What's the difference between mach_vm_allocate and vm_allocate?

Question

I would like to know what's the difference betweenmach_vm_allocate and vm_allocate. I know mach_vm_allocate is only available in OS X and not iOS, but I'm not sure why. The file that has all the function prototypes in for the mach_vm_... functions (mach/mach_vm.h) only has #error mach_vm.h unsupported. in iOS.

Kithara

Answer 1

the new Mach VM API that is introduced in Mac OS X 10.4. The new API is essentially the same as the old API from the programmer's standpoint, with the following key differences.

-Routine names have the mach_ prefixfor example, vm_allocate() becomes mach_vm_allocate() .

-Data types used in routines have been updated to support both 64-bit and 32-bit tasks. Consequently, the new API can be used with any task.

The new and old APIs are exported by different MIG subsystems: mach_vm and vm_map , respectively. The corresponding header files are <mach/mach_vm.h> and <mach/vm_map.h> , respectively.

Vonvona

Thanks, @Ravindra. Can you specify the header files? It looks like they are missing on your last sentence. – Kithara

Answer 2

The new Mach VM API that is introduced in Mac OS X 10.4. The new API is essentially the same as the old API from the programmer's standpoint, with the following key differences:

• Routine names have the mach_ prefixfor example, vm_allocate() becomes mach_vm_allocate();

• Data types used in routines have been updated to support both 64-bit and 32-bit tasks. Consequently, the new API can be used with any task.

The new and old APIs are exported by different MIG subsystems: mach_vm and vm_map respectively. The corresponding header files are <mach/mach_vm.h> and <mach/vm_map.h> respectively.

The information is derived from the book OS X Internals A System Approach.

Chlorobenzene

Answer 3

I'm not happy with the other answers. Two seem to quote from the same source (one doesn't correctly attribute it) and this source rather misleading.

The data types in the old API are variable size. If you are compiling for i386 CPU architecture, they are 32 bit, if you compile for x86_64 CPU architecture, they are 64 bit.

The data type in the new API are fixed size and always 64 bit.

As a 32 bit process only requires 32 bit data types and a 64 bit process already had 64 bit types even with the old API, it isn't obvious why the new API was required at all. But it will become obvious if you look at Apples kernel design:

The real kernel of macOS is a Mach 3 micro kernel. Being a micro kernel, it only takes care of process and thread management, IPC, virtual memory management and process/thread scheduling. That's it. Usually everything else would be done in userspace when using a micros kernel but that is slow and thus Apple took a different approach.

Apple took the FreeBSD monolithic kernel and wrapped it around the micro kernel which executes the FreeBSD kernel as a single process (a task) with special privileges (despite being a task, the code runs within kernel space not userspace as all the other tasks in the system). The combined kernel has the name XNU.

And in the past, Apple supported any combination of mixing 32 and 64 bits within a system: Your system could run a 32 bit kernel and run 32 or 64 bit processes on it or it could run a 64 bit kernel and run 32 or 64 bit processes on it. You might already be seeing where this going, aren't you?

If process and kernel are of different bit width, the old API is of no use. E.g., a 32 bit kernel cannot use it to interact with the memory of a 64 bit process because all data types of the API calls would only be 32 bit in the 32 bit kernel task but a 64 bit process has a 64 bit memory space even if the kernel itself has not.

Actually there is even a 3rd version of that API. To quote a comment from Apple's kernel source:

 * There are three implementations of the "XXX_allocate" functionality in
 * the kernel: mach_vm_allocate (for any task on the platform), vm_allocate
 * (for a task with the same address space size, especially the current task),
 * and vm32_vm_allocate (for the specific case of a 32-bit task). vm_allocate
 * in the kernel should only be used on the kernel_task. vm32_vm_allocate only
 * makes sense on platforms where a user task can either be 32 or 64, or the kernel
 * task can be 32 or 64. mach_vm_allocate makes sense everywhere, and is preferred
 * for new code.

As long as you are only using that API in userspace and only for memory management of your old process, using the old API is still fine, even for a 64 bit process as all data types will be 64 bit in that case.

The new API is only required when working across process boundaries and only if it's not certain that both processes are both 32 or 64 bit. However, Apple has dropped 32 bit kernel support long ago and meanwhile they have also dropped 32 bit userspace support as all system libraries only ship as 64 bit library starting with 10.15 (Catalina).

On iOS the new API was never required as you could never write kernel code for iOS and the security concept of iOS forbids direct interaction with other process spaces. On iOS you can only use that API to interact with your own process space and for that it will always have correct data types.

Misshape

Answer 4

Those would be /usr/include/mach/mach_vm.h and /usr/include/mach/vm_map.h. You can grep in /usr/include/mach to get those.

In the kernel, the APIs are basically using the same implementation. On iOS, you can use the "old" APIs perfectly well.

Pullman