socket

SOCKET(9)                  Kernel Developer's Manual                 SOCKET(9)

NAME
     socket — kernel socket interface

SYNOPSIS
     #include <sys/socket.h>
     #include <sys/socketvar.h>

     void
     soabort(struct socket *so);

     int
     soaccept(struct socket *so, struct sockaddr **nam);

     int
     socheckuid(struct socket *so, uid_t uid);

     int
     sobind(struct socket *so, struct sockaddr *nam, struct thread *td);

     void
     soclose(struct socket *so);

     int
     soconnect(struct socket *so, struct sockaddr *nam, struct thread *td);

     int
     socreate(int dom, struct socket **aso, int type, int proto,
         struct ucred *cred, struct thread *td);

     int
     sodisconnect(struct socket *so);

     void
     sodtor_set(struct socket *so, void (*func)(struct socket *));

     struct sockaddr *
     sodupsockaddr(const struct sockaddr *sa, int mflags);

     void
     sofree(struct socket *so);

     void
     sohasoutofband(struct socket *so);

     int
     solisten(struct socket *so, int backlog, struct thread *td);

     void
     solisten_proto(struct socket *so, int backlog);

     int
     solisten_proto_check(struct socket *so);

     struct socket *
     sonewconn(struct socket *head, int connstatus);

     int
     sopoll(struct socket *so, int events, struct ucred *active_cred,
         struct thread *td);

     int
     sopoll_generic(struct socket *so, int events, struct ucred *active_cred,
         struct thread *td);

     int
     soreceive(struct socket *so, struct sockaddr **psa, struct uio *uio,
         struct mbuf **mp0, struct mbuf **controlp, int *flagsp);

     int
     soreceive_stream(struct socket *so, struct sockaddr **paddr,
         struct uio *uio, struct mbuf **mp0, struct mbuf **controlp,
         int *flagsp);

     int
     soreceive_dgram(struct socket *so, struct sockaddr **paddr,
         struct uio *uio, struct mbuf **mp0, struct mbuf **controlp,
         int *flagsp);

     int
     soreceive_generic(struct socket *so, struct sockaddr **paddr,
         struct uio *uio, struct mbuf **mp0, struct mbuf **controlp,
         int *flagsp);

     int
     soreserve(struct socket *so, u_long sndcc, u_long rcvcc);

     void
     sorflush(struct socket *so);

     int
     sosend(struct socket *so, struct sockaddr *addr, struct uio *uio,
         struct mbuf *top, struct mbuf *control, int flags,
         struct thread *td);

     int
     sosend_dgram(struct socket *so, struct sockaddr *addr, struct uio *uio,
         struct mbuf *top, struct mbuf *control, int flags,
         struct thread *td);

     int
     sosend_generic(struct socket *so, struct sockaddr *addr, struct uio *uio,
         struct mbuf *top, struct mbuf *control, int flags,
         struct thread *td);

     int
     soshutdown(struct socket *so, int how);

     void
     sotoxsocket(struct socket *so, struct xsocket *xso);

     void
     soupcall_clear(struct socket *so, int which);

     void
     soupcall_set(struct socket *so, int which,
         int (*func)(struct socket *, void *, int), void *arg);

     void
     sowakeup(struct socket *so, struct sockbuf *sb);

     #include <sys/sockopt.h>

     int
     sosetopt(struct socket *so, struct sockopt *sopt);

     int
     sogetopt(struct socket *so, struct sockopt *sopt);

     int
     sooptcopyin(struct sockopt *sopt, void *buf, size_t len, size_t minlen);

     int
     sooptcopyout(struct sockopt *sopt, const void *buf, size_t len);

DESCRIPTION
     The kernel socket programming interface permits in-kernel consumers to
     interact with local and network socket objects in a manner similar to
     that permitted using the socket(2) user API.  These interfaces are
     appropriate for use by distributed file systems and other network-aware
     kernel services.  While the user API operates on file descriptors, the
     kernel interfaces operate directly on struct socket pointers.  Some
     portions of the kernel API exist only to implement the user API, and are
     not expected to be used by kernel code.  The portions of the socket API
     used by socket consumers and implementations of network protocols will
     differ; some routines are only useful for protocol implementors.

     Except where otherwise indicated, socket functions may sleep, and are not
     appropriate for use in an ithread(9) context or while holding non-
     sleepable kernel locks.

   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.

SEE ALSO
     bind(2), close(2), connect(2), getsockopt(2), recv(2), send(2),
     setsockopt(2), shutdown(2), socket(2), ng_ksocket(4), ithread(9),
     msleep(9), ucred(9)

HISTORY
     The socket(2) system call appeared in 4.2BSD.  This manual page was
     introduced in FreeBSD 7.0.

AUTHORS
     This manual page was written by Robert Watson and
     Benjamin Kaduk.

BUGS
     The use of explicitly passed credentials, credentials hung from
     explicitly passed threads, the credential on curthread, and the cached
     credential from socket creation time is inconsistent, and may lead to
     unexpected behaviour.  It is possible that several of the td arguments
     should be cred arguments, or simply not be present at all.

     The caller may need to manually clear SS_ISCONNECTING if soconnect()
     returns an error.

     The MSG_DONTWAIT flag is not implemented for sosend(), and may not always
     work with soreceive() when zero copy sockets are enabled.

     This manual page does not describe how to register socket upcalls or
     monitor a socket for readability/writability without using blocking I/O.

     The soref() and sorele() functions are not described, and in most cases
     should not be used, due to confusing and potentially incorrect
     interactions when sorele() is last called after soclose().

                               October 18, 2018