These functions may be used to interrogate the status of an existing database connection object.
Tip: libpq application programmers should be careful to maintain the PGconn abstraction. Use the accessor functions described below to get at the contents of PGconn. Avoid directly referencing the fields of the PGconn structure because they are subject to change in the future. (Beginning in PostgreSQL release 6.4, the definition of the struct behind PGconn is not even provided in libpq-fe.h. If you have old code that accesses PGconn fields directly, you can keep using it by including libpq-int.h too, but you are encouraged to fix the code soon.)
The following functions return parameter values established at connection. These values are fixed for the life of the PGconn object.
Returns the database name of the connection.
char *PQdb(const PGconn *conn);
Returns the user name of the connection.
char *PQuser(const PGconn *conn);
Returns the password of the connection.
char *PQpass(const PGconn *conn);
Returns the server host name of the connection.
char *PQhost(const PGconn *conn);
Returns the port of the connection.
char *PQport(const PGconn *conn);
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 (This is obsolete, since the server no longer pays attention to the TTY setting, but the function remains for backwards compatibility.)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);
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.
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
See the entry for
PQconnectPoll with regards to other
status codes that might be seen.
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.
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
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, and integer_datetimes. (server_encoding, TimeZone, and integer_datetimes were not reported by releases before 8.0.) 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
PQparameterStatus.) For server_version, see also
PQserverVersion, which returns the
information in a numeric form that is much easier to
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.
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.)
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 7.4.2 will be returned as 70402, and version 8.1 will be returned as 80100 (leading zeroes are not shown). Zero is returned if the connection is bad.
char *PQerrorMessage(const PGconn *conn);
Nearly all libpq
functions will set a message for
PQerrorMessage if they fail. Note that by
libpq convention, a
result will include a trailing newline. The caller should
not free the result directly. It will be freed when the
associated PGconn handle is
result string should not be expected to remain the same
across operations on the PGconn
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);
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!
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.