| PostgreSQL 8.2.6 Documentation | ||||
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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 . 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 (This is obsolete, since the server no longer pays attention to the TTY setting, but the function remains for backwards compatibility.)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.
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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 .