PostgreSQL 7.1.3 Documentation
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# Chapter 3. libpq++ - C++ Binding Library

`libpq++` is the C++ API to Postgres. `libpq++` is a set of classes that 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 the 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.

# 3.1. Control and Initialization

## 3.1.1. Environment Variables

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:

> Note: Refer to Section 1.9 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.

> Note: `libpq++` uses only environment variables or `libpq`'s `PQconnectdb` conninfo style strings.

- `PGHOST` sets the default server name. If this begins with a slash, it specifies Unix-domain communication rather than TCP/IP communication; the value is the name of the directory in which the socket file is stored (default "/tmp").

- `PGPORT` sets the default TCP port number or 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.

Refer to the SET SQL command for information on correct values for these environment variables.

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