So, it looks like in Linux I can't use the constructor

string (const char* s, size_t n)

(as Kevin suggested above).

This leaves me with two options. If I want to truncate the read line (remove the trailing newline), I can either truncate the C-string prior to using it to create the C++ string:

        if !keep_newlines {
                line![readCount - 1] = 0
            }
         let cpps = std.string(line)
         free(line)
         return cpps

or I can truncate the C++ string after it was created:

            var cpps = std.string(line)
            free(line)
            if !keep_newlines {
                cpps.pop_back()
            }
            return cpps

Both options work and the performance is pretty much the same. @PaulMcKenzie, @Kevin, @Remy Lebeau - Thanks for your suggestions.