This was reported in jextract-dev alias

https://mail.openjdk.org/pipermail/jextract-dev/2024-July/001843.html

The test file struct.h

#include <stdint.h>

typedef enum
{
    EnumDef_Undefined = -1,
    EnumDef_Big = 0x80000000

} EnumDef;

typedef struct structType
{
        EnumDef someEnum;
        uint64_t bigNumber;
} value;

On Mac and Linux, the layout generated is as follows:

private static final GroupLayout $LAYOUT = MemoryLayout.structLayout(
        foo_h.C_INT.withName("someEnum"),
        foo_h.C_LONG.withName("bigNumber")
    ).withName("structType");

Layout on Windows is as follows:

private static final GroupLayout $LAYOUT = MemoryLayout.structLayout(
        struct_h.C_INT.withName("someEnum"),
        MemoryLayout.paddingLayout(4),
        struct_h.C_LONG_LONG.withName("bigNumber")
    ).withName("structType");