Mac 64位系统调用

Question

How to execute a 64-bit syscall on Mac in C++.

I need the below to work:

#include <unistd.h>
#include <sys/syscall.h>
#include <sys/types.h>

int main() {
  long* addr = (long*) syscall(SYS_mmap, 0, 100, 1 | 2, 2 | 4096, -1, 0);
}

The problem is that addr below should be 64-bit as it is a pointer, but it truncates the result to 32-bit value.

I compile with:

g++ ./mmap.cc -o ./mmap 

PS I know there exists mmap function, the above is just an example, I need to get syscall function to work.

PPS The problem is that on 64-bit systems syscall should return 64-bit value, but in Mac unistd.h it is defined as int :

int  syscall(int, ...);

Is that a bug? The mmap system call return correctly void* :

void *  mmap(void *, size_t, int, int, int, off_t) __DARWIN_ALIAS(mmap);

How is mmap actually implemented?

PPPS

On Linux it is defined correctly as long:

long syscall(long number, ...);

Answer 1

After some searching I did not find a way to execute 64-bit system calls on Mac.

So, I implemented those myself, you can find the functions here .

Also, if you use those functions don't forget to add 0x2000000 Unix system call class shift to your system call numbers:

int SYS_write = 4;
int STDOUT = 1;
char* str = "Hello world\n";

syscall3(0x2000000 + SYS_write, STDOUT, str, 12);

Answer 2

The syscall function is deprecated on OS X as Ken Thomases pointed out.

For your specific example, you should use mmap instead of syscall . The mmap function is not implemented in terms of the syscall function but in terms of __mmap :

libsystem_kernel.dylib`mmap:
    0x7fff643fa69e <+87>:  callq  0x7fff643fe998            ; __mmap

which, in turn, does the actual syscall and is probably implemented in assembly:

libsystem_kernel.dylib`__mmap:
    0x7fff643fe998 <+0>:  movl   $0x20000c5, %eax          ; imm = 0x20000C5
    0x7fff643fe99d <+5>:  movq   %rcx, %r10
    0x7fff643fe9a0 <+8>:  syscall
    0x7fff643fe9a2 <+10>: jae    0x7fff643fe9ac            ; <+20>