struct sockaddr_un vs. sockaddr

"struct sockaddr" is a generic definition. It's used by any socket function that requires an address.

"struct sockaddr_un" (a "Unix sockets" address) is a specific kind of address family.

The more commonly seen "struct sockaddr_in" (an "Internet socket" address) is another specific kind of address family.

The cast is what allows the sockets APIs to accept a common parameter type that will actually be any one of several different actual types.

Here's a good link that shows several different address family definitions:

http://www.cas.mcmaster.ca/~qiao/courses/cs3mh3/tutorials/socket.html

A struct sockaddr should generally only be used as the base type for a pointer. It is a structure intended to cover the common initial sequence of the members in the address family specific socket address types (struct sockaddr_un, struct sockaddr_in, struct sockaddr_in6 etc.)

The only member that you can rely on struct sockaddr having is a single sa_family_t, indicating the socket address family. The idea is that to obtain a sort of polymorphism - you can have a function that can operate on several different socket address types:

void foo(struct sockaddr *sa){    switch(sa->sa_family)    {    case AF_INET: {        struct sockaddr_in *sa_in = (struct sockaddr_in *)sa;        /* AF_INET processing */    }    case AF_UNIX: {        struct sockaddr_un *sa_un = (struct sockaddr_un *)sa;        /* AF_UNIX processing */    }/* ... */

Note though that code like the above is generally considered to break the "strict aliasing" rule in C - if you want to do that in your own code, you are supposed to use a union type:

union sockaddr {    struct sockaddr sa;    struct sockaddr_in sin;    struct sockaddr_un sun;    /* ... */};void foo(union sockaddr *sa_union){    struct sockaddr *sa = (struct sockaddr *)sa_union;    switch(sa->sa_family)    {    case AF_INET: {        struct sockaddr_in *sa_in = (struct sockaddr_in *)sa;        /* AF_INET processing */    }    case AF_UNIX: {        struct sockaddr_un *sa_un = (struct sockaddr_un *)sa;        /* AF_UNIX processing */    }/* ... */