This chapter is from the book
This chapter is from the book
This chapter is from the book
3.5 sockaddr Structure
Addressing information for an interface consists of more than a single host address. Net/3 maintains host, broadcast, and network masks in structures derived from a generic sockaddr structure. By using a generic structure, hardware and protocol-specific addressing details are hidden from the interface layer.
Figure 3.17 shows the current definition of the structure as well as the definition from earlier BSD releases—an osockaddr structure.
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Figure 3.17. sockaddr and osockaddr structures.
Figure 3.18 illustrates the organization of these structures.
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Figure 3.18. sockaddr and osockaddr structures (sa_ prefix dropped).
In many figures, we omit the common prefix in member names. In this case, we've dropped the sa_ prefix.
sockaddr structure
102-124
Every protocol has its own address format. Net/3 handles generic addresses in a sockaddr structure. sa_len specifies the length of the address (OSI and Unix domain protocols have variable-length addresses) and sa_family specifies the type of address. Figure 3.19 lists the address family constants that we encounter.
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Figure 3.19. sa_family constants.
The contents of a sockaddr when AF_UNSPEC is specified depends on the context. In most cases, it contains an Ethernet hardware address.
The sa_len and sa_family members allow protocol-independent code to manipulate variable-length sockaddr structures from multiple protocol families. The remaining member, sa_data, contains the address in a protocol-dependent format. sa_data is defined to be an array of 14 bytes, but when the sockaddr structure overlays a larger area of memory sa_data may be up to 253 bytes long. sa_len is only a single byte, so the size of the entire address including sa_len and sa_family must be less than 256 bytes.
This is a common C technique that allows the programmer to consider the last member in a structure to have a variable length.
Each protocol defines a specialized sockaddr structure that duplicates the sa_len and sa_family members but defines the sa_data member as required for that protocol. The address stored in sa_data is a transport address; it contains enough information to identify multiple communication end points on the same host. In Chapter 6 we look at the Internet address structure sockaddr_in, which consists of an IP address and a port number.
osockaddr structure
271-274
The osockaddr structure is the definition of a sockaddr before the 4.3BSD Reno release. Since the length of an address was not explicitly available in this definition, it was not possible to write protocol-independent code to handle variable-length addresses. The desire to include the OSI protocols, which utilize variable-length addresses, motivated the change in the sockaddr definition seen in Net/3. The osockaddr structure is supported for binary compatibility with previously compiled programs.
We have omitted the binary compatibility code from this text.