Communicating with PostgreSQL Using LibPQ - PostgreSQL Developer's Guide - page 174

Establishing a connection
asynchronously
The libpq library provides functions to connect to the PostgreSQL database
asynchronously; these functions are called nonblocking functions. The nonblocking
functions do not block and return immediately. The libpq library has functions
that are both variant blocking and nonblocking connection functions. We've already
discussed the blocking functions, and now it's time to discuss nonblocking functions.
Using PQconnectStartParams
The PQconnectStartParams function is a nonblocking variant of the
PQconnectdbParams function. The complete connection is made in an application
loop rather than inside the library function. The syntax for PQconnectStartParams
is as follows:
PGconn *PQconnectStartParams(const char **keywords,
const char **values,
int expand_dbname);
Using PQconnectStart
The PQconnectStart function is a nonblocking variant of the PQconnectdb function.
Like the PQconnectStartParams function, it also does not block. The syntax for
PQconnectdbParams is as follows:
PGconn *PQconnectStart(const char *conninfo);
Using PQconnectPoll
The PQconnectPoll function is used to poll the connection status of a connection.
The syntax for PQconnectPoll is as follows:
PostgresPollingStatusType PQconnectPoll(PGconn *conn);