Next: Setting Address, Up: Socket Addresses
The functions bind
and getsockname
use the generic data
type 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 bind
or getsockname
.
The one piece of information that you can get from the struct
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.
The
struct sockaddr
type 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.
char sa_data[14]
- 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_data
is 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:
AF_LOCAL
PF_LOCAL
is the name of that namespace.) See Local Namespace Details, for information about this address format.
AF_UNIX
AF_LOCAL
. Although AF_LOCAL
is
mandated by POSIX.1g, AF_UNIX
is portable to more systems.
AF_UNIX
was 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 PF_UNIX
vs. PF_LOCAL
).
AF_FILE
AF_LOCAL
, for compatibility.
(PF_FILE
is likewise a synonym for PF_LOCAL
.)
AF_INET
PF_INET
is the name of that namespace.)
See Internet Address Formats.
AF_INET6
AF_INET
, but refers to the IPv6 protocol.
(PF_INET6
is the name of the corresponding namespace.)
AF_UNSPEC
The corresponding namespace designator symbol PF_UNSPEC
exists
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.