What is the type of the errno variable in C++?

I was doing some system programming in C++, and I want to pass the errno variable in linux systems to my exception handler. You can see example code in here https://pastebin.com/ppgMc8Hj

#include <sys/stat.h>
#include <cerrno>
#include <iostream>
#include <cstring>

std::string errorString(int errno){
    return std::strerror(errno);
}

int main(){
    struct stat sb;
    errno = 0;
    if(stat("/jdfj/", &sb)){
        errorString(errno);
    }
    else{
        std::cout << std::boolalpha << S_ISDIR(sb.st_mode) << std::endl;
    }
}

But it returns an error like this

In function 'int main()': 
 14:26: error: invalid conversion from 'int' to 'int* (*)()' [-fpermissive] 
  6:13: note: initializing argument 1 of 'std::string errorString(int* (*)())'.

I have seen here http://en.cppreference.com/w/cpp/string/byte/strerror that the standard function that returns a string from errno accepts an int as argument. My question is

1. If it is an int type why doesn't it work in the above code?
2. If not what is it's type?