translate-c: correct translation of global variables

* externs with intializers are translated as exports
* non extern without explicit initialization are zero initalized

lib/std/mem.zig

@@ -221,9 +221,19 @@ pub fn zeroes(comptime T: type) T {
         .Vector => |info| {
             return @splat(info.len, zeroes(info.child));
         },
+        .Union => |info| {
+            if (comptime meta.containerLayout(T) == .Extern) {
+                // The C language specification states that (global) unions
+                // should be zero initialized to the first named member.
+                var item: T = undefined;
+                @field(item, info.fields[0].name) = zeroes(@TypeOf(@field(item, info.fields[0].name)));
+                return item;
+            }
+
+            @compileError("Can't set a " ++ @typeName(T) ++ " to zero.");
+        },
         .ErrorUnion,
         .ErrorSet,
-        .Union,
         .Fn,
         .BoundFn,
         .Type,
@@ -312,6 +322,14 @@ test "mem.zeroes" {
     for (b.sentinel) |e| {
         testing.expectEqual(@as(u8, 0), e);
     }
+
+    const C_union = extern union {
+        a: u8,
+        b: u32,
+    };
+
+    var c = zeroes(C_union);
+    testing.expectEqual(@as(u8, 0), c.a);
 }
 
 /// Sets a slice to zeroes.

src-self-hosted/translate_c.zig

@@ -701,8 +701,14 @@ fn visitVarDecl(c: *Context, var_decl: *const ZigClangVarDecl) Error!void {
     const qual_type = ZigClangVarDecl_getTypeSourceInfo_getType(var_decl);
     const storage_class = ZigClangVarDecl_getStorageClass(var_decl);
     const is_const = ZigClangQualType_isConstQualified(qual_type);
+    const has_init = ZigClangVarDecl_hasInit(var_decl);
 
-    const extern_tok = if (storage_class == .Extern)
+    // In C extern variables with initializers behave like Zig exports.
+    // extern int foo = 2;
+    // does the same as:
+    // extern int foo;
+    // int foo = 2;
+    const extern_tok = if (storage_class == .Extern and !has_init)
         try appendToken(c, .Keyword_extern, "extern")
     else if (storage_class != .Static)
         try appendToken(c, .Keyword_export, "export")
@@ -730,7 +736,7 @@ fn visitVarDecl(c: *Context, var_decl: *const ZigClangVarDecl) Error!void {
     // If the initialization expression is not present, initialize with undefined.
     // If it is an integer literal, we can skip the @as since it will be redundant
     // with the variable type.
-    if (ZigClangVarDecl_hasInit(var_decl)) {
+    if (has_init) {
         eq_tok = try appendToken(c, .Equal, "=");
         init_node = if (ZigClangVarDecl_getInit(var_decl)) |expr|
             transExprCoercing(rp, &c.global_scope.base, expr, .used, .r_value) catch |err| switch (err) {
@@ -745,7 +751,22 @@ fn visitVarDecl(c: *Context, var_decl: *const ZigClangVarDecl) Error!void {
             try transCreateNodeUndefinedLiteral(c);
     } else if (storage_class != .Extern) {
         eq_tok = try appendToken(c, .Equal, "=");
-        init_node = try transCreateNodeIdentifierUnchecked(c, "undefined");
+        // The C language specification states that variables with static or threadlocal
+        // storage without an initializer are initialized to a zero value.
+
+        // @import("std").mem.zeroes(T)
+        const import_fn_call = try c.createBuiltinCall("@import", 1);
+        const std_node = try transCreateNodeStringLiteral(c, "\"std\"");
+        import_fn_call.params()[0] = std_node;
+        import_fn_call.rparen_token = try appendToken(c, .RParen, ")");
+        const inner_field_access = try transCreateNodeFieldAccess(c, &import_fn_call.base, "mem");
+        const outer_field_access = try transCreateNodeFieldAccess(c, inner_field_access, "zeroes");
+
+        const zero_init_call = try c.createCall(outer_field_access, 1);
+        zero_init_call.params()[0] = type_node;
+        zero_init_call.rtoken = try appendToken(c, .RParen, ")");
+
+        init_node = &zero_init_call.base;
     }
 
     const linksection_expr = blk: {

test/translate_c.zig

@@ -466,7 +466,7 @@ pub fn addCases(cases: *tests.TranslateCContext) void {
     , &[_][]const u8{
         \\pub extern var extern_var: c_int;
         \\pub const int_var: c_int = 13;
-        \\pub export var foo: c_int = undefined;
+        \\pub export var foo: c_int = @import("std").mem.zeroes(c_int);
     });
 
     cases.add("const ptr initializer",
@@ -1327,7 +1327,7 @@ pub fn addCases(cases: *tests.TranslateCContext) void {
         \\static char arr1[] = "hello";
         \\char arr2[] = "hello";
     , &[_][]const u8{
-        \\pub extern var arr0: [*c]u8 = "hello";
+        \\pub export var arr0: [*c]u8 = "hello";
         \\pub var arr1: [*c]u8 = "hello";
         \\pub export var arr2: [*c]u8 = "hello";
     });