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This technique is used in Berkley sockets: the `sockaddr` type (https://pubs.opengroup.org/onlinepubs/009695399/basedefs/sys...) is the "base" type for other types such as `sockaddr_ip` (for IPv4). But it's undefined behaviour according to ISO C due to the "strict aliasing" rule: https://en.cppreference.com/w/c/language/object#Strict_alias...

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Strict aliasing allows you to convert between a pointer to a struct and a pointer to its first member, since two objects exist at that address, one with the type of the struct and one with the type of the first member.

> A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

Yes, that's the workaround described by @spacedcowboy and @leni53. I should have clarified that my comment on undefined behaviour was specifically for the case of two structs that start the same (e.g. "int x; int y;") as opposed to composition.

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