Advanced Topics

For most programmers, the mechanisms already described are enough to build distributed applications. Others need some of the additional features in this section.

Out-of-Band Data

The stream socket abstraction includes out-of-band data. Out-of-band data is a logically independent transmission channel between a pair of connected stream sockets. Out-of-band data is delivered independent of normal data. The out-of-band data facilities must support the reliable delivery of at least one out-of-band message at a time. This message can contain at least one byte of data, and at least one message can be pending delivery at any time.

For communications protocols that support only in-band signaling (that is, urgent data is delivered in sequence with normal data), the message is extracted from the normal data stream and stored separately. This lets users choose between receiving the urgent data in order and receiving it out of sequence, without having to buffer the intervening data.

You can peek (with MSG_PEEK) at out-of-band data. If the socket has a process group, a SIGURG signal is generated when the protocol is notified of its existence. A process can set the process group or process ID to be informed by SIGURG with the appropriate fcntl(2) call, as described in "Interrupt-Driven Socket I/O" for SIGIO. If multiple sockets have out-of-band data waiting delivery, a select(3C) call for exceptional conditions can be used to determine the sockets with such data pending.

A logical mark is placed in the data stream at the point at which the out-of-band data was sent. The remote login and remote shell applications use this facility to propagate signals between client and server processes. When a signal is received, all data up to the mark in the data stream is discarded.

To send an out-of-band message, the MSG_OOB flag is applied to send(3SOCKET) or sendto(3SOCKET). To receive out-of-band data, specify MSG_OOB to recvfrom(3SOCKET) or recv(3SOCKET) (unless out-of-band data is taken in line, in which case the MSG_OOB flag is not needed). The SIOCATMARK ioctl(2) tells whether the read pointer currently points at the mark in the data stream:

int yes;
ioctl(s, SIOCATMARK, &yes);

If yes is 1 on return, the next read returns data after the mark. Otherwise, assuming out-of-band data has arrived, the next read provides data sent by the client before sending the out-of-band signal. The routine in the remote login process that flushes output on receipt of an interrupt or quit signal is shown in Example 2-12. This code reads the normal data up to the mark (to discard it), then reads the out-of-band byte.

A process can also read or peek at the out-of-band data without first reading up to the mark. This is more difficult when the underlying protocol delivers the urgent data in-band with the normal data, and only sends notification of its presence ahead of time (for example, TCP, the protocol used to provide socket streams in the Internet family). With such protocols, the out-of-band byte might not yet have arrived when a recv(3SOCKET) is done with the MSG_OOB flag. In that case, the call returns the error of EWOULDBLOCK. Also, there might be enough in-band data in the input buffer that normal flow control prevents the peer from sending the urgent data until the buffer is cleared. The process must then read enough of the queued data before the urgent data can be delivered.

---

Example 2-12 Flushing Terminal I/O on Receipt of Out-of-Band Data

#include <sys/ioctl.h>
#include <sys/file.h>
...
oob()
{
		int out = FWRITE;
		char waste[BUFSIZ];
		int mark = 0;
 
		/* flush local terminal output */
		ioctl(1, TIOCFLUSH, (char *) &out);
		while(1) {
			if (ioctl(rem, SIOCATMARK, &mark) == -1) {
				perror("ioctl");
				break;
			}
			if (mark)
				break;
			(void) read(rem, waste, sizeof waste);
		}
		if (recv(rem, &mark, 1, MSG_OOB) == -1) {
			perror("recv");
			...
		}
		...
}

---

There is also a facility to retain the position of urgent in-line data in the socket stream. This is available as a socket-level option, SO_OOBINLINE. See the getsockopt(3SOCKET) manpage for usage. With this option, the position of urgent data (the mark) is retained, but the urgent data immediately follows the mark in the normal data stream returned without the MSG_OOB flag. Reception of multiple urgent indications causes the mark to move, but no out-of-band data are lost.

Nonblocking Sockets

Some applications require sockets that do not block. For example, requests that cannot complete immediately and would cause the process to be suspended (awaiting completion) are not executed. An error code would be returned. After a socket is created and any connection to another socket is made, it can be made nonblocking by issuing a fcntl(2) call, as shown in Example 2-13.

