GCC does not provide the globalref, globaldef and
globalvalue keywords of VAX-C. You can get the same effect with
an obscure feature of GAS, the GNU assembler. (This requires GAS
version 1.39 or later.) The following macros allow you to use this
feature in a fairly natural way:
#ifdef __GNUC__
#define GLOBALREF(TYPE,NAME) \
TYPE NAME \
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME)
#define GLOBALDEF(TYPE,NAME,VALUE) \
TYPE NAME \
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \
= VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME)
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \
= {VALUE}
#else
#define GLOBALREF(TYPE,NAME) \
globalref TYPE NAME
#define GLOBALDEF(TYPE,NAME,VALUE) \
globaldef TYPE NAME = VALUE
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
globalvalue TYPE NAME = VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
globalvalue TYPE NAME
#endif
(The _$$PsectAttributes_GLOBALSYMBOL prefix at the start of the
name is removed by the assembler, after it has modified the attributes
of the symbol). These macros are provided in the VMS binaries
distribution in a header file GNU_HACKS.H. An example of the
usage is:
GLOBALREF (int, ijk);
GLOBALDEF (int, jkl, 0);
The macros GLOBALREF and GLOBALDEF cannot be used
straightforwardly for arrays, since there is no way to insert the array
dimension into the declaration at the right place. However, you can
declare an array with these macros if you first define a typedef for the
array type, like this:
typedef int intvector[10];
GLOBALREF (intvector, foo);
Array and structure initializers will also break the macros; you can
define the initializer to be a macro of its own, or you can expand the
GLOBALDEF macro by hand. You may find a case where you wish to
use the GLOBALDEF macro with a large array, but you are not
interested in explicitly initializing each element of the array. In
such cases you can use an initializer like: {0,}, which will
initialize the entire array to 0.
A shortcoming of this implementation is that a variable declared with
GLOBALVALUEREF or GLOBALVALUEDEF is always an array. For
example, the declaration:
GLOBALVALUEREF(int, ijk);
declares the variable ijk as an array of type int [1].
This is done because a globalvalue is actually a constant; its “value”
is what the linker would normally consider an address. That is not how
an integer value works in C, but it is how an array works. So treating
the symbol as an array name gives consistent results—with the
exception that the value seems to have the wrong type. Don't
try to access an element of the array. It doesn't have any elements.
The array “address” may not be the address of actual storage.
The fact that the symbol is an array may lead to warnings where the variable is used. Insert type casts to avoid the warnings. Here is an example; it takes advantage of the ISO C feature allowing macros that expand to use the same name as the macro itself.
GLOBALVALUEREF (int, ss$_normal);
GLOBALVALUEDEF (int, xyzzy,123);
#ifdef __GNUC__
#define ss$_normal ((int) ss$_normal)
#define xyzzy ((int) xyzzy)
#endif
Don't use globaldef or globalref with a variable whose
type is an enumeration type; this is not implemented. Instead, make the
variable an integer, and use a globalvaluedef for each of the
enumeration values. An example of this would be:
#ifdef __GNUC__
GLOBALDEF (int, color, 0);
GLOBALVALUEDEF (int, RED, 0);
GLOBALVALUEDEF (int, BLUE, 1);
GLOBALVALUEDEF (int, GREEN, 3);
#else
enum globaldef color {RED, BLUE, GREEN = 3};
#endif