Creating and Destroying Sockets

A new socket may be created using socreate(). As with socket(2), arguments specify the requested domain, type, and protocol via dom, type, and proto. The socket is returned via aso on success. In addition, the credential used to authorize operations associated with the socket will be passed via cred (and will be cached for the lifetime of the socket), and the thread performing the operation via td. Warning: authorization of the socket creation operation will be performed using the thread credential for some protocols (such as raw sockets).

Sockets may be closed and freed using soclose(), which has similar semantics to close(2).

In certain circumstances, it is appropriate to destroy a socket without waiting for it to disconnect, for which soabort() is used. This is only appropriate for incoming connections which are in a partially connected state. It must be called on an unreferenced socket, by the thread which removed the socket from its listen queue, to prevent races. It will call into protocol code, so no socket locks may be held over the call. The caller of soabort() is responsible for setting the VNET context. The normal path to freeing a socket is sofree(), which handles reference counting on the socket. It should be called whenever a reference is released, and also whenever reference flags are cleared in socket or protocol code. Calls to sofree() should not be made from outside the socket layer; outside callers should use soclose() instead.

Connections and Addresses

The sobind() function is equivalent to the bind(2) system call, and binds the socket so to the address nam. The operation would be authorized using the credential on thread td.

The soconnect() function is equivalent to the connect(2) system call, and initiates a connection on the socket so to the address nam. The operation will be authorized using the credential on thread td. Unlike the user system call, soconnect() returns immediately; the caller may msleep(9) on so->so_timeo while holding the socket mutex and waiting for the SS_ISCONNECTING flag to clear or so->so_error to become non-zero. If soconnect() fails, the caller must manually clear the SS_ISCONNECTING flag.

A call to sodisconnect() disconnects the socket without closing it.

The soshutdown() function is equivalent to the shutdown(2) system call, and causes part or all of a connection on a socket to be closed down.

Sockets are transitioned from non-listening status to listening with solisten().

Socket Options

The sogetopt() function is equivalent to the getsockopt(2) system call, and retrieves a socket option on socket so. The sosetopt() function is equivalent to the setsockopt(2) system call, and sets a socket option on socket so.

The second argument in both sogetopt() and sosetopt() is the sopt pointer to a struct sopt describing the socket option operation. The caller-allocated structure must be zeroed, and then have its fields initialized to specify socket option operation arguments:

sopt_dir

Set to SOPT_SET or SOPT_GET depending on whether this is a get or set operation.

sopt_level

Specify the level in the network stack the operation is targeted at; for example, SOL_SOCKET.

sopt_name

Specify the name of the socket option to set.

sopt_val

Kernel space pointer to the argument value for the socket option.

sopt_valsize

Size of the argument value in bytes.

Socket Upcalls

In order for the owner of a socket to be notified when the socket is ready to send or receive data, an upcall may be registered on the socket. The upcall is a function that will be called by the socket framework when a socket buffer associated with the given socket is ready for reading or writing. soupcall_set() is used to register a socket upcall. The function func is registered, and the pointer arg will be passed as its second argument when it is called by the framework. The possible values for which are SO_RCV and SO_SND, which register upcalls for receive and send events, respectively. The upcall function func() must return either SU_OK or SU_ISCONNECTED, depending on whether or not a call to soisconnected should be made by the socket framework after the upcall returns. The upcall func cannot call soisconnected itself due to lock ordering with the socket buffer lock. Only SO_RCV upcalls should return SU_ISCONNECTED. When a SO_RCV upcall returns SU_ISCONNECTED, the upcall will be removed from the socket.

Upcalls are removed from their socket by soupcall_clear(). The which argument again specifies whether the sending or receiving upcall is to be cleared, with SO_RCV or SO_SND.

Socket Destructor Callback

A kernel system can use the sodtor_set() function to set a destructor for a socket. The destructor is called when the socket is is about to be freed. The destructor is called before the protocol detach routine. The destructor can serve as a callback to initiate additional cleanup actions.

