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NAME
socket — kernel socket interface
SYNOPSIS
#include <sys/socket.h>
#include <sys/socketvar.h>
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
sogetopt(struct socket *so, struct sockopt *sopt);
int
soreceive(struct socket *so, struct sockaddr **psa, struct uio *uio, struct mbuf **mp0,
struct mbuf **controlp, int *flagsp);
int
sosetopt(struct socket *so, struct sockopt *sopt);
int
sosend(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);
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.
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).
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.
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.
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 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. Only one of the uio or mp0 pointers may be non-NULL. 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.
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.
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 December 14, 2006 SOCKET(9)