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(libc.info)Address Formats

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Address Formats
---------------

   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'.

 - Data Type: struct sockaddr
     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'
     This designates the address format that goes with the local
     namespace.  (`PF_LOCAL' is the name of that namespace.)  Note:
     Local Namespace Details, for information about this address
     format.

`AF_UNIX'
     This is a synonym for `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'
     This is another synonym for `AF_LOCAL', for compatibility.
     (`PF_FILE' is likewise a synonym for `PF_LOCAL'.)

`AF_INET'
     This designates the address format that goes with the Internet
     namespace.  (`PF_INET' is the name of that namespace.)  Note:
     Internet Address Formats.

`AF_INET6'
     This is similar to `AF_INET', but refers to the IPv6 protocol.
     (`PF_INET6' is the name of the corresponding namespace.)

`AF_UNSPEC'
     This designates no particular address format.  It is used only in
     rare cases, such as to clear out the default destination address
     of a "connected" datagram socket.  Note: Sending Datagrams.

     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.