OSX gettimeofday syscall on x86_64 seems to not work - macos

Question

I'm making a call to gettimeofday via the syscall instruction using 64bit code.
I can't get any results back and am getting told via Dtrace that the call worked with no errors, but the registers I get back from the call are garbage.
I do the following:
lea rdi, [rel timeval] ;buffer for 16bytes
mov rsi, 0 ;no need of timezone
mov rax, 0x2000074 ;gettimeofday
syscall
On return rax is neither 0 nor -1, and the buffer never gets any data.
Please can somebody check this and see if they can get a working call. I've no idea what is going on.
Best Regards
Chris
ps this is the example code I just tried, it dosn't return anything but 0.
SECTION .text
global _main
_main:
lea rdi, [rel buffer]
mov rsi, 0
mov rax, 0x2000074
syscall
mov rdi, [rel buffer]
mov rax, 0x2000001
syscall
SECTION .data
buffer:
times 16 db 0

Answer 1

According to the APIs, you need to point:
1) RDI to the timeval structure:
_STRUCT_TIMEVAL
{
__darwin_time_t tv_sec; /* seconds */
__darwin_suseconds_t tv_usec; /* and microseconds */
};
DarwinTime is a QWORD and DarwinSuSeconds is a DWORD.
2) RSI to the timezone structure.
Just point that to a 2xQWORD scratch buffer if you don't like its output.
3) RDX to the mach_absolute_time structure
This is a 1xQWORD buffer
Setting either of the above RSI, RDX, RDI to 0x0 rather than pointers might be getting rejected as it can trigger an internal exception. (Check if the return value is -EFAULT.)
Also note that POSIX deprecates gettimeofday in favour of clock_gettime (which has a seconds / nanosecond time struct), so you might want to use that instead.