34.2. 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 thePGconn abstraction. Use the accessor functions described below to get at the contents ofPGconn. Reference to internalPGconn fields usinglibpq-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 ofPQhost,PQport, andPQpass can change if a new connection is established using the samePGconn object. Other values are fixed for the lifetime of thePGconn object.
PQdbReturns the database name of the connection.
char *PQdb(const PGconn *conn);
PQuserReturns the user name of the connection.
char *PQuser(const PGconn *conn);
PQpassReturns the password of the connection.
char *PQpass(const PGconn *conn);
PQpasswill return either the password specified in the connection parameters, or if there was none and the password was obtained from thepassword 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 ofPQpassuntil the connection is established. The status of the connection can be checked using the functionPQstatus.PQhostReturns 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
hostandhostaddr, thenPQhostwill return thehostinformation. If onlyhostaddrwas specified, then that is returned. If multiple hosts were specified in the connection parameters,PQhostreturns the host actually connected to.PQhostreturnsNULLif theconnargument isNULL. 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
PQhostuntil the connection is established. The status of the connection can be checked using the functionPQstatus.PQhostaddrReturns 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
hostaddrparameter.char *PQhostaddr(const PGconn *conn);
PQhostaddrreturnsNULLif theconnargument isNULL. 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.PQportReturns the port of the active connection.
char *PQport(const PGconn *conn);
If multiple ports were specified in the connection parameters,
PQportreturns the port actually connected to.PQportreturnsNULLif theconnargument isNULL. 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
PQportuntil the connection is established. The status of the connection can be checked using the functionPQstatus.PQttyThis function no longer does anything, but it remains for backwards compatibility. The function always return an empty string, or
NULLif theconnargument isNULL.char *PQtty(const PGconn *conn);
PQoptionsReturns 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 thePGconn object.
PQstatusReturns 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_OKandCONNECTION_BAD. A good connection to the database has the statusCONNECTION_OK. A failed connection attempt is signaled by statusCONNECTION_BAD. Ordinarily, an OK status will remain so untilPQfinish, but a communications failure might result in the status changing toCONNECTION_BADprematurely. In that case the application could try to recover by callingPQreset.See the entry for
PQconnectStartParams,PQconnectStartandPQconnectPollwith regards to other status codes that might be returned.PQtransactionStatusReturns 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), orPQTRANS_INERROR(idle, in a failed transaction block).PQTRANS_UNKNOWNis reported if the connection is bad.PQTRANS_ACTIVEis reported only when a query has been sent to the server and not yet completed.PQparameterStatusLooks 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.
PQparameterStatuscan be used to interrogate these settings. It returns the current value of a parameter if known, orNULLif the parameter is not known.Parameters reported as of the current release include
server_version,server_encoding,client_encoding,application_name,default_transaction_read_only,in_hot_standby,is_superuser,session_authorization,DateStyle,IntervalStyle,TimeZone,integer_datetimes, andstandard_conforming_strings. (server_encoding,TimeZone, andinteger_datetimeswere not reported by releases before 8.0;standard_conforming_stringswas not reported by releases before 8.1;IntervalStylewas not reported by releases before 8.4;application_namewas not reported by releases before 9.0;default_transaction_read_onlyandin_hot_standbywere not reported by releases before 14.) Note thatserver_version,server_encodingandinteger_datetimescannot change after startup.If no value for
standard_conforming_stringsis reported, applications can assume it isoff, 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 thePGconnstructure. It is unwise to assume the pointer will remain valid across queries.PQprotocolVersionInterrogates 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 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 is supported byPostgreSQL server versions 7.4 and above.
PQserverVersionReturns 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,
PQserverVersionuses 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
PQserverVersionby 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).PQerrorMessageReturns the error message most recently generated by an operation on the connection.
char *PQerrorMessage(const PGconn *conn);
Nearly alllibpq functions will set a message for
PQerrorMessageif they fail. Note that bylibpq convention, a nonemptyPQerrorMessageresult 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 associatedPGconnhandle is passed toPQfinish. The result string should not be expected to remain the same across operations on thePGconnstructure.PQsocketObtains 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);
PQbackendPIDReturns the processID (PID) of the backend process handling this connection.
int PQbackendPID(const PGconn *conn);
The backendPID is useful for debugging purposes and for comparison to
NOTIFYmessages (which include thePID of the notifying backend process). Note that thePID belongs to a process executing on the database server host, not the local host!PQconnectionNeedsPasswordReturns 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.
PQconnectionUsedPasswordReturns 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.
PQsslInUseReturns true (1) if the connection uses SSL, false (0) if not.
int PQsslInUse(const PGconn *conn);
PQsslAttributeReturns 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. Returns NULL if the connection does not use SSL or the specified attribute name is not defined for the library in use.
The following attributes are commonly available:
libraryName of the SSL implementation in use. (Currently, only
"OpenSSL"is implemented)protocolSSL/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.key_bitsNumber of key bits used by the encryption algorithm.
cipherA short name of the ciphersuite used, e.g.,
"DHE-RSA-DES-CBC3-SHA". The names are specific to each SSL implementation.compressionReturns "on" if SSL compression is in use, else it returns "off".
As a special case, the
libraryattribute may be queried without a connection by passing NULL as theconnargument. The result will be the default SSL library name, or NULL iflibpq was compiled without any SSL support. (Prior toPostgreSQL version 15, passing NULL as theconnargument always resulted in NULL. Client programs needing to differentiate between the newer and older implementations of this case may check theLIBPQ_HAS_SSL_LIBRARY_DETECTIONfeature macro.)PQsslAttributeNamesReturns an array of SSL attribute names available. The array is terminated by a NULL pointer.
const char * const * PQsslAttributeNames(const PGconn *conn);
PQsslStructReturns a pointer to an SSL-implementation-specific object describing the connection. Returns NULL if the connection is not encrypted or the requested type of object is not available from the connection's SSL implementation.
void *PQsslStruct(const PGconn *conn, const char *struct_name);
The struct(s) available depend on the SSL implementation in use. ForOpenSSL, there is one struct, available under the name
OpenSSL, and it returns a pointer toOpenSSL'sSSLstruct. 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 theOpenSSL documentation for information about this structure.
PQgetsslReturns 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 toOpenSSL and will not be available if anotherSSL implementation is used. To check if a connection uses SSL, callPQsslInUseinstead, and for more details about the connection, usePQsslAttribute.