The Design and Implementation of the Anykernel and Rump Kernels

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[ .... ]
2194: e8 fc ff ff ff call 2195
2199: 85 db test %ebx,%ebx
219b: 75 0c jne 21a9
219d: 8b 45 f4 mov 0xfffffff4(%ebp),%eax
21a0: 8b 55 f8 mov 0xfffffff8(%ebp),%edx
21a3: 83 c4 24 add $0x24,%esp
21a6: 5b pop %ebx
21a7: 5d pop %ebp
21a8: c3 ret
[ .... ]
Figure 3.13: Compile-time optimized sizeof() check. The assembly of the
generated code compiled for i386 is presented.
4. The second exception to the above is the lseek() call on 32bit architectures.
The call returns a 64bit off_t 10 with the low bits occupying one register and
the high bits the other one. Since NetBSD supports all combinations of 32bit,
64bit, little endian and big endian architectures, care had to be taken to have
the translation from a two-element register_t vector to a variable work for
all calls on all architectures. We use a compile-time check for data type sizes
and typecast accordingly. To see why the check is required, consider the
following. If the typecast is never done, lseek breaks on 32bit architectures.
If the typecast to the return type is done for all calls, system calls returning
an integer break on 64bit big-endian architectures.
The above is not the only way to solve the problem. The makesyscalls.sh
script detects 64bit return values and sets the SYCALL_RET_64 flag in a system
call’s description. We could have hooked into the facility and created a special
wrapper for lseek without the “if (sizeof())” clause. The compiled code
is the same for both approaches (Figure 3.13), so the choice is a matter of
taste instead of runtime performance.
10 off_t is always 64bit on NetBSD instead of depending on the value of _FILE_OFFSET_BITS
which used on for example Linux and Solaris.