libpq++ is the C++ API to Postgres. libpq++ is a set of classes which allow client programs to connect to the Postgres backend server. These connections come in two forms: a Database Class and a Large Object class. The Database Class is intended for manipulating a database. You can send all sorts of SQL queries to the Postgres backend server and retrieve the responses of the server. The Large Object Class is intended for manipulating a large object in a database. Although a Large Object instance can send normal queries to the Postgres backend server it is only intended for simple queries that do not return any data. A large object should be seen as a file stream. In future it should behave much like the C++ file streams cin, cout and cerr. This chapter is based on the documentation for the libpq C library. Three short programs are listed at the end of this section as examples of libpq++ programming (though not necessarily of good programming). There are several examples of libpq++ applications in src/libpq++/examples, including the source code for the three examples in this chapter. The following environment variables can be used to set up default values for an environment and to avoid hard-coding database names into an application program: Refer to the for a complete list of available connection options. The following environment variables can be used to select default connection parameter values, which will be used by PQconnectdb or PQsetdbLogin if no value is directly specified by the calling code. These are useful to avoid hard-coding database names into simple application programs. PGHOST sets the default server name. If a non-zero-length string is specified, TCP/IP communication is used. Without a host name, libpq will connect using a local Unix domain socket. PGPORT sets the default port or local Unix domain socket file extension for communicating with the Postgres backend. PGDATABASE sets the default Postgres database name. PGUSER sets the username used to connect to the database and for authentication. PGPASSWORD sets the password used if the backend demands password authentication. PGREALM sets the Kerberos realm to use with Postgres, if it is different from the local realm. If PGREALM is set, Postgres applications will attempt authentication with servers for this realm and use separate ticket files to avoid conflicts with local ticket files. This environment variable is only used if Kerberos authentication is selected by the backend. PGOPTIONS sets additional runtime options for the Postgres backend. PGTTY sets the file or tty on which debugging messages from the backend server are displayed. The following environment variables can be used to specify user-level default behavior for every Postgres session: PGDATESTYLE sets the default style of date/time representation. PGTZ sets the default time zone. The following environment variables can be used to specify default internal behavior for every Postgres session: PGGEQO sets the default mode for the genetic optimizer. PGRPLANS sets the default mode to allow or disable right-sided plans in the optimizer. PGCOSTHEAP sets the default cost for heap searches for the optimizer. PGCOSTINDEX sets the default cost for indexed searches for the optimizer. PGQUERY_LIMIT sets the maximum number of rows returned by a query. Refer to the SET SQL command for information on correct values for these environment variables. The database environment class provides C++ objects for manipulating the above environment variables: PGenv creates an environment for the running program. PGenv() PGenv(char* auth, char* host, char* port, char* option, char* tty) The first form of this object's constructor sets up the defaults for the program from the environment variables listed above. The second allows the programmer to hardcode the values into the program. The values of the second form relate directly to the environment variables above. The database class is a provides C++ objects that have a connection to a backend server. To create such an object one first need the apropriate environment for the backend to access. The following constructors deal with making a connection to a backend server from a C++ program. PGdatabase makes a new connection to a backend database server. PGdatabase(PGenv *env, char *dbName) After a PGdatabase has been created it should be checked to make sure the connection to the database succeded before sending queries to the object. This can easily be done by retrieving the current status of the PGdatabase object with the status method. status returns the status of the PGdatabase object. ConnStatus PGdatabase::status() The following values are allowed: CONNECTION_OK CONNECTION_BAD PGdatabase::exec submits a query to Postgres and returns result status. In case of an error PGdatabase::errormessage can be used to get more information on the error. void ExecStatusType PGdatabase::exec(char *query); The following status results can be expected: PGRES_EMPTY_QUERY PGRES_COMMAND_OK, if the query was a command PGRES_TUPLES_OK, if the query successfully returned tuples PGRES_COPY_OUT PGRES_COPY_IN PGRES_BAD_RESPONSE, if an unexpected response was received PGRES_NONFATAL_ERROR PGRES_FATAL_ERROR If the result status is PGRES_TUPLES_OK, then the following routines can be used to retrieve the tuples returned by the query. PGdatabase::ntuples returns the number of tuples (instances) in the query result. int PGdatabase::ntuples() PGdatabase::nfields returns the number of fields (attributes) in the query result. int PGdatabase::nfields() PGdatabase::fieldname returns the field (attribute) name associated with the given field index. Field indices start at zero. char* PGdatabase::fieldname(int field_index) PGdatabase::fieldnum returns the field (attribute) index associated with the given field name. int PGdatabase::fieldnum(char* field_name) PGdatabase::fieldtype returns the field type of associated with the given field index or name. The integer returned is an internal coding