mmap Mac: Segmentation fault - c++

Question

The following on my Mac succeeds:
int main() {
int* addr = (int*) mmap(0, 100, 1 | 2, 2 | 4096, -1, 0);
*addr = 25;
return 0;
}
However the below code is identical but fails when I try to write to *addr with segmentation fault:
int main() {
int* addr = (int*) syscall(SYS_mmap, 0, 100, 1 | 2, 2 | 4096, -1, 0);
*addr = 25;
return 0;
}
I.e. syscall successfully returns me a memory address, but when I try writing to it it fails.
I compile it like this:
g++ ./c++/mmap.cc -o ./mmap && ./mmap
If I run both versions with dtruss:
g++ ./c++/mmap.cc -o ./mmap && sudo dtruss ./mmap
then both version succeed and I see identical mmap call for both:
mmap(0x0, 0x64, 0x3, 0x1002, 0xFFFFFFFF, 0x0) = 0xXXXXXXX 0
Why does the syscall version give me segmentation fault, what am I missing?
P.S. If I do something similar on Linux it works fine.
So, as I understand the mmap function on Mac does not execute syscall(SYS_mmap, .... What does it do then? Can anyone please give me some links where I can see implementation.
EDIT:
It looks like syscall on Mac returns only first 4 bytes. Is there a 64-bit syscall version?
DISASSEMBLED:
mmap version:
_main:
0000000100000cf0 pushq %rbp
0000000100000cf1 movq %rsp, %rbp
0000000100000cf4 subq $0x30, %rsp
0000000100000cf8 xorl %eax, %eax
0000000100000cfa movl %eax, %ecx
0000000100000cfc movl $0x64, %eax
0000000100000d01 movl %eax, %esi
0000000100000d03 movl $0x3, %edx
0000000100000d08 movl $0x1002, %eax
0000000100000d0d movl $0xffffffff, %r8d
0000000100000d13 movl $0x0, -0x14(%rbp)
0000000100000d1a movq %rcx, %rdi
0000000100000d1d movq %rcx, -0x28(%rbp)
0000000100000d21 movl %eax, %ecx
0000000100000d23 movq -0x28(%rbp), %r9
0000000100000d27 callq 0x100000ed6 ## symbol stub for: _mmap
0000000100000d2c movq 0x2cd(%rip), %rdi ## literal pool symbol address: __ZNSt3__14coutE
0000000100000d33 movq %rax, -0x20(%rbp)
0000000100000d37 movq -0x20(%rbp), %rax
0000000100000d3b movq %rax, %rsi
syscall version:
_main:
0000000100000cf0 pushq %rbp
0000000100000cf1 movq %rsp, %rbp
0000000100000cf4 subq $0x30, %rsp
0000000100000cf8 movl $0xc5, %edi
0000000100000cfd xorl %esi, %esi
0000000100000cff movl $0x64, %edx
0000000100000d04 movl $0x3, %ecx
0000000100000d09 movl $0x1002, %r8d
0000000100000d0f movl $0xffffffff, %r9d
0000000100000d15 movl $0x0, -0x14(%rbp)
0000000100000d1c movl $0x0, (%rsp)
0000000100000d23 movb $0x0, %al
0000000100000d25 callq 0x100000ed6 ## symbol stub for: _syscall
0000000100000d2a movq 0x2cf(%rip), %rdi ## literal pool symbol address: __ZNSt3__14coutE
0000000100000d31 movslq %eax, %r10
0000000100000d34 movq %r10, -0x20(%rbp)
0000000100000d38 movq -0x20(%rbp), %r10
0000000100000d3c movq %r10, %rsi

Answer 1

Apparently Mac does not have a 64-bit syscall function, here a is simple implementation:
#include <sys/types.h>
#define CARRY_FLAG_BIT 1
inline int64_t syscall6(int64_t num, int64_t arg1, int64_t arg2, int64_t arg3, int64_t arg4, int64_t arg5, int64_t arg6) {
int64_t result;
int64_t flags;
__asm__ __volatile__ (
"movq %6, %%r10;\n"
"movq %7, %%r8;\n"
"movq %8, %%r9;\n"
"syscall;\n"
"movq %%r11, %1;\n"
: "=a" (result), "=r" (flags)
: "a" (num), "D" (arg1), "S" (arg2), "d" (arg3), "r" (arg4), "r" (arg5), "r" (arg6)
: "%r10", "%r8", "%r9", "%rcx", "%r11"
);
return (flags & CARRY_FLAG_BIT) ? -result : result;
}
And you use it on mac by shifting system call numbers by 0x2000000:
int* addr = (int*) syscall6(0x2000000 + SYS_mmap, 0, 100, 1 | 2, 2 | 4096, -1, 0);
You can find more here.