---

Example 2-13 Set Nonblocking Socket

#include <fcntl.h>
#include <sys/file.h>
...
int fileflags;
int s;
...
s = socket(AF_INET6, SOCK_STREAM, 0);
...
if (fileflags = fcntl(s, F_GETFL, 0) == -1)
		perror("fcntl F_GETFL");
		exit(1);
}
if (fcntl(s, F_SETFL, fileflags | FNDELAY) == -1)
		perror("fcntl F_SETFL, FNDELAY");
		exit(1);
}
...

---

When doing I/O on a nonblocking socket, check for the error EWOULDBLOCK (in errno.h), which occurs when an operation would normally block. accept(3SOCKET), connect(3SOCKET), send(3SOCKET), recv(3SOCKET), read(2), and write(2) can all return EWOULDBLOCK. If an operation such as a send(3SOCKET) cannot be done in its entirety, but partial writes work (such as when using a stream socket), the data that can be sent immediately are processed, and the return value is the amount actually sent.

Asynchronous Socket I/O

Asynchronous communication between processes is required in applications that handle multiple requests simultaneously. Asynchronous sockets must be SOCK_STREAM type. To make a socket asynchronous, you issue a fcntl(2) call, as shown in Example 2-14.

---

Example 2-14 Making a Socket Asynchronous

#include <fcntl.h>
#include <sys/file.h>
...
int fileflags;
int s;
...
s = socket(AF_INET6, SOCK_STREAM, 0);
...
if (fileflags = fcntl(s, F_GETFL ) == -1)
		perror("fcntl F_GETFL");
		exit(1);
}
if (fcntl(s, F_SETFL, fileflags | FNDELAY | FASYNC) == -1)
		perror("fcntl F_SETFL, FNDELAY | FASYNC");
		exit(1);
}
...

---

After sockets are initialized, connected, and made asynchronous, communication is similar to reading and writing a file asynchronously. A send(3SOCKET), write(2), recv(3SOCKET), or read(2) initiates a data transfer. A data transfer is completed by a signal-driven I/O routine, described in the next section.

Interrupt-Driven Socket I/O

The SIGIO signal notifies a process when a socket (actually any file descriptor) has finished a data transfer. The steps in using SIGIO are:

• Set up a SIGIO signal handler with thesignal(3C) or sigvec(3UCB) calls.

• Use fcntl(2) to set the process ID or process group ID to route the signal to its own process ID or process group ID (the default process group of a socket is group 0).

• Convert the socket to asynchronous, as shown in "Asynchronous Socket I/O".

Example 2-15 shows some sample code to allow a given process to receive information on pending requests as they occur for a socket. With the addition of a handler for SIGURG, this code can also be used to prepare for receipt of SIGURG signals.

---

Example 2-15 Asynchronous Notification of I/O Requests

#include <fcntl.h>
#include <sys/file.h>
 ...
signal(SIGIO, io_handler);
/* Set the process receiving SIGIO/SIGURG signals to us. */
if (fcntl(s, F_SETOWN, getpid()) < 0) {
		perror("fcntl F_SETOWN");
		exit(1);
}

---

Signals and Process Group ID

For SIGURG and SIGIO, each socket has a process number and a process group ID. These values are initialized to zero, but can be redefined at a later time with the F_SETOWN fcntl(2), as in the previous example. A positive third argument to fcntl(2) sets the socket's process ID. A negative third argument to fcntl(2) sets the socket's process group ID. The only allowed recipient of SIGURG and SIGIO signals is the calling process. A similar fcntl(2), F_GETOWN, returns the process number of a socket.

Reception of SIGURG and SIGIO can also be enabled by using ioctl(2) to assign the socket to the user's process group:

/* oobdata is the out-of-band data handling routine */
sigset(SIGURG, oobdata);
int pid = -getpid();
if (ioctl(client, SIOCSPGRP, (char *) &pid) < 0) {
		perror("ioctl: SIOCSPGRP");
}

Another signal that is useful in server processes is SIGCHLD. This signal is delivered to a process when any child process changes state. Normally, servers use the signal to "reap" child processes that have exited without explicitly awaiting their termination or periodically polling for exit status. For example, the remote login server loop shown previously can be augmented as shown in Example 2-16.

---

Example 2-16 SIGCHLD Signal

int reaper();
...
sigset(SIGCHLD, reaper);
listen(f, 5);
while (1) {
		int g, len = sizeof from;
		g = accept(f, (struct sockaddr *) &from, &len);
		if (g < 0) {
			if (errno != EINTR)
				syslog(LOG_ERR, "rlogind: accept: %m");
			continue;
		}
		...
}
 
#include <wait.h>
 
reaper()
{
		int options;
		int error;
		siginfo_t info;
 
		options = WNOHANG | WEXITED;
		bzero((char *) &info, sizeof(info));
		error = waitid(P_ALL, 0, &info, options);
}

---

If the parent server process fails to reap its children, zombie processes result.

