In fact, this is not a C language library function, but a function defined by the programmer himself.
In C language, whether you are naming a function or a variable, we strive to be able to know the meaning just by looking at the name.
The meaning of arrout can be understood like this:
arrout = arr + out
arr = array array
out = output output
The combination of p>
means outputting an array.
So the purpose of the programmer defining this function name is to output an array. Nothing else!