libpq

libpq is the C client application programming interface for PostgreSQL. It is distributed as part of the PostgreSQL project and provides the primary means for applications to connect to PostgreSQL servers, issue SQL commands, and obtain query results.

Key capabilities include connection management (establishing and maintaining a database session, with statuses accessible via PQstatus), authentication, and optional TLS/SSL encryption. The library supports both blocking and non-blocking operation, using functions such as PQconnectdb, PQconnectdbParams, PQsendQuery, and PQgetResult.

Query execution and results are handled through high-level functions like PQexec, PQexecParams, and PQprepare for prepared statements. Results are represented by PGresult objects, with accessors such as PQntuples, PQnfields, PQfname, and PQgetvalue. Error reporting is available via PQresultErrorMessage, and resources are freed with PQclear.

libpq provides data transfer features for large objects and the COPY protocol, including functions for COPY operations and binary data transfer. It supports transparent type conversion and binary transfer when requested, and offers an asynchronous API alongside the traditional synchronous style.

Security and platform support: libpq supports SSL/TLS, GSSAPI/SSPI authentication, Kerberos, and certificate verification. It runs on major Unix-like systems and Windows, and is designed to be portable across platforms. The library handles encoding conversions and error reporting in a consistent way to simplify client development.

License and ecosystem: libpq is released under the PostgreSQL License, a liberal open-source license. It is the standard C interface used by many PostgreSQL client libraries and tools; language bindings exist for other languages but often rely on libpq for low-level communication. Compatibility is generally aligned with corresponding PostgreSQL server versions.