PostgreSQL

33.2. Connection Status Functions
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33.2. Connection Status Functions

These functions can 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 connection. If a multi-host connection string is used, the values of PQhost, PQport, and PQpass can change if a new connection is established using the same PGconn object. Other values are fixed for the lifetime of the PGconn object.

[.term]#PQdb: Returns the database name of the connection. +

char *PQdb(const PGconn *conn);

[.term]#PQuser: Returns the user name of the connection. +

char *PQuser(const PGconn *conn);

[.term]#PQpass: Returns the password of the connection. +

char *PQpass(const PGconn *conn);

  +
  `+PQpass+` will return either the password specified in the connection parameters, or if there was none and the password was obtained from the link:libpq-pgpass.html[password file], it will return that. In the latter case, if multiple hosts were specified in the connection parameters, it is not possible to rely on the result of `+PQpass+` until the connection is established. The status of the connection can be checked using the function `+PQstatus+`.
[.term]#`+PQhost+` ::
  Returns the server host name of the active connection. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. (The path case can be distinguished because it will always be an absolute path, beginning with `+/+`.)
  +

char *PQhost(const PGconn *conn);

  +
  If the connection parameters specified both `+host+` and `+hostaddr+`, then `+PQhost+` will return the `+host+` information. If only `+hostaddr+` was specified, then that is returned. If multiple hosts were specified in the connection parameters, `+PQhost+` returns the host actually connected to.
  +
  `+PQhost+` returns `+NULL+` if the _`+conn+`_ argument is `+NULL+`. Otherwise, if there is an error producing the host information (perhaps if the connection has not been fully established or there was an error), it returns an empty string.
  +
  If multiple hosts were specified in the connection parameters, it is not possible to rely on the result of `+PQhost+` until the connection is established. The status of the connection can be checked using the function `+PQstatus+`.
[.term]#`+PQhostaddr+` ::
  Returns the server IP address of the active connection. This can be the address that a host name resolved to, or an IP address provided through the `+hostaddr+` parameter.
  +

char *PQhostaddr(const PGconn *conn);

  +
  `+PQhostaddr+` returns `+NULL+` if the _`+conn+`_ argument is `+NULL+`. Otherwise, if there is an error producing the host information (perhaps if the connection has not been fully established or there was an error), it returns an empty string.
[.term]#`+PQport+` ::
  Returns the port of the active connection.
  +

char *PQport(const PGconn *conn);

  +
  If multiple ports were specified in the connection parameters, `+PQport+` returns the port actually connected to.
  +
  `+PQport+` returns `+NULL+` if the _`+conn+`_ argument is `+NULL+`. Otherwise, if there is an error producing the port information (perhaps if the connection has not been fully established or there was an error), it returns an empty string.
  +
  If multiple ports were specified in the connection parameters, it is not possible to rely on the result of `+PQport+` until the connection is established. The status of the connection can be checked using the function `+PQstatus+`.
[.term]#`+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 backward compatibility.)
  +

char *PQtty(const PGconn *conn);

[.term]#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.

[.term]#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 `+PQconnectStartParams+`, `+PQconnectStart+` and `+PQconnectPoll+` with regards to other status codes that might be returned.
[.term]#`+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.
[.term]#`+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+`, `+application_name+`, `+is_superuser+`, `+session_authorization+`, `+DateStyle+`, `+IntervalStyle+`, `+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; `+IntervalStyle+` was not reported by releases before 8.4; `+application_name+` was not reported by releases before 9.0.) 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 [.foreignphrase]#_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 can assume it is `+off+`, that is, backslashes are treated as escapes in string literals. Also, the presence of this parameter can 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.
[.term]#`+PQprotocolVersion+` ::
Interrogates the frontend/backend protocol being used.
+

int PQprotocolVersion(const PGconn *conn);

  +
  Applications might wish to use this function to determine whether certain features are supported. Currently, the possible values are 2 (2.0 protocol), 3 (3.0 protocol), or zero (connection bad). The protocol version 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.)
[.term]#`+PQserverVersion+` ::
  Returns an integer representing the server version.
  +

int PQserverVersion(const PGconn *conn);