Selecting Specific Protocols

If the third argument of the socket(3SOCKET) call is 0, socket(3SOCKET) selects a default protocol to use with the returned socket of the type requested. The default protocol is usually correct, and alternate choices are not usually available. When using "raw" sockets to communicate directly with lower-level protocols or hardware interfaces, it can be important for the protocol argument to set up de-multiplexing. For example, raw sockets in the Internet family can be used to implement a new protocol on IP, and the socket receives packets only for the protocol specified. To obtain a particular protocol, determine the protocol number as defined in the protocol family. For the Internet family, use one of the library routines discussed in "Standard Routines", such as getprotobyname(3SOCKET):

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
 ...
pp = getprotobyname("newtcp");
s = socket(AF_INET6, SOCK_STREAM, pp->p_proto);

This results in a socket s using a stream-based connection, but with protocol type of newtcp instead of the default tcp.

Address Binding

TCP and UDP use a 4-tuple of local IP address, local port number, foreign IP address, and foreign port number to do their addressing. TCP requires these 4-tuples to be unique. UDP does not. It is unrealistic to expect user programs to always know proper values to use for the local address and local port, since a host can reside on multiple networks and the set of allocated port numbers is not directly accessible to a user. To avoid these problems, you can leave parts of the address unspecified and let the system assign the parts appropriately when needed. Various portions of these tuples may be specified by various parts of the sockets API.

bind(3SOCKET)

Local address or local port or both

connect(3SOCKET)

Foreign address and foreign port

A call to accept(3SOCKET) retrieves connection information from a foreign client, so it causes the local address and port to be specified to the system (even though the caller of accept(3SOCKET) didn't specify anything), and the foreign address and port to be returned.

A call to listen(3SOCKET) can cause a local port to be chosen. If no explicit bind(3SOCKET) has been done to assign local information, listen(3SOCKET) causes an ephemeral port number to be assigned.

A service that resides at a particular port, but which does not care what local address is chosen, can bind(3SOCKET) itself to its port and leave the local address unspecified (set to in6addr_any, a variable with a constant value in <netinet/in.h>). If the local port need not be fixed, a call to listen(3SOCKET) causes a port to be chosen. Specifying an address of in6addr_any or a port number of 0 is known as wildcarding. (For AF_INET, INADDR_ANY is used in place of in6addr_any.)

The wildcard address simplifies local address binding in the Internet family. The sample code below binds a specific port number, MYPORT, to a socket, and leaves the local address unspecified.

---

Example 2-17 Bind Port Number to Socket

#include <sys/types.h>
#include <netinet/in.h>
...
struct sockaddr_in6 sin;
...
		s = socket(AF_INET6, SOCK_STREAM, 0);
		bzero (&sin6, sizeof (sin6));
		sin.sin6_family = AF_INET6;
		sin.sin6_addr.s6_addr = in6addr_any;
		sin.sin6_port = htons(MYPORT);
		bind(s, (struct sockaddr *) &sin, sizeof sin);

---

Each network interface on a host typically has a unique IP address. Sockets with wildcard local addresses can receive messages directed to the specified port number and sent to any of the possible addresses assigned to a host. For example, if a host has two interfaces with addresses 128.32.0.4 and 10.0.0.78, and a socket is bound as in Example 2-17, the process can accept connection requests addressed to 128.32.0.4 or 10.0.0.78. To allow only hosts on a specific network to connect to it, a server binds the address of the interface on the appropriate network.

Similarly, a local port number can be left unspecified (specified as 0), in which case the system selects a port number. For example, to bind a specific local address to a socket, but to leave the local port number unspecified:

bzero (&sin, sizeof (sin));
(void) inet_pton (AF_INET6, ":ffff:127.0.0.1", sin.sin6_addr.s6_addr);
sin.sin6_family = AF_INET6;
sin.sin6_port = htons(0);
bind(s, (struct sockaddr *) &sin, sizeof sin);

The system uses two criteria to select the local port number:

• The first is that Internet port numbers less than 1024 (IPPORT_RESERVED) are reserved for privileged users (that is, the superuser). Nonprivileged users can use any Internet port number greater than 1024. The largest Internet port number is 65535.

