c extension with ruby issue

gistfile1.txt

I have a C extension that uses StringValueCStr to convert a passed-in ruby argument into a c-str like so:

VALUE login(VALUE self, VALUE rb_user, VALUE rb_password) {
    char *user = StringValueCStr(rb_user);
    char *passsword = StringValueCStr(rb_password);
    {other stuff using these}
}

VALUE cmd(VALUE self, VALUE rb_user, VALUE rb_cmd) {
    char *user = StringValueCStr(rb_user);
    char *cmd = StringValueCStr(rb_cmd);
}

I call this from ruby like:

login "blah", "1234"
cmd "blah, "ls"

This works fine.

However, it is preferred to have a variable already set up like:

USER = "blah"
PASS = "1234"

login USER, PASS
cmd USER, "ls"


When I try that I get: 
test.rb:11:in `send_cmd': string contains null byte (ArgumentError)


So it seems that the null byte add by StringValueCStr is sticking around and causing problems when I call it a second time in cmd.