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6.  Socket Interfaces Advanced Socket Topics Interrupt-Driven Socket I/O  Previous   Contents   Next 

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) command, 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.

You can also enable reception of SIGURG and SIGIO 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 the following example.

Example 6-13 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");
#include <wait.h>
		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, set up de-multiplexing with the protocol argument.

Using raw sockets in the Internet family to implement a new protocol on IP ensures that the socket only receives packets for the specified protocol. 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);

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

Address Binding

For addressing, TCP and UDP use a 4-tuple of:

  • Local IP address

  • Local port number

  • Foreign IP address

  • Foreign port number

TCP requires these 4-tuples to be unique. UDP does not. User programs do not always know proper values to use for the local address and local port, because 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, leave parts of the address unspecified and let the system assign the parts appropriately when needed. Various portions of these tuples can be specified by various parts of the sockets API:


Local address or local port or both


Foreign address and foreign port

A call to accept(3SOCKET) retrieves connection information from a foreign client. This causes the local address and port to be specified to the system even though the caller of accept(3SOCKET) didn't specify anything. The foreign address and port are 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) assigns an ephemeral port number.

A service that resides at a particular port can bind(3SOCKET) itself to its port and leave the local address unspecified if the service does not require local address information. The local address is set to in6addr_any, a variable with a constant value in <netinet/in.h>. If the local port does not need to 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 that was returned by a call to getaddrinfo(3SOCKET) to a socket and leaves the local address unspecified. For example:

#include <sys/types.h>
#include <netinet/in.h>
    struct addrinfo		*aip;
    if (bind(sock, aip->ai_addr, aip->ai_addrlen) == -1) {
        (void) close(sock);
        return (-1);

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. To allow only hosts on a specific network to connect to the server, 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, you could use bind as follows:

bzero (&sin, sizeof (sin));
(void) inet_pton (AF_INET6, ":ffff:", 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:

  • 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 port number is not currently bound to some other socket.

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

In certain cases, the algorithm used by the system to select port numbers is unsuitable for an application due to the two-step creation process for associations. 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.

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