freebsd/share/man/man9/socket.9

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.Dd December 14, 2006
.Dt SOCKET 9
.Os
.Sh NAME
.Nm socket
.Nd "kernel socket interface"
.Sh SYNOPSIS
.In sys/socket.h
.In sys/socketvar.h
.Ft int
.Fn sobind "struct socket *so" "struct sockaddr *nam" "struct thread *td"
.Ft void
.Fn soclose "struct socket *so"
.Ft int
.Fn soconnect "struct socket *so" "struct sockaddr *nam" "struct thread *td"
.Ft int
.Fo socreate
.Fa "int dom" "struct socket **aso" "int type" "int proto"
.Fa "struct ucred *cred" "struct thread *td"
.Fc
.Ft int
.Fn sogetopt "struct socket *so" "struct sockopt *sopt"
.Ft int
.Fo soreceive
.Fa "struct socket *so" "struct sockaddr **psa" "struct uio *uio"
.Fa "struct mbuf **mp0" "struct mbuf **controlp" "int *flagsp"
.Fc
.Ft int
.Fn sosetopt "struct socket *so" "struct sockopt *sopt"
.Ft int
.Fo sosend
.Fa "struct socket *so" "struct sockaddr *addr" "struct uio *uio"
.Fa "struct mbuf *top" "struct mbuf *control" "int flags" "struct thread *td"
.Fc
.Ft int
.Fn soshutdown "struct socket *so" "int how"
.Sh DESCRIPTION
The kernel
.Nm
programming interface permits in-kernel consumers to interact with
local and network socket objects in a manner similar to that permitted using
the
.Xr 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
.Vt "struct socket"
pointers.
.Pp
Except where otherwise indicated,
.Nm
functions may sleep, and are not appropriate for use in an
.Xr ithread 9
context or while holding non-sleepable kernel locks.
.Ss Creating and Destroying Sockets
A new socket may be created using
.Fn socreate .
As with
.Xr socket 2 ,
arguments specify the requested domain, type, and protocol via
.Fa dom , type ,
and
.Fa proto .
The socket is returned via
.Fa aso
on success.
In addition, the credential used to authorize operations associated with the
socket will be passed via
.Fa cred
(and will be cached for the lifetime of the socket), and the thread
performing the operation via
.Fa td .
.Em Warning :
authorization of the socket creation operation will be performed
using the thread credential for some protocols (such as raw sockets).
.Pp
Sockets may be closed and freed using
.Fn soclose ,
which has similar semantics to
.Xr close 2 .
.Ss Connections and Addresses
The
.Fn sobind
function is equivalent to the
.Xr bind 2
system call, and binds the socket
.Fa so
to the address
.Fa nam .
The operation would be authorized using the credential on thread
.Fa td .
.Pp
The
.Fn soconnect
function is equivalent to the
.Xr connect 2
system call, and initiates a connection on the socket
.Fa so
to the address
.Fa nam .
The operation will be authorized using the credential on thread
.Fa td .
Unlike the user system call,
.Fn soconnect
returns immediately; the caller may
.Xr msleep 9
on
.Fa so->so_timeo
while holding the socket mutex and waiting for the
.Dv SS_ISCONNECTING
flag to clear or
.Fa so->so_error
to become non-zero.
If
.Fn soconnect
fails, the caller must manually clear the
.Dv SS_ISCONNECTING
flag.
.Pp
The
.Fn soshutdown
function is equivalent to the
.Xr shutdown 2
system call, and causes part or all of a connection on a socket to be closed
down.
.Ss Socket Options
The
.Fn sogetopt
function is equivalent to the
.Xr getsockopt 2
system call, and retrieves a socket option on socket
.Fa so .
The
.Fn sosetopt
function is equivalent to the
.Xr setsockopt 2
system call, and sets a socket option on socket
.Fa so .
.Pp
The second argument in both
.Fn sogetopt
and
.Fn sosetopt
is the
.Fa sopt
pointer to a
.Vt "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:
.Bl -tag -width ".Va sopt_valsize"
.It Va sopt_dir
Set to
.Dv SOPT_SET
or
.Dv SOPT_GET
depending on whether this is a get or set operation.
