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    <p>Functions that expect a pointer to <code>struct sockaddr</code> probably typecast the pointer you send them to <code>sockaddr</code> when you send them a pointer to <code>struct sockaddr_storage</code>. In that way, they access it as if it was a <code>struct sockaddr</code>. </p> <p><code>struct sockaddr_storage</code> is designed to fit in both a <code>struct sockaddr_in</code> and <code>struct sockaddr_in6</code> </p> <p>You don't create your own <code>struct sockaddr</code>, you usually create a <code>struct sockaddr_in</code> or a <code>struct sockaddr_in6</code> depending on what IP version you're using. In order to avoid trying to know what IP version you will be using, you can use a <code>struct sockaddr_storage</code> which can hold either. This will in turn be typecasted to <code>struct sockaddr</code> by the connect(), bind(), etc functions and accessed that way.</p> <p>You can see all of these structs below (the padding is implementation specific, for alignment purposes):</p> <pre><code>struct sockaddr { unsigned short sa_family; // address family, AF_xxx char sa_data[14]; // 14 bytes of protocol address }; struct sockaddr_in { short sin_family; // e.g. AF_INET, AF_INET6 unsigned short sin_port; // e.g. htons(3490) struct in_addr sin_addr; // see struct in_addr, below char sin_zero[8]; // zero this if you want to }; struct sockaddr_in6 { u_int16_t sin6_family; // address family, AF_INET6 u_int16_t sin6_port; // port number, Network Byte Order u_int32_t sin6_flowinfo; // IPv6 flow information struct in6_addr sin6_addr; // IPv6 address u_int32_t sin6_scope_id; // Scope ID }; struct sockaddr_storage { sa_family_t ss_family; // address family // all this is padding, implementation specific, ignore it: char __ss_pad1[_SS_PAD1SIZE]; int64_t __ss_align; char __ss_pad2[_SS_PAD2SIZE]; }; </code></pre> <p>So as you can see, if the function expects an IPv4 address, it will just read the first 4 bytes (because it assumes the struct is of type <code>struct sockaddr</code>. Otherwise it will read the full 16 bytes for IPv6).</p>
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    1. POReasoning behind C sockets sockaddr and sockaddr_storage
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    1. USJonathon Reinhart
    1. UStheprole
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    1. COSo suppose I had a pointer to a `struct sockaddr_storage` called `sas` and all the fields in the structure have been duly populated with the address and address family. I now perform this: `struct sockaddr *s = (struct sockaddr*)sas`. How would I get the address off of `s` now?
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      1. USMatt Vaughan
    2. COyou wouldn't use a `struct sockaddr` directly. What you would do is typecast it back to `sockaddr_storage` or `sockaddr_in` and then read it. As you can see from my post `sockaddr` has enough space to fit an IPv4 or v6 address. Having said that, I don't know why you would want to typecast to `struct sockaddr` for anything other than for parameters who's functions require it. `struct sockaddr` is not meant for 'programmer use'.
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    3. COI am reading a book that says sockaddr isn't large enough to hold sockaddr_in6.
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      1. USMatt Vaughan
 

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