getsockname use the generic data
struct sockaddr * to represent a pointer to a socket
address. You can't use this data type effectively to interpret an
address or construct one; for that, you must use the proper data type
for the socket's namespace.
Thus, the usual practice is to construct an address of the proper
namespace-specific type, then cast a pointer to
struct sockaddr *
when you call
The one piece of information that you can get from the
sockaddr data type is the address format designator. This tells
you which data type to use to understand the address fully.
The symbols in this section are defined in the header file sys/socket.h.
struct sockaddrtype itself has the following members:
short int sa_family
- This is the code for the address format of this address. It identifies the format of the data which follows.
- This is the actual socket address data, which is format-dependent. Its length also depends on the format, and may well be more than 14. The length 14 of
sa_datais essentially arbitrary.
Each address format has a symbolic name which starts with ‘AF_’. Each of them corresponds to a ‘PF_’ symbol which designates the corresponding namespace. Here is a list of address format names:
PF_LOCALis the name of that namespace.) See Local Namespace Details, for information about this address format.
AF_LOCALis mandated by POSIX.1g,
AF_UNIXis portable to more systems.
AF_UNIXwas the traditional name stemming from BSD, so even most POSIX systems support it. It is also the name of choice in the Unix98 specification. (The same is true for
AF_LOCAL, for compatibility. (
PF_FILEis likewise a synonym for
PF_INETis the name of that namespace.) See Internet Address Formats.
AF_INET, but refers to the IPv6 protocol. (
PF_INET6is the name of the corresponding namespace.)
The corresponding namespace designator symbol
for completeness, but there is no reason to use it in a program.
sys/socket.h defines symbols starting with ‘AF_’ for many different kinds of networks, most or all of which are not actually implemented. We will document those that really work as we receive information about how to use them.