Socket I/O

The soreceive() function is equivalent to the recvmsg(2) system call, and attempts to receive bytes of data from the socket so, optionally blocking awaiting for data if none is ready to read. Data may be retrieved directly to kernel or user memory via the uio argument, or as an mbuf chain returned to the caller via mp0, avoiding a data copy. The uio must always be non-NULL. If mp0 is non-NULL, only the uio_resid of uio is used. The caller may optionally retrieve a socket address on a protocol with the PR_ADDR capability by providing storage via non-NULL psa argument. The caller may optionally retrieve control data mbufs via a non-NULL controlp argument. Optional flags may be passed to soreceive() via a non-NULL flagsp argument, and use the same flag name space as the recvmsg(2) system call.

The sosend() function is equivalent to the sendmsg(2) system call, and attempts to send bytes of data via the socket so, optionally blocking if data cannot be immediately sent. Data may be sent directly from kernel or user memory via the uio argument, or as an mbuf chain via top, avoiding a data copy. Only one of the uio or top pointers may be non-NULL. An optional destination address may be specified via a non-NULL addr argument, which may result in an implicit connect if supported by the protocol. The caller may optionally send control data mbufs via a non-NULL control argument. Flags may be passed to sosend() using the flags argument, and use the same flag name space as the sendmsg(2) system call.

Kernel callers running in ithread(9) context, or with a mutex held, will wish to use non-blocking sockets and pass the MSG_DONTWAIT flag in order to prevent these functions from sleeping.

A socket can be queried for readability, writability, out-of-band data, or end-of-file using sopoll(). The possible values for events are as for poll(2), with symbolic values POLLIN, POLLPRI, POLLOUT, POLLRDNORM, POLLWRNORM, POLLRDBAND, and POLLINGEOF taken from <sys/poll.h>.

Calls to soaccept() pass through to the protocol's accept routine to accept an incoming connection.

Socket Utility Functions

The uid of a socket's credential may be compared against a uid with socheckuid().

A copy of an existing struct sockaddr may be made using sodupsockaddr().

Protocol implementations notify the socket layer of the arrival of out-of-band data using sohasoutofband(), so that the socket layer can notify socket consumers of the available data.

An “external-format” version of a struct socket can be created using sotoxsocket(), suitable for isolating user code from changes in the kernel structure.

Protocol Implementations

Protocols must supply an implementation for solisten(); such protocol implementations can call back into the socket layer using solisten_proto_check() and solisten_proto() to check and set the socket-layer listen state. These callbacks are provided so that the protocol implementation can order the socket layer and protocol locks as necessary. Protocols must supply an implementation of soreceive(); the functions soreceive_stream(), soreceive_dgram(), and soreceive_generic() are supplied for use by such implementations.

Protocol implementations can use sonewconn() to create a socket and attach protocol state to that socket. This can be used to create new sockets available for soaccept() on a listen socket. The returned socket has a reference count of zero.

Protocols must supply an implementation for sopoll(); sopoll_generic() is provided for the use by protocol implementations.

The functions sosend_dgram() and sosend_generic() are supplied to assist in protocol implementations of sosend().

When a protocol creates a new socket structure, it is necessary to reserve socket buffer space for that socket, by calling soreserve(). The rough inverse of this reservation is performed by sorflush(), which is called automatically by the socket framework.

When a protocol needs to wake up threads waiting for the socket to become ready to read or write, variants of sowakeup() are used. The sowakeup() function should not be called directly by protocol code, instead use the wrappers sorwakeup(), sorwakeup_locked(), sowwakeup(), and sowwakeup_locked() for readers and writers, with the corresponding socket buffer lock not already locked, or already held, respectively.

The functions sooptcopyin() and sooptcopyout() are useful for transferring struct sockopt data between user and kernel code.