In X11/Intrinsic.h, we have this:

typedef char *String;

typedef struct {
    String      name;
    XtArgVal    value;
} Arg, *ArgList;

As a result, when the XtSetArg macro is used to populate one
of these structures, it is often the case that a constant string
(e.g. 'const char *') is assigned to the name field.  Since name
does not have the const qualifier, a compiler warning is issued.

Is there any reason not to use 'const char *' for the name field?
I cannot see why any code would have a reason to modify the name
string...