The libpq library is a PostgreSQL client library to communicate with the PostgreSQL server. The purpose of this chapter is to write the C program to connect to the PostgreSQL server and execute queries in the C programming language. In this chapter, we will explore communication with PostgreSQL using libpq, which includes learning about all the library functions and their utilization with sufficient C code examples. We will also discuss the blocking and nonblocking behavior of libpq functions.

Connecting and disconnecting to PostgreSQL

The libpq library provides multiple functions and mechanisms to connect to the PostgreSQL server. We will discuss each and every function that is involved in establishing a connection to the backend. An application can have multiple connections with the PostgreSQL database, but needs to maintain the PGconn pointer for each and every connection. The PGconn pointer is a connection object and a function used to establish a connection. It returns the pointer that the application needs to store and use for subsequent functions to query the database. This object must be closed when the connection to the server is no longer needed because the code must release resources that the PostgreSQL runtime allocates.

Using PQconnectdb

The PQconnectdb function is the most basic function to connect to the PostgreSQL backend. This function takes only one parameter: the conninfo string. The syntax for PQconnectdb is as follows:

PGconn *PQconnectdb (const char *conninfo);

The conninfo string is a space-delimited string that contains the keyword value pairs, for
example,keyword = 'foo'.Hereisthelistofallconninfokeywords:

• hostaddr: This is the IP address of the PostgreSQL server
• host: This is the hostname of the PostgreSQL server; it can be the name of the path of the Unix domain socket. The default value is the /tmp directory

Note

If the operating system does not support Unix domain sockets, the default value is localhost.

• port: This is the port number of the PostgreSQL server; the - default port is 5432 dbname: This is the name of the database we need to connect to; the default value for this is the username
• user: This is the username to connect as; the default is the - operating system user password: This is the password used during authentication
• connect_timeout: The connect_timeout keyword shows how long the PQconnectdb function waits before giving up

Here are some more options that are not commonly used:

- client_encoding
- options
- application_name
- fallback_application_name
- keepalives
- keepalives_idle
- keepalives_interval
- keepalives_count
- tty
- sslmode
- disable
- allow
- verify-ca
- verify-full
- requiressl
- sslcert
- sslkey
- sslrootcert
- sslcrl
- requirepeer
- krbsrvname
- gsslib
- service

The details of these values can be found at http://www.postgresql.org/docs/9.4/static/libpq- connect.html#LIBPQ-PARAMKEYWORDS.

Note

If you see the following kind of error, then it is due to an error in the conninfo string: missing "=" after "port" in connection info string.

We'll explore other connection methods in the next article.