• The second criterion is that the port number is not currently bound to some other socket.

The port number and IP address of the client is found through either accept(3SOCKET) (the from result) or getpeername(3SOCKET).

In certain cases, the algorithm used by the system to select port numbers is unsuitable for an application. This is because associations are created in a two-step process. For example, the Internet file transfer protocol specifies that data connections must always originate from the same local port. However, duplicate associations are avoided by connecting to different foreign ports. In this situation, the system would disallow binding the same local address and port number to a socket if a previous data connection's socket still existed. To override the default port selection algorithm, you must perform an option call before address binding:

 ...
int on = 1;
...
setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, sizeof on);
bind(s, (struct sockaddr *) &sin, sizeof sin);

With this call, local addresses already in use can be bound. This does not violate the uniqueness requirement, because the system still verifies at connect time that any other sockets with the same local address and port do not have the same foreign address and port. If the association already exists, the error EADDRINUSE is returned.

Using Multicast

IP multicasting is only supported on AF_INET6 and AF_INETsockets of type SOCK_DGRAM and SOCK_RAW, and only on subnetworks for which the interface driver supports multicasting.

Sending IPv4 Multicast Datagrams

To send a multicast datagram, specify an IP multicast address in the range 224.0.0.0 to 239.255.255.255 as the destination address in a sendto(3SOCKET) call.

By default, IP multicast datagrams are sent with a time-to-live (TTL) of 1, which prevents them from being forwarded beyond a single subnetwork. The socket option IP_MULTICAST_TTL allows the TTL for subsequent multicast datagrams to be set to any value from 0 to 255, to control the scope of the multicasts:

    u_char ttl;
    setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl,sizeof(ttl))

Multicast datagrams with a TTL of 0 are not transmitted on any subnet, but can be delivered locally if the sending host belongs to the destination group and if multicast loopback has not been disabled on the sending socket (see below). Multicast datagrams with TTL greater than one can be delivered to more than one subnet if one or more multicast routers are attached to the first-hop subnet. To provide meaningful scope control, the multicast routers support the notion of TTL "thresholds", which prevent datagrams with less than a certain TTL from traversing certain subnets. The thresholds enforce the following convention that multicast datagrams with initial TTL:

0	Are restricted to the same host
1	Are restricted to the same subnet
32	Are restricted to the same site
64	Are restricted to the same region
128	Are restricted to the same continent
255	Are unrestricted in scope

"Sites" and "regions" are not strictly defined, and sites can be subdivided into smaller administrative units, as a local matter.

An application can choose an initial TTL other than the ones listed above. For example, an application might perform an "expanding-ring search" for a network resource by sending a multicast query, first with a TTL of 0, and then with larger and larger TTLs, until a reply is received, using (for example) the TTL sequence 0, 1, 2, 4, 8, 16, 32.

The multicast router refuses to forward any multicast datagram with a destination address between 224.0.0.0 and 224.0.0.255, inclusive, regardless of its TTL. This range of addresses is reserved for the use of routing protocols and other low-level topology discovery or maintenance protocols, such as gateway discovery and group membership reporting.

Each multicast transmission is sent from a single network interface, even if the host has more than one multicast-capable interface. (If the host is also a multicast router and the TTL is greater than 1, a multicast can be forwarded to interfaces other than originating interface.) A socket option is available to override the default for subsequent transmissions from a given socket:

    struct in_addr addr;
    setsockopt(sock, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr))

where addr is the local IP address of the outgoing interface you want. Revert to the default interface by specifying the address INADDR_ANY. The local IP address of an interface is obtained with the SIOCGIFCONF ioctl. To determine if an interface supports multicasting, fetch the interface flags with the SIOCGIFFLAGS ioctl and test if the IFF_MULTICAST flag is set. (This option is intended primarily for multicast routers and other system services specifically concerned with internet topology.)

If a multicast datagram is sent to a group to which the sending host itself belongs (on the outgoing interface), a copy of the datagram is, by default, looped back by the IP layer for local delivery. Another socket option gives the sender explicit control over whether or not subsequent datagrams are looped back:

    u_char loop;
    setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop))  

where loop is 0 to disable loopback, and 1 to enable loopback. This option provides a performance benefit for applications that have only one instance on a single host (such as a router or a mail demon), by eliminating the overhead of receiving their own transmissions. It should not normally be used by applications that can have more than one instance on a single host (such as a conferencing program) or for which the sender does not belong to the destination group (such as a time querying program).

