POSIX leaves it ambiguous whether you need to clear it or not. About sockaddr_in, POSIX-2001, 2004 edition[1] has to say: 10004 The <netinet/in.h> hea

POSIX leaves it ambiguous whether you need to clear it or not. About sockaddr_in, POSIX-2001, 2004 edition[1] has to say: 10004 The <netinet/in.h> header shall define the sockaddr_in structure that includes at least the 10005 following members: 10006 sa_family_t sin_family AF_INET. 10007 in_port_t sin_port Port number. 10008 struct in_addr sin_addr IP address. So implementations are free to add whatever extra members they feel like. However, about IPv6 it says: 10016 The <netinet/in.h> header shall define the sockaddr_in6 structure that includes at least the 10017 following members: 10018 sa_family_t sin6_family AF_INET6. 10019 in_port_t sin6_port Port number. 10020 uint32_t sin6_flowinfo IPv6 traffic class and flow information. 10021 struct in6_addr sin6_addr IPv6 address. 10022 uint32_t sin6_scope_id Set of interfaces for a scope. 10023 The sin6_port and sin6_addr members shall be in network byte order. 10024 The sockaddr_in6 structure shall be set to zero by an application prior to using it, since 10025 implementations are free to have additional, implementation-defined fields in sockaddr_in6. So raimue's point is explicitly true for sockaddr_in6; while it is left ambiguous whether the (allowed) implementation-defined members of sockaddr_in must be initialized. [1]: The current UNIX standard is UNIX V7, which corresponds to SUSv4, which corresponds to the POSIX-2008. However, macOS is certified UNIX under the previous version, UNIX 03, which corresponds to SUSv3, which corresponds to POSIX-2001.