Nil, then Unix domain sockets are disabled. An empty string will result in an error at some point, it will not disable use of Unix domain sockets.
When enabled, curl will connect to the Unix domain socket instead of establishing a TCP connection to a host. Since no TCP connection is created, curl does not need to resolve the DNS hostname in the URL.
The maximum path length on Cygwin, Linux and Solaris is 107. On other platforms it might be even less.
Proxy and TCP options such as
#CURLOPT_TCP_NODELAYare not supported. Proxy options such as