str[ln]cat is like memcpy, but it does unnecessary work (search for the end of the string which the caller should long have figured out). It's like memcpy but less general and more confusing to use due to the implicit string length of the target operand. It's a completely unnecessary API, likely a remainder from the times when people used C for scripting.
It's also a complete idiocy to possibly not copy everything. I've never wanted that. It's begging for bugs. If there's not enough room to copy all the source bytes, you either know that in advance or you want to crash hard.
Even memcpy itself is hardly necessary as a library function. It's a simple loop. But at least it's what you would write anyway, and in some cases compilers are able to optimize it (with questionable benefits).
If you use memcpy and your destination buffer is too small you don’t crash hard with any certainty, your program might even appear to work.
Moreover, most of the time I’m either certain the output buffer is large enough (maybe because I just allocated it) or I really do want the output buffer to have a truncated string. Strncpy’s two dumb behaviors that I almost never want is to pad the destination buffer with zeros if the source string is smaller and to leave the destination I terminated if the source is longer.
Sure, however the problem I'm highlighting is most programmers are not aware this std library function is not like the other str[n] functions. There are plenty of solutions, but it was an objectively poor decision to define strncat the way it was.
