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Expose BitOffset from the clang AST #1

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Merged
craigwarne-rw merged 3 commits into from
Jan 28, 2026
Merged

Expose BitOffset from the clang AST #1

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6de760f
Expose BitOffset, update Offset to just be a getter which divides by 8
craigwarne-rw Jan 27, 2026
bd05a98
Fix for anonymous field offsets (need to convert between byte and bit…
craigwarne-rw Jan 28, 2026
a4008c1
Add basic unit test for bit offsets
craigwarne-rw Jan 28, 2026
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41 changes: 41 additions & 0 deletions src/CppAst.Tests/TestStructs.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -310,5 +310,46 @@ struct HelloWorld
}
);
}

[Test]
public void TestBitOffsets()
{
ParseAssert(@"
struct NormalFields
{
int a;
short b;
float c;
};

struct Bitfields
{
int a : 1;
int b : 2;
int c : 3;
};
",
compilation =>
{
Assert.False(compilation.HasErrors);

void Test(int classIndex, int fieldIndex, int expectedOffset, int expectedBitOffset)
{
var cppStruct = compilation.Classes[classIndex];
var field = cppStruct.Fields[fieldIndex];
Assert.AreEqual(expectedOffset, field.Offset);
Assert.AreEqual(expectedBitOffset, field.BitOffset);
}

Test(0, 0, 0, 0);
Test(0, 1, 4, 32);
Test(0, 2, 8, 64);

Test(1, 0, 0, 0);
Test(1, 1, 0, 1);
Test(1, 2, 0, 3);
}
);
}
}
}
11 changes: 8 additions & 3 deletions src/CppAst/CppField.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -71,11 +71,16 @@ public CppField(CppType type, string name)
/// Gets or sets the number of bits for this bit field. Only valid if <see cref="IsBitField"/> is <c>true</c>.
/// </summary>
public int BitFieldWidth { get; set; }


/// <summary>
/// Gets the offset of the field in bytes.
/// </summary>
public long Offset => BitOffset / 8;

/// <summary>
/// Gets or sets the offset of the field in bytes.
/// Gets or sets the offset of the field in bits.
/// </summary>
public long Offset { get; set; }
public long BitOffset { get; set; }

/// <inheritdoc />
public override string ToString()
Expand Down
13 changes: 7 additions & 6 deletions src/CppAst/CppModelBuilder.cs
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Expand Up @@ -531,13 +531,14 @@ private CXChildVisitResult VisitMember(CXCursor cursor, CXCursor parent, void* d
// Might not be always correct (with alignment rules),
// but not sure how to recover the offset without recalculating the entire offsets
// BF: disregard the above comment? as we are now aligning using the anonymous struct's AlignOf...
var offset = 0L;
var bitOffset = 0L;
var cppClassContainer = containerContext.Container as CppClass;
if (cppClassContainer is object && cppClassContainer.Fields.Count > 0)
{
var lastField = cppClassContainer.Fields[cppClassContainer.Fields.Count - 1];
offset = (int)lastField.Offset + lastField.Type.SizeOf;
var offset = lastField.Offset + lastField.Type.SizeOf;
offset = Align(offset, cppClass.AlignOf);
bitOffset = offset * 8;

static long Align(long offset, long alignment)
{
Expand All @@ -551,7 +552,7 @@ static long Align(long offset, long alignment)
Visibility = containerContext.CurrentVisibility,
StorageQualifier = GetStorageQualifier(cursor),
IsAnonymous = true,
Offset = offset,
BitOffset = bitOffset,
};
ParseAttributes(cursor, cppField, true);
containerContext.DeclarationContainer.Fields.Add(cppField);
Expand Down Expand Up @@ -1154,7 +1155,7 @@ private CppField VisitFieldOrVariable(CppContainerContext containerContext, CXCu
cppField = previousField;
cppField.Name = fieldName;
cppField.Type = type;
cppField.Offset = cursor.OffsetOfField / 8;
cppField.BitOffset = cursor.OffsetOfField;
}
else
{
Expand All @@ -1164,7 +1165,7 @@ private CppField VisitFieldOrVariable(CppContainerContext containerContext, CXCu
StorageQualifier = GetStorageQualifier(cursor),
IsBitField = cursor.IsBitField,
BitFieldWidth = cursor.FieldDeclBitWidth,
Offset = cursor.OffsetOfField / 8,
BitOffset = cursor.OffsetOfField,
};
containerContext.DeclarationContainer.Fields.Add(cppField);
ParseAttributes(cursor, cppField, true);
Expand All @@ -1188,7 +1189,7 @@ private void AddAnonymousTypeWithField(CppContainerContext containerContext, CXC
Visibility = GetVisibility(cursor.CXXAccessSpecifier),
StorageQualifier = GetStorageQualifier(cursor),
IsAnonymous = true,
Offset = cursor.OffsetOfField / 8,
BitOffset = cursor.OffsetOfField,
};
containerContext.DeclarationContainer.Fields.Add(cppField);
ParseAttributes(cursor, cppField, true);
Expand Down
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