If the sending host belongs to the destination group on another interface, a multicast datagram sent with an initial TTL greater than 1 can be delivered to the sending host on the other interface. The loopback control option has no effect on such delivery.

Receiving IPv4 Multicast Datagrams

Before a host can receive IP multicast datagrams, it must become a member of one, or more, IP multicast group. A process can ask the host to join a multicast group by using the following socket option:

    struct ip_mreq mreq;
    setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)) 

where mreq is the structure

    struct ip_mreq {
        struct in_addr imr_multiaddr;   /* multicast group to join */
        struct in_addr imr_interface;   /* interface to join on */
    }  

Each membership is associated with a single interface, and it is possible to join the same group on more than one interface. Specify imr_interface to be in6addr_any to choose the default multicast interface, or one of the host's local addresses to choose a particular (multicast-capable) interface.

To drop a membership, use

    struct ip_mreq mreq;
    setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq))  

where mreq contains the same values used to add the membership. The memberships associated with a socket are also dropped when the socket is closed or the process holding the socket is killed. More than one socket can claim a membership in a particular group, and the host remains a member of that group until the last claim is dropped.

Incoming multicast packets are accepted by the kernel IP layer if any socket has claimed a membership in the destination group of the datagram. Delivery of a multicast datagram to a particular socket is based on the destination port and the memberships associated with the socket (or protocol type, for raw sockets), just as with unicast datagrams. To receive multicast datagrams sent to a particular port, bind to the local port, leaving the local address unspecified (such as, INADDR_ANY).

More than one process can bind to the same SOCK_DGRAM UDP port if the bind(3SOCKET) is preceded by:

    int one = 1;
    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one))  

In this case, every incoming multicast or broadcast UDP datagram destined to the shared port is delivered to all sockets bound to the port. For backwards compatibility reasons, this does not apply to incoming unicast datagrams. Unicast datagrams are never delivered to more than one socket, regardless of how many sockets are bound to the datagram's destination port. SOCK_RAW sockets do not require the SO_REUSEADDR option to share a single IP protocol type.

The definitions required for the new, multicast-related socket options are found in <netinet/in.h>. All IP addresses are passed in network byte-order.

Sending IPv6 Multicast Datagrams

To send a multicast datagram, specify an IP multicast address in the range ff00::0/8 as the destination address in a sendto(3SOCKET) call.

By default, IP multicast datagrams are sent with a hop limit of 1, which prevents them from being forwarded beyond a single subnetwork. The socket option IPV6_MULTICAST_HOPS allows the hoplimit for subsequent multicast datagrams to be set to any value from 0 to 255, to control the scope of the multicasts:

    uint_l;
    setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &hops,sizeof(hops))

Multicast datagrams with a hoplimit of 0 are not transmitted on any subnet, but can be delivered locally if the sending host belongs to the destination group and if multicast loopback has not been disabled on the sending socket (see below). Multicast datagrams with hoplimit greater than one can be delivered to more than one subnet if one or more multicast routers are attached to the first-hop subnet. The IPv6 multicast addresses, unlike their IPv4 counterparts, contain explicit scope information encoded in the first part of the address. The defined scopes are (where X is unspecified):

ffX1::0/16

Node-local scope -- restricted to the same node

ffX2::0/16

Link-local scope

ffX5::0/16

Site-local scope

ffX8::0/16

Organization-local scope

ffXe::0/16

Global scope

An application can, separately from the scope of the multicast address, use different hoplimit values. For example, an application might perform an "expanding-ring search" for a network resource by sending a multicast query, first with a hoplimit of 0, and then with larger and larger hoplimits, until a reply is received, using (for example) the hoplimit sequence 0, 1, 2, 4, 8, 16, 32.

Each multicast transmission is sent from a single network interface, even if the host has more than one multicast-capable interface. (If the host is also a multicast router and the hoplimit is greater than 1, a multicast can be forwarded to interfaces other than originating interface.) A socket option is available to override the default for subsequent transmissions from a given socket:

    uint_t ifindex;

    ifindex = if_nametoindex )"hme3");
    setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_IF, &ifindex, sizeof(ifindex))

where ifindex is the interface index for the desired outgoing interface. Revert to the default interface by specifying the value 0.

