I am trying to convert an ObjC stackoverflow answer to Swift and failing. It looks like I am passing a UnsafeMutablePointer<mach_msg_type_number_t> when I should be passing an inout mach_msg_type_number_t and I can't seem to work out my problem. From what I understand of the Swift pointer documentation (not much) these should be interchangeable..?
Further info below.
Here's the Objective C:
struct task_basic_info info;
mach_msg_type_number_t size = sizeof(info);
kern_return_t kerr = task_info(mach_task_self(), TASK_BASIC_INFO, (task_info_t)&info, &size);
and here's as far as I got in Swift (many lines for easier type checking)
let name: task_name_t = mach_task_self_
let flavor: task_flavor_t = task_flavor_t(MACH_TASK_BASIC_INFO)
var info: mach_task_basic_info
var size: mach_msg_type_number_t = UnsignedFixed(sizeof(mach_task_basic_info_t))
let kerr = task_info(name, flavor, info as task_info_t, &size)
The task_info signature is:
func task_info(target_task: task_name_t, flavor: task_flavor_t, task_info_out: task_info_t, task_info_outCnt: UnsafeMutablePointer<mach_msg_type_number_t>) -> kern_return_t
and the error on the last line is:
Cannot convert the expression's type '(@!lvalue task_name_t, task_flavor_t, task_info_t, inout mach_msg_type_number_t)' to type 'kern_return_t'
When interacting with C functions, you can't rely on the compiler's error messages - break it down parameter by parameter, command-clicking until you know what you're working with. To start with, the types you're running into are:
task_name_t: UInt32task_flavor_t: UInt32task_info_t: UnsafeMutablePointer<Int32>UnsafeMutablePointer<mach_msg_type_number_t>: UnsafeMutablePointer<UInt