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packed struct fix example and clarify least to most significant ordering

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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.
This commit is contained in:
Jonathan Marler 2022-11-16 23:41:54 -07:00 committed by Andrew Kelley
parent 91b8d9e370
commit 9f215318b8
1 changed files with 10 additions and 7 deletions
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17
doc/langref.html.in
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@ -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);
},
}
}