Commit 58d9004cea

Jonathan Marler <johnnymarler@gmail.com>
2022-11-17 07:41:54
packed struct fix example and clarify least to most significant ordering
The packed struct example was mistakenly applying endianness where it shouldn't have been. This wasn't being caught because we don't currently test the examples on Big-endian systems. I updated the test to remove the endianness where it didn't apply, and added a new part of the test to demonstrate when it would apply.
master
1 parent a09a5ad
Changed files (1)
doc
langref.html.in
doc/langref.html.in
@@ -3277,7 +3277,7 @@ test "default struct initialization fields" {
       Unlike normal structs, {#syntax#}packed{#endsyntax#} structs have guaranteed in-memory layout:
       </p>
       <ul>
-        <li>Fields remain in the order declared.</li>
+        <li>Fields remain in the order declared, least to most significant.</li>
         <li>There is no padding between fields.</li>
         <li>Zig supports arbitrary width {#link|Integers#} and although normally, integers with fewer
         than 8 bits will still use 1 byte of memory, in packed structs, they use
@@ -3320,16 +3320,19 @@ fn doTheTest() !void {
     try expect(@sizeOf(Divided) == 2);
     var full = Full{ .number = 0x1234 };
     var divided = @bitCast(Divided, full);
+    try expect(divided.half1 == 0x34);
+    try expect(divided.quarter3 == 0x2);
+    try expect(divided.quarter4 == 0x1);
+
+    var ordered = @bitCast([2]u8, full);
     switch (native_endian) {
         .Big => {
-            try expect(divided.half1 == 0x12);
-            try expect(divided.quarter3 == 0x3);
-            try expect(divided.quarter4 == 0x4);
+            try expect(ordered[0] == 0x12);
+            try expect(ordered[1] == 0x34);
         },
         .Little => {
-            try expect(divided.half1 == 0x34);
-            try expect(divided.quarter3 == 0x2);
-            try expect(divided.quarter4 == 0x1);
+            try expect(ordered[0] == 0x34);
+            try expect(ordered[1] == 0x12);
         },
     }
 }