  +
  Applications might use this function to determine the version of the database server they are connected to. The result is formed by multiplying the server's major version number by 10000 and adding the minor version number. For example, version 10.1 will be returned as 100001, and version 11.0 will be returned as 110000. Zero is returned if the connection is bad.
  +
  Prior to major version 10, PostgreSQL used three-part version numbers in which the first two parts together represented the major version. For those versions, `+PQserverVersion+` uses two digits for each part; for example version 9.1.5 will be returned as 90105, and version 9.2.0 will be returned as 90200.
  +
  Therefore, for purposes of determining feature compatibility, applications should divide the result of `+PQserverVersion+` by 100 not 10000 to determine a logical major version number. In all release series, only the last two digits differ between minor releases (bug-fix releases).
[.term]#`+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 can consist of multiple lines, and 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.
[.term]#`+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);

[.term]#PQbackendPID: Returns the process ID (PID) of the backend 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!
[.term]#`+PQconnectionNeedsPassword+`::
  Returns true (1) if the connection authentication method required a password, but none was available. Returns false (0) if not.
  +

int PQconnectionNeedsPassword(const PGconn *conn);

  +
  This function can be applied after a failed connection attempt to decide whether to prompt the user for a password.
[.term]#`+PQconnectionUsedPassword+`::
  Returns true (1) if the connection authentication method used a password. Returns false (0) if not.
  +

int PQconnectionUsedPassword(const PGconn *conn);

+
This function can be applied after either a failed or successful connection attempt to detect whether the server demanded a password.

The following functions return information related to SSL. This information usually doesn’t change after a connection is established.

[.term]#PQsslInUse: Returns true (1) if the connection uses SSL, false (0) if not. +

int PQsslInUse(const PGconn *conn);

[.term]#PQsslAttribute: Returns SSL-related information about the connection. +

const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);

  +
  The list of available attributes varies depending on the SSL library being used, and the type of connection. If an attribute is not available, returns NULL.
  +
  The following attributes are commonly available:
  +
  [.term]#`+library+`#;;
    Name of the SSL implementation in use. (Currently, only `+"OpenSSL"+` is implemented)
  [.term]#`+protocol+`#;;
    SSL/TLS version in use. Common values are `+"TLSv1"+`, `+"TLSv1.1"+` and `+"TLSv1.2"+`, but an implementation may return other strings if some other protocol is used.
  [.term]#`+key_bits+`#;;
    Number of key bits used by the encryption algorithm.
  [.term]#`+cipher+`#;;
    A short name of the ciphersuite used, e.g., `+"DHE-RSA-DES-CBC3-SHA"+`. The names are specific to each SSL implementation.
  [.term]#`+compression+`#;;
    Returns "on" if SSL compression is in use, else it returns "off".
[.term]#`+PQsslAttributeNames+`::
  Return an array of SSL attribute names available. The array is terminated by a NULL pointer.
  +

const char * const * PQsslAttributeNames(const PGconn *conn);

[.term]#PQsslStruct: Return a pointer to an SSL-implementation-specific object describing the connection. +

void *PQsslStruct(const PGconn *conn, const char *struct_name);

+
The struct(s) available depend on the SSL implementation in use. For OpenSSL, there is one struct, available under the name "OpenSSL", and it returns a pointer to the OpenSSL `+SSL+` struct. To use this function, code along the following lines could be used:
+

#include <libpq-fe.h>
#include <openssl/ssl.h>

...

    SSL *ssl;

    dbconn = PQconnectdb(...);
    ...

    ssl = PQsslStruct(dbconn, "OpenSSL");
    if (ssl)
    {
        /* use OpenSSL functions to access ssl */
    }

  +
  This structure can be used to verify encryption levels, check server certificates, and more. Refer to the OpenSSL documentation for information about this structure.
[.term]#`+PQgetssl+`::
   Returns the SSL structure used in the connection, or null if SSL is not in use.
  +

void *PQgetssl(const PGconn *conn);

+
This function is equivalent to `+PQsslStruct(conn, "OpenSSL")+`. It should not be used in new applications, because the returned struct is specific to OpenSSL and will not be available if another SSL implementation is used. To check if a connection uses SSL, call `+PQsslInUse+` instead, and for more details about the connection, use `+PQsslAttribute+`.

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