If a multicast datagram is sent to a group to which the sending host itself belongs (on the outgoing interface), a copy of the datagram is, by default, looped back by the IP layer for local delivery. Another socket option gives the sender explicit control over whether or not subsequent datagrams are looped back:

    uint_t loop;
    setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &loop, sizeof(loop))  

where loop is 0 to disable loopback, and 1 to enable loopback. This option provides a performance benefit for applications that have only one instance on a single host (such as a router or a mail demon), by eliminating the overhead of receiving their own transmissions. It should not normally be used by applications that can have more than one instance on a single host (such as a conferencing program) or for which the sender does not belong to the destination group (such as a time querying program).

If the sending host belongs to the destination group on another interface, a multicast datagram sent with an initial hoplimit greater than 1 can be delivered to the sending host on the other interface. The loopback control option has no effect on such delivery.

Receiving IPv6 Multicast Datagrams

Before a host can receive IP multicast datagrams, it must become a member of one, or more, IP multicast group. A process can ask the host to join a multicast group by using the following socket option:

    struct ipv6_mreq mreq;
    setsockopt(sock, IPPROTO_IPV6, IPV6_JOIN_GROUP, &mreq, sizeof(mreq)) 

where mreq is the structure

    struct ipv6_mreq {
        struct in6_addr ipv6mr_multiaddr;   /* IPv6 multicast addr */
        unsigned int    ipv6mr_interface;   /* interface index */
    }  

Each membership is associated with a single interface, and it is possible to join the same group on more than one interface. Specify ipv6_interface to be 0 to choose the default multicast interface, or an interface index for one of the host's interfaces to choose that (multicast capable) interface.

To leave a group, use

    struct ipv6_mreq mreq;
    setsockopt(sock, IPPROTO_IPV6, IP_LEAVE_GROUP, &mreq, sizeof(mreq))  

where mreq contains the same values used to add the membership. The memberships associated with a socket are also dropped when the socket is closed or the process holding the socket is killed. More than one socket can claim a membership in a particular group, and the host remains a member of that group until the last claim is dropped.

Incoming multicast packets are accepted by the kernel IP layer if any socket has claimed a membership in the destination group of the datagram. Delivery of a multicast datagram to a particular socket is based on the destination port and the memberships associated with the socket (or protocol type, for raw sockets), just as with unicast datagrams. To receive multicast datagrams sent to a particular port, bind to the local port, leaving the local address unspecified (such as, INADDR_ANY).

More than one process can bind to the same SOCK_DGRAM UDP port if the bind(3SOCKET) is preceded by:

    int one = 1;
    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one))  

In this case, every incoming multicast UDP datagram destined to the shared port is delivered to all sockets bound to the port. For backwards compatibility reasons, this does not apply to incoming unicast datagrams. Unicast datagrams are never delivered to more than one socket, regardless of how many sockets are bound to the datagram's destination port. SOCK_RAW sockets do not require the SO_REUSEADDR option to share a single IP protocol type.

The definitions required for the new, multicast-related socket options are found in <netinet/in.h>. All IP addresses are passed in network byte-order.

Zero Copy and Checksum Off-load

In SunOS 5.6 and later, the TCP/IP protocol stack has been enhanced to support two new features: zero copy and TCP checksum off-load.

• Zero copy uses virtual memory MMU remapping and a copy-on-write technique to move data between the application and the kernel space.

• Checksum off-loading relies on special hardware logic to off-load the TCP checksum calculation.

---

Note -

Although zero copy and checksum off-loading are functionally independent of one another, they have to work together to obtain the optimal performance. Checksum off-loading requires hardware support from the network interface and, without this hardware support, zero copy is not enabled.

---

Zero copy requires that the applications supply page-aligned buffers before VM page remapping can be applied. Applications should use large, circular buffers on the transmit side to avoid expensive copy-on-write faults. A typical buffer allocation is sixteen 8K buffers.

Socket Options

You can set and get several options on sockets through setsockopt(3SOCKET) and getsockopt(3SOCKET); for example by changing the send or receive buffer space. The general forms of the calls are:

setsockopt(s, level, optname, optval, optlen);

and

getsockopt(s, level, optname, optval, optlen);

In some cases, such as setting the buffer sizes, these are only hints to the operating system. The operating system reserves the right to adjust the values appropriately.

Table 2-4 shows the arguments of the calls.

