PostgreSQL 8.0.0rc2 Documentation
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# 27.2. Connection Status Functions

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.

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 (This is obsolete, since the server no longer pays attention to the TTY setting, but the function remains for backwards compatibility.)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

  Caution
  `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`, 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 reflected by `PQparameterStatus`.) For `server_version`, see also `PQserverVersion`, which returns the information in a numeric form that is much easier to compare against.

  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 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 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.

`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`.

`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 prototype for this function. Doing this will also automatically include `ssl.h` from OpenSSL.

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