struct sockaddr_un vs. sockaddr
Solution 1
"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
Solution 2
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 */
}
/* ... */
Pavitar
I have pursued a Diploma in Software Development , in which I have learnt multiple languages. I'm also a photography enthusiast,as it runs in the Family :) Interests: Java, Asp.Net, PHP, Angular, NodeJS, Vuejs, Embedded-C, IOT
Updated on December 19, 2020Comments
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Pavitar over 3 years
How is
struct sockaddr
different fromstruct sockaddr_un
?I know that we use these structures in client-server modules, for binding the socket to the socket address. And we use a cast operator for it to accept struct
sockaddr_un
.I want to know how different/similar they are, and why the cast operator?
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Admin almost 9 yearsProvided link is gives clear view for diff in unix and internet sockets