of the type. Field indices start at zero. Oid PGdatabase::fieldtype(int field_index) Oid PGdatabase::fieldtype(char* field_name) PGdatabase::fieldsize returns the size in bytes of the field associated with the given field index or name. If the size returned is -1, the field is a variable length field. Field indices start at zero. int2 PGdatabase::fieldsize(int field_index) int2 PGdatabase::fieldsize(char* field_name) PGdatabase::getvalue returns the field (attribute) value. For most queries, the values returned by PGdatabase::getvalue is a null-terminated ASCII string representation of the attribute value. If the query was a result of a BINARY cursor, then the values returned by PGdatabase::getvalue is the binary representation of the type in the internal format of the backend server. It is the programmer's responsibility to cast and convert the data to the correct C++ type. The value return by PGdatabase::getvalue points to storage that is part of the PGdatabase structure. One must explicitly copy the value into other storage if it is to be used past the next query. char* PGdatabase::getvalue(int tup_num, int field_index) char* PGdatabase::getvalue(int tup_num, char* field_name) PGdatabase::getlength returns the length of a field (attribute) in bytes. If the field is a struct varlena, the length returned here does not include the size field of the varlena, i.e., it is 4 bytes less. int PGdatabase::getlength(int tup_num, int field_index) int PGdatabase::getlength(int tup_num, char* field_name) PGdatabase::printtuples prints out all the tuples and, optionally, the attribute names to the specified output stream. void PGdatabase::printtuples( FILE* fout, /* output stream */ int printAttName,/* print attribute names or not*/ int terseOutput, /* delimiter bars or not?*/ int width /* width of column, variable width if 0*/ ); Postgres supports asynchronous notification via the LISTEN and NOTIFY commands. A backend registers its interest in a particular semaphore with the LISTEN command. All backends that are listening on a particular named semaphore will be notified asynchronously when a NOTIFY of that name is executed by another backend. No additional information is passed from the notifier to the listener. Thus, typically, any actual data that needs to be communicated is transferred through the relation. In the past, the documentation has associated the names used for asyncronous notification with relations or classes. However, there is in fact no direct linkage of the two concepts in the implementation, and the named semaphore in fact does not need to have a corresponding relation previously defined. libpq++ applications are notified whenever a connected backend has received an asynchronous notification. However, the communication from the backend to the frontend is not asynchronous. The libpq++ application must poll the backend to see if there is any pending notification information. After the execution of a query, a frontend may call PGdatabase::notifies to see if any notification data is currently available from the backend. PGdatabase::notifies returns the notification from a list of unhandled notifications from the backend. The function eturns NULL if there is no pending notifications from the backend. PGdatabase::notifies behaves like the popping of a stack. Once a notification is returned from PGdatabase::notifies, it is considered handled and will be removed from the list of notifications. PGdatabase::notifies retrieves pending notifications from the server. PGnotify* PGdatabase::notifies() The second sample program gives an example of the use of asynchronous notification. The copy command in Postgres has options to read from or write to the network connection used by libpq++. Therefore, functions are necessary to access this network connection directly so applications may take full advantage of this capability. PGdatabase::getline reads a newline-terminated line of characters (transmitted by the backend server) into a buffer string of size length. int PGdatabase::getline(char* string, int length) Like the Unix system routine fgets (3), this routine copies up to length-1 characters into string. It is like gets (3), however, in that it converts the terminating newline into a null character. PGdatabase::getline returns EOF at end of file, 0 if the entire line has been read, and 1 if the buffer is full but the terminating newline has not yet been read. Notice that the application must check to see if a new line consists of a single period ("."), which indicates that the backend server has finished sending the results of the copy. Therefore, if the application ever expects to receive lines that are more than length-1 characters long, the application must be sure to check the return value of PGdatabase::getline very carefully. PGdatabase::putline Sends a null-terminated string to the backend server. void PGdatabase::putline(char* string) The application must explicitly send a single period character (".") to indicate to the backend that it has finished sending its data. PGdatabase::endcopy syncs with the backend. int PGdatabase::endcopy() This function waits until the backend has finished processing the copy. It should either be issued when the last string has been sent to the backend using PGdatabase::putline or when the last string has been received from the backend using PGdatabase::getline. It must be issued or the backend may get out of sync with the frontend. Upon return from this function, the backend is ready to receive the next query. The return value is 0 on successful completion, nonzero otherwise. As an example: PGdatabase data; data.exec("create table foo (a int4, b char16, d float8)"); data.exec("copy foo from stdin"); data.putline("3\etHello World\et4.5\en"); data.putline("4\etGoodbye World\et7.11\en"); \&... data.putline(".\en"); data.endcopy(); The query buffer is 8192 bytes long, and queries over that length will be silently truncated. The PGlobj class is largely untested. Use with caution.