Is zeroing out the "sockaddr_in" structure necessary?

Short answer:

The IEEE Standard doesn't require it.

But, I think it's best to be safe and zero out everything.

---

Long(er) answer:

The IEEE Standard 1003.1 specifies that the definition of sockaddr_in is (Emphasis mine):

The <netinet/in.h> header shall define the sockaddr_in structure that includes at least the following members:

sa_family_t sin_family AF_INET.

in_port_t sin_port Port number.

struct in_addr sin_addr IP address.

Note that, unlike the definition for sockaddr_in6, which specifies that it should be zeroed out:

The sockaddr_in6 structure shall be set to zero by an application prior to using it, since implementations are free to have additional, implementation-defined fields in sockaddr_in6.

There is no similar wording for sockaddr_in. However, such lack of wording gives platform implementors enough ambiguity to make their own requirements about zeroing out all or part of sockaddr_in.

Note that the definition of sockaddr_in used to require a sin_zero field to pad out the structure to make it compatible with sockaddr structures:

The sin_zero member was removed from the sockaddr_in structure as per The Open Group Base Resolution bwg2001-004.

And it's with sin_zero we find a discrepancy between Windows and Linux. Even though the field was removed from the official definition, both Windows and Linux implementation still include it (because it's not explicitly illegal thanks to the wording "at least").

Whether sin_zero always requires zeroing out or not for Windows platforms isn't clear, but in this blog post the writer did some digging and came up with the following:

On some architectures, it wont cause any problems not clearing sin_zero. But on other architectures it might. Its required by specification to clear sin_zero, so you must do this if you intend your code to be bug free for now and in the future.

For the part about "It's required by specification to clear sin_zero", I could only find the following Windows documentation (for Winsock Kernel) appearing to support the claim:

A WSK application should set the contents of this array to zero.

However, I can find no similar wording for Linux.

So to conclude, it appears that in some architectures you need to zero out at least one field, while in others you don't. I think it's best to be safe and zero out everything.