Set on non-blocking sockets (timeout != 0) on which the previous read() did not fill a buffer completely. the next apr_socket_recv() will first call select()/poll() rather than going straight into read(). (Can also be set by an application to force a select()/poll() call before the next read, in cases where the app expects that an immediate read would fail.)
Don't accept IPv4 connections on an IPv6 listening socket.
#define APR_RESET_NODELAY 2048
This flag is ONLY set internally when we set APR_TCP_NOPUSH with APR_TCP_NODELAY set to tell us that APR_TCP_NODELAY should be turned on again when NOPUSH is turned off
#define APR_SO_DEBUG 4
Debug
#define APR_SO_DISCONNECTED 256
Disconnected
#define APR_SO_KEEPALIVE 2
Keepalive
#define APR_SO_LINGER 1
Linger
#define APR_SO_NONBLOCK 8
Non-blocking IO
#define APR_SO_RCVBUF 128
Receive buffer
#define APR_SO_REUSEADDR 16
Reuse addresses
#define APR_SO_SNDBUF 64
Send buffer
#define APR_TCP_DEFER_ACCEPT 32768
Delay accepting of new connections until data is available.