An archive library is loaded by dynamically loading a shared library with the archive_library's name as the library base name. The normal library search path is used to locate the library. To provide the required archive module callbacks and to indicate that the library is actually an archive module, it needs to provide a function named _PG_archive_module_init. This function is passed a struct that needs to be filled with the callback function pointers for individual actions.

typedef struct ArchiveModuleCallbacks
{
    ArchiveCheckConfiguredCB check_configured_cb;
    ArchiveFileCB archive_file_cb;
    ArchiveShutdownCB shutdown_cb;
} ArchiveModuleCallbacks;
typedef void (*ArchiveModuleInit) (struct ArchiveModuleCallbacks *cb);

Only the archive_file_cb callback is required. The others are optional.