2.3.t

早期freebsd实现

Sending and receiving data
.PP
Messages may be sent from a socket by:
.DS
cc = sendto(s, buf, len, flags, to, tolen);
result int cc; int s; caddr_t buf; int len, flags; caddr_t to; int tolen;
.DE
if the socket is not connected or:
.DS
cc = send(s, buf, len, flags);
result int cc; int s; caddr_t buf; int len, flags;
.DE
if the socket is connected.
The corresponding receive primitives are:
.DS
msglen = recvfrom(s, buf, len, flags, from, fromlenaddr);
result int msglen; int s; result caddr_t buf; int len, flags;
result caddr_t from; result int *fromlenaddr;
.DE
and
.DS
msglen = recv(s, buf, len, flags);
result int msglen; int s; result caddr_t buf; int len, flags;
.DE
.PP
In the unconnected case,
the parameters \fIto\fP and \fItolen\fP
specify the destination or source of the message, while
the \fIfrom\fP parameter stores the source of the message,
and \fI*fromlenaddr\fP initially gives the size of the \fIfrom\fP
buffer and is updated to reflect the true length of the \fIfrom\fP
address.
.PP
All calls cause the message to be received in or sent from
the message buffer of length \fIlen\fP bytes, starting at address \fIbuf\fP.
The \fIflags\fP specify
peeking at a message without reading it or sending or receiving
high-priority out-of-band messages, as follows:
.DS
._d
#define	MSG_PEEK	0x1	/* peek at incoming message */
#define	MSG_OOB	0x2	/* process out-of-band data */
.DE
.NH 4
Scatter/gather and exchanging access rights
.PP
It is possible scatter and gather data and to exchange access rights
with messages.  When either of these operations is involved,
the number of parameters to the call becomes large.
Thus the system defines a message header structure, in \fI<sys/socket.h>\fP,
which can be
used to conveniently contain the parameters to the calls:
.DS
.if t .ta .5i 1.25i 2i 2.7i
.if n ._f
struct msghdr {
	caddr_t	msg_name;		/* optional address */
	int	msg_namelen;	/* size of address */
	struct	iov *msg_iov;	/* scatter/gather array */
	int	msg_iovlen;		/* # elements in msg_iov */
	caddr_t	msg_accrights;	/* access rights sent/received */
	int	msg_accrightslen;	/* size of msg_accrights */
};
.DE
Here \fImsg_name\fP and \fImsg_namelen\fP specify the source or destination
address if the socket is unconnected; \fImsg_name\fP may be given as
a null pointer if no names are desired or required.
The \fImsg_iov\fP and \fImsg_iovlen\fP describe the scatter/gather
locations, as described in section 2.1.3.
Access rights to be sent along with the message are specified
in \fImsg_accrights\fP, which has length \fImsg_accrightslen\fP.
In the ``unix'' domain these are an array of integer descriptors,
taken from the sending process and duplicated in the receiver.
.PP
This structure is used in the operations \fIsendmsg\fP and \fIrecvmsg\fP:
.DS
sendmsg(s, msg, flags);
int s; struct msghdr *msg; int flags;

msglen = recvmsg(s, msg, flags);
result int msglen; int s; result struct msghdr *msg; int flags;
.DE
.NH 4
Using read and write with sockets
.PP
The normal UNIX \fIread\fP and \fIwrite\fP calls may be
applied to connected sockets and translated into \fIsend\fP and \fIreceive\fP
calls from or to a single area of memory and discarding any rights
received.  A process may operate on a virtual circuit socket, a terminal
or a file with blocking or non-blocking input/output
operations without distinguishing the descriptor type.
.NH 4
Shutting down halves of full-duplex connections
.PP
A process that has a full-duplex socket such as a virtual circuit
and no longer wishes to read from or write to this socket can
give the call:
.DS
shutdown(s, direction);
int s, direction;
.DE
where \fIdirection\fP is 0 to not read further, 1 to not
write further, or 2 to completely shut the connection down.
If the underlying protocol supports unidirectional or bidirectional shutdown,
this indication will be passed to the peer.
For example, a shutdown for writing might produce an end-of-file
condition at the remote end.
.NH 4
Socket and protocol options
.PP
Sockets, and their underlying communication protocols, may
support \fIoptions\fP.  These options may be used to manipulate
implementation- or protocol-specific facilities. 
The \fIgetsockopt\fP
and \fIsetsockopt\fP calls are used to control options:
.DS
getsockopt(s, level, optname, optval, optlen)
int s, level, optname; result caddr_t optval; result int *optlen;

setsockopt(s, level, optname, optval, optlen)
int s, level, optname; caddr_t optval; int optlen;
.DE
The option \fIoptname\fP is interpreted at the indicated
protocol \fIlevel\fP for socket \fIs\fP.  If a value is specified
with \fIoptval\fP and \fIoptlen\fP, it is interpreted by
the software operating at the specified \fIlevel\fP.  The \fIlevel\fP
SOL_SOCKET is reserved to indicate options maintained
by the socket facilities.  Other \fIlevel\fP values indicate
a particular protocol which is to act on the option request;
these values are normally interpreted as a ``protocol number''.
.NH 3
UNIX domain
.PP
This section describes briefly the properties of the UNIX communications
domain.
.NH 4
Types of sockets
.PP
In the UNIX domain,
the SOCK_STREAM abstraction provides pipe-like
facilities, while SOCK_DGRAM provides (usually)
reliable message-style communications.
.NH 4
Naming
.PP
Socket names are strings and may appear in the UNIX file
system name space through portals\(dg.
.FS
\(dg The 4.3BSD implementation of the UNIX domain embeds
bound sockets in the UNIX file system name space;
this may change in future releases.
.FE
.NH 4
Access rights transmission
.PP
The ability to pass UNIX descriptors with messages in this domain
allows migration of service within the system and allows
user processes to be used in building system facilities.
.NH 3
INTERNET domain
.PP
This section describes briefly how the Internet domain is
mapped to the model described in this section.  More
information will be found in the document describing the
network implementation in 4.3BSD.
.NH 4
Socket types and protocols
.PP
SOCK_STREAM is supported by the Internet TCP protocol;
SOCK_DGRAM by the UDP protocol.
Each is layered atop the transport-level Internet Protocol (IP).
The Internet Control Message Protocol is implemented atop/beside IP
and is accessible via a raw socket.
The SOCK_SEQPACKET
has no direct Internet family analogue; a protocol
based on one from the XEROX NS family and layered on
top of IP could be implemented to fill this gap.
.NH 4
Socket naming
.PP
Sockets in the Internet domain have names composed of the 32 bit
Internet address, and a 16 bit port number.
Options may be used to
provide IP source routing or security options.
The 32-bit address is composed of network and host parts;
the network part is variable in size and is frequency encoded.
The host part may optionally be interpreted as a subnet field
plus the host on subnet; this is is enabled by setting a network address
mask at boot time.
.NH 4
Access rights transmission
.PP
No access rights transmission facilities are provided in the Internet domain.
.NH 4
Raw access
.PP
The Internet domain allows the super-user access to the raw facilities
of IP.
These interfaces are modeled as SOCK_RAW sockets.
Each raw socket is associated with one IP protocol number,
and receives all traffic received for that protocol.
This allows administrative and debugging
functions to occur,
and enables user-level implementations of special-purpose protocols
such as inter-gateway routing protocols.