Sockets

Sockets in Scalability Protocols provide the handle for communication between peers. Sockets also encapsulate protocol specific semantics, such as filtering subscriptions, or automatically retrying requests.

Socket Structure

#define NNG_SOCKET_INITIALIZER // opaque value

typedef struct nng_socket_s nng_socket;

The nng_socket structure represents socket. This is a handle, and the members of it are opaque. However, unlike a pointer, it is usually passed by value.

A socket may be initialized statically with the NNG_SOCKET_INITIALIZER macro, to ensure that it cannot be confused with a valid open socket.

Socket Identity

int nng_socket_id(nng_socket s);
int nng_socket_raw(nng_socket s, bool *raw);
int nng_socket_proto_id(nng_socket s, uint16_t *proto);
int nng_socket_peer_id(nng_socket s, uint16_t *proto);
int nng_socket_proto_name(nng_socket s, const char **name);
int nng_socket_peer_name(nng_socket s, const char **name);

These functions are used to provide fundamental information about the socket s. Most applications will not need to use these functions.

The nng_socket_id function returns the numeric id, which will be a non-negative value, associated with the socket. If the socket is uninitialized (has never been opened), then the return value may be -1.

The nng_socket_proto_id and nng_socket_peer_id functions provide the 16-bit protocol identifier for the socket’s protocol, and of the protocol peers will use when communicating with the socket.

The nng_socket_proto_name and nng_socket_peer_name functions provide the ASCII names of the socket’s protocol, and of the protocol peers of the socket use. The value stored in name is a fixed string located in program text, and must not be freed or altered. It is guaranteed to remain valid while this library is present.

The nng_socket_raw function determines whether the socket is in raw mode or not, storing true in raw if it is, or false if it is not.

Polling Socket Events

int nng_socket_get_recv_poll_fd(nng_socket s, int *fdp);
int nng_socket_get_send_poll_fd(nng_socket s, int *fdp);

Sometimes it is necessary to integrate a socket into a poll or select driven event loop. (Or, on Linux, epoll, or on BSD derived systems like macOS kqueue).

For these occasions, a suitable file descriptor for polling is provided by these two functions.

The nng_socket_get_recv_poll_fd function obtains a file descriptor that will poll as readable when a message is ready for receiving for the socket.

The nng_socket_get_send_poll_fd function obtains a file descriptor that will poll as readable when the socket can accept a message for sending.

These file descriptors should only be polled for readability, and no other operation performed on them. The socket will read from, or write to, these file descriptors to provide a level-signaled behavior automatically.

Additionally the socket will close these file descriptors when the socket itself is closed.

These functions replace the NNG_OPT_SENDFD and NNG_OPT_RECVFD socket options that were available in previous versions of NNG.

Examples

Example 1: Initializing a Socket

nng_socket s = NNG_SOCKET_INITIALIZER;