Table 2-4 setsockopt(3SOCKET) and getsockopt(3SOCKET) Arguments

Arguments	Description
s	Socket on which the option is to be applied
level	Specifies the protocol level, such as socket level, indicated by the symbolic constant SOL_SOCKET in sys/socket.h
optname	Symbolic constant defined in sys/socket.h that specifies the option
optval	Points to the value of the option
optlen	Points to the length of the value of the option

For getsockopt(3SOCKET), optlen is a value-result argument, initially set to the size of the storage area pointed to by optval and set on return to the length of storage used.

It is sometimes useful to determine the type (for example, stream or datagram) of an existing socket. Programs invoked by inetd(1M) can do this by using the SO_TYPE socket option and the getsockopt(3SOCKET) call:

#include <sys/types.h>
#include <sys/socket.h>
 
int type, size;
 
size = sizeof (int);
if (getsockopt(s, SOL_SOCKET, SO_TYPE, (char *) &type, &size) <0) {
 	...
}

After getsockopt(3SOCKET), type is set to the value of the socket type, as defined in sys/socket.h. For a datagram socket, type would be SOCK_DGRAM.

inetd(1M) Daemon

One of the daemons provided with the system is inetd(1M). It is invoked at start-up time, and gets the services for which it listens from the /etc/inet/inetd.conf file. The daemon creates one socket for each service listed in /etc/inet/inetd.conf, binding the appropriate port number to each socket. See the inetd(1M) man page for details.

inetd(1M) polls each socket, waiting for a connection request to the service corresponding to that socket. For SOCK_STREAM type sockets, inetd(1M) does an accept(3SOCKET) on the listening socket, fork(2)s, dup(2)s the new socket to file descriptors 0 and 1 (stdin and stdout), closes other open file descriptors, and exec(2)s the appropriate server.

The primary benefit of inetd(1M) is that services that are not in use are not taking up machine resources. A secondary benefit is that inetd(1M) does most of the work to establish a connection. The server started by inetd(1M) has the socket connected to its client on file descriptors 0 and 1, and can immediately read(2), write(2), send(3SOCKET), or recv(3SOCKET). Servers can use buffered I/O as provided by the stdio conventions, as long as they use fflush(3C) when appropriate.

getpeername(3SOCKET) returns the address of the peer (process) connected to a socket; it is useful in servers started by inetd(1M). For example, to log the Internet address (such as fec0::56:a00:20ff:fe7d:3dd2, which is conventional for representing the IPv6 address of a client), an inetd(1M) server could use the following:

    struct sockaddr_storage name;
    int namelen = sizeof (name);
    char abuf[INET6_ADDRSTRLEN];
    struct in6_addr addr6;
    struct in_addr addr;

    if (getpeername(fd, (struct sockaddr *)&name, &namelen) == -1) {
        perror("getpeername");
        exit(1);
    } else {
        addr = ((struct sockaddr_in *)&name)->sin_addr;
        addr6 = ((struct sockaddr_in6 *)&name)->sin6_addr;
        if (name.ss_family == AF_INET) {
                (void) inet_ntop(AF_INET, &addr, abuf, sizeof (abuf));
        } else if (name.ss_family == AF_INET6 && IN6_IS_ADDR_V4MAPPED(&addr6)) {
                /* this is a IPv4-mapped IPv6 address */
                IN6_MAPPED_TO_IN(&addr6, &addr);
                (void) inet_ntop(AF_INET, &addr, abuf, sizeof (abuf));
        } else if (name.ss_family == AF_INET6) {
                (void) inet_ntop(AF_INET6, &addr6, abuf, sizeof (abuf));

        }
        syslog("Connection from %s\n", abuf);
    }

Broadcasting and Determining Network Configuration

Broadcasting is not supported in IPv6. It is supported only in IPv4.

Messages sent by datagram sockets can be broadcast to reach all of the hosts on an attached network. The network must support broadcast; the system provides no simulation of broadcast in software. Broadcast messages can place a high load on a network since they force every host on the network to service them. Broadcasting is usually used for either of two reasons: to find a resource on a local network without having its address, or functions like routing require that information be sent to all accessible neighbors.

To send a broadcast message, create an Internet datagram socket:

s = socket(AF_INET, SOCK_DGRAM, 0);

and bind a port number to the socket:

sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sin, sizeof sin);

The datagram can be broadcast on only one network by sending to the network's broadcast address. A datagram can also be broadcast on all attached networks by sending to the special address INADDR_BROADCAST, defined in netinet/in.h.

