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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().
Debian October 18, 2018 SOCKET(9)