prototype for this function. Doing this will also
automatically include ssl.h
from OpenSSL.