.It Va sopt_level
Specify the level in the network stack the operation is targeted at; for
example,
.Dv SOL_SOCKET .
.It Va sopt_name
Specify the name of the socket option to set.
.It Va sopt_val
Kernel space pointer to the argument value for the socket option.
.It Va sopt_valsize
Size of the argument value in bytes.
.El
.Ss Socket I/O
The
.Fn soreceive
function is equivalent to the
.Xr recvmsg 2
system call, and attempts to receive bytes of data from the socket
.Fa so ,
optionally blocking awaiting for data if none is ready to read.
Data may be retrieved directly to kernel or user memory via the
.Fa uio
argument, or as an mbuf chain returned to the caller via
.Fa mp0 ,
avoiding a data copy.
Only one of the
.Fa uio
or
.Fa mp0
pointers may be
.Pf non- Dv NULL .
The caller may optionally retrieve a socket address on a protocol with the
.Dv PR_ADDR
capability by providing storage via
.Pf non- Dv NULL
.Fa psa
argument.
The caller may optionally retrieve control data mbufs via a
.Pf non- Dv NULL
.Fa controlp
argument.
Optional flags may be passed to
.Fn soreceive
via a
.Pf non- Dv NULL
.Fa flagsp
argument, and use the same flag name space as the
.Xr recvmsg 2
system call.
.Pp
The
.Fn sosend
function is equivalent to the
.Xr sendmsg 2
system call, and attempts to send bytes of data via the socket
.Fa so ,
optionally blocking if data cannot be immediately sent.
Data may be sent directly from kernel or user memory via the
.Fa uio
argument, or as an mbuf chain via
.Fa top ,
avoiding a data copy.
Only one of the
.Fa uio
or
.Fa top
pointers may be
.Pf non- Dv NULL .
An optional destination address may be specified via a
.Pf non- Dv NULL
.Fa addr
argument, which may result in an implicit connect if supported by the
protocol.
The caller may optionally send control data mbufs via a
.Pf non- Dv NULL
.Fa control
argument.
Flags may be passed to
.Fn sosend
using the
.Fa flags
argument, and use the same flag name space as the
.Xr sendmsg 2
system call.
.Pp
Kernel callers running in
.Xr ithread 9
context, or with a mutex held, will wish to use non-blocking sockets and pass
the
.Dv MSG_DONTWAIT
flag in order to prevent these functions from sleeping.
.Sh SEE ALSO
.Xr bind 2 ,
.Xr close 2 ,
.Xr connect 2 ,
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr setsockopt 2 ,
.Xr shutdown 2 ,
.Xr socket 2 ,
.Xr ng_ksocket 4 ,
.Xr ithread 9 ,
.Xr msleep 9 ,
.Xr ucred 9
.Sh HISTORY
The
.Xr socket 2
system call appeared in
.Bx 4.2 .
This manual page was introduced in
.Fx 7.0 .
.Sh AUTHORS
This manual page was written by
.An Robert Watson .
.Sh BUGS
The use of explicitly passed credentials, credentials hung from explicitly
passed threads, the credential on
.Dv curthread ,
and the cached credential from
socket creation time is inconsistent, and may lead to unexpected behaviour.
It is possible that several of the
.Fa td
arguments should be
.Fa cred
arguments, or simply not be present at all.
.Pp
The caller may need to manually clear
.Dv SS_ISCONNECTING
if
.Fn soconnect
returns an error.
.Pp
The
.Dv MSG_DONTWAIT
flag is not implemented for
.Fn sosend ,
and may not always work with
.Fn soreceive
when zero copy sockets are enabled.
.Pp
This manual page does not describe how to register socket upcalls or monitor
a socket for readability/writability without using blocking I/O.
.Pp
The
.Fn soref
and
.Fn sorele
functions are not described, and in most cases should not be used, due to
confusing and potentially incorrect interactions when
.Fn sorele
is last called after
.Fn soclose .