char_cast.c

typedef unsigned char char_u;

void some_function(char *argument)
{
  return;
}

int main()
{
  char_u *test = (char_u *)"example";
  some_function(test);
  return 0;
}

clang_output

stefan at air in ~/Desktop
% clang char_cast.c 
char_cast.c:12:17: warning: passing 'char_u *' (aka 'unsigned char *') to
      parameter of type 'char *' converts between pointers to integer types with
      different sign [-Wpointer-sign]
  some_function(test);
                ^~~~
char_cast.c:4:26: note: passing argument to parameter 'argument' here
void some_function(char *argument)
                         ^
1 warning generated.