#### A.1.7 Accessing Global Variables in Oct-Files

Global variables allow variables in the global scope to be accessed. Global variables can be accessed within oct-files by using the support functions `get_global_value` and `set_global_value`. `get_global_value` takes two arguments, the first is a string representing the variable name to obtain. The second argument is a boolean argument specifying what to do if no global variable of the desired name is found. An example of the use of these two functions is

    #include <octave/oct.h>

    DEFUN_DLD (globaldemo, args, , "Global Demo")
    {
      if (args.length () != 1)
        print_usage ();

      octave_value retval;

      std::string s = args(0).string_value ();

      octave_value tmp = get_global_value (s, true);

      if (tmp.is_defined ())
        retval = tmp;
      else
        retval = "Global variable not found";

      set_global_value ("a", 42.0);

      return retval;
    }

An example of its use is

    global a b
    b = 10;
    globaldemo ("b")
    ⇒ 10
    globaldemo ("c")
    ⇒ "Global variable not found"
    num2str (a)
    ⇒ 42