The system provides a mechanism to determine a number of pieces of information (including the IP address and broadcast address) about the network interfaces on the system. The SIOCGIFCONF ioctl(2) call returns the interface configuration of a host in a single ifconf structure. This structure contains an array of ifreq structures, one for each address family supported by each network interface to which the host is connected. Example 2-18 shows these structures defined in net/if.h.

---

Example 2-18 net/if.h Header File

struct ifreq {
#define IFNAMSIZ 16
char ifr_name[IFNAMSIZ]; /* if name, e.g., "en0" */
union {
		struct sockaddr ifru_addr;
		struct sockaddr ifru_dstaddr;
		char ifru_oname[IFNAMSIZ]; /* other if name */
		struct sockaddr ifru_broadaddr;
		short ifru_flags;
		int ifru_metric;
		char ifru_data[1]; /* interface dependent data */
		char ifru_enaddr[6];
} ifr_ifru;
#define ifr_addr ifr_ifru.ifru_addr
#define ifr_dstaddr ifr_ifru.ifru_dstaddr
#define ifr_oname ifr_ifru.ifru_oname
#define ifr_broadaddr ifr_ifru.ifru_broadaddr
#define ifr_flags ifr_ifru.ifru_flags
#define ifr_metric ifr_ifru.ifru_metric
#define ifr_data ifr_ifru.ifru_data
#define ifr_enaddr ifr_ifru.ifru_enaddr
};

---

The call that obtains the interface configuration is:

/*
 * Do SIOCGIFNUM ioctl to find the number of interfaces
 *
 * Allocate space for number of interfaces found
 *
 * Do SIOCGIFCONF with allocated buffer
 *
 */
if (ioctl(s, SIOCGIFNUM, (char *)&numifs) == -1) {
        numifs = MAXIFS;
}
bufsize = numifs * sizeof(struct ifreq);
reqbuf = (struct ifreq *)malloc(bufsize);
if (reqbuf == NULL) {
        fprintf(stderr, "out of memory\n");
        exit(1);
}
ifc.ifc_buf = (caddr_t)&reqbuf[0];
ifc.ifc_len = bufsize;
if (ioctl(s, SIOCGIFCONF, (char *)&ifc) == -1) {
        perror("ioctl(SIOCGIFCONF)");
        exit(1);
}
...
}

After this call, buf contains an array of ifreq structures, one for each network to which the host is connected. These structures are ordered first by interface name, then by supported address families. ifc.ifc_len is set to the number of bytes used by the ifreq structures.

Each structure has a set of interface flags that tell whether the corresponding network is up or down, point-to-point or broadcast, and so on. Example 2-19 shows the SIOCGIFFLAGS ioctl(2) returning these flags for an interface specified by an ifreq structure.

---

Example 2-19 Obtaining Interface Flags

struct ifreq *ifr;
ifr = ifc.ifc_req;
for (n = ifc.ifc_len/sizeof (struct ifreq); --n >= 0; ifr++) {
   /*
    * Be careful not to use an interface devoted to an address
    * family other than those intended.
    */
   if (ifr->ifr_addr.sa_family != AF_INET)
      continue;
   if (ioctl(s, SIOCGIFFLAGS, (char *) ifr) < 0) {
      ...
   }
   /* Skip boring cases */
   if ((ifr->ifr_flags & IFF_UP) == 0 ||
      (ifr->ifr_flags & IFF_LOOPBACK) ||
      (ifr->ifr_flags & (IFF_BROADCAST | IFF_POINTOPOINT)) == 0)
      continue;
}

---

Example 2-20 shows the broadcast of an interface can be obtained with the SIOGGIFBRDADDR ioctl(2).

---

Example 2-20 Broadcast Address of an Interface

if (ioctl(s, SIOCGIFBRDADDR, (char *) ifr) < 0) {
		...
}
memcpy((char *) &dst, (char *) &ifr->ifr_broadaddr,
		sizeof ifr->ifr_broadaddr);

---

The SIOGGIFBRDADDR ioctl(2) can also be used to get the destination address of a point-to-point interface.

After the interface broadcast address is obtained, transmit the broadcast datagram with sendto(3SOCKET):

sendto(s, buf, buflen, 0, (struct sockaddr *)&dst, sizeof dst);

Use one sendto(3SOCKET) for each interface to which the host is connected that supports the broadcast or point-to-point addressing.