alt Hacker Newsstrncpy is fairly easy, that's a special-purpose function for copying a C string into a fixed-width string, like typically used in old C applications for on-disk formats. E.g. you might have a char username[20] field which can contain up to 20 characters, with unused characters filled with NULs. That's what strncpy is for. The destination argument should always be a fixed-size char array.
A couple years ago we got a new manual page courtesy of Alejandro Colomar just about this: https://man.archlinux.org/man/string_copying.7.en
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Yes, these were also common in several wire formats I had to use for market data/entry.
You would think char symbol[20] would be inefficient for such performance sensitive software, but for the vast majority of exchanges, their technical competencies were not there to properly replace these readable symbol/IDs with a compact/opaque integer ID like a u32. Several exchanges tried and they had numerous issues with IDs not being "properly" unique across symbol types, or time (restarts intra-day or shortly before the open were a common nightmare), etc. A char symbol[20] and strncpy was a dream by comparison.
A big footgun with strncpy is that the output string may not be null terminated.
strncpy doesn’t handle overlapping buffers (undefined behavior). Better to use strncpy_s (if you can) as it is safer overall. See: https://en.cppreference.com/w/c/string/byte/strncpy.html.
As an aside, this is part of the reason why there are so many C successor languages: you can end up with undefined behavior if you don’t always carefully read the docs.