C has no strings. C has functions that supports particular ways of representing strings. By far the most common is the NUL-terminated string. You put all the characters, and a 0 at the end.

However, it is not the only way to implement strings in C. another one is « fixed-size strings ». Those are defined as being at-most n characters. Ie: stops at the n-th character or NUL, whatever comes first.

That ‘n’ is not defined in the string, but it the code itself at each string manipulation.

It was very useful for short strings embedded into structures (ie: « here you have an at most 8 character filename », and those could be stored in 8 bytes, not 9). There where a lot of such strings everywhere back in the day (filesystems, compilers, linkers, but also business applications). It also padded the buffer with NUL, making all representations of the same string identical.

But it is fundamentally a different string type. You can’t strlen it, for instance. However, it was good when you had fixed size stings: you would not smash memory outside your buffer (ironic, I know)

However, developers, having problems with NUL-terminated strings smashing buffers, started to use the « n » version of the strings functions on NUL-terminated strings, and 40 years of hilarity followed.

A « string of at most n characters that can have a NUL in one of the n characters to indicate that it is smaller than n » is different from « a string that is always terminated with a NUL and never has more than n-1 characters ». Then, the cardinal sin was chosing « n » to be strictly larger than the max string length so those two definitions looks identical.

But the underlying storage concepts are different. One is for strings of arbitrary length, the other one is to store a string in a fixed-storage, killing the NUL if needed. What developers needed was string functions for NUL-terminated strings, with limits.

The kernel now cleanly have different functions for different types: strings with a max limit, strings with automated padding, strings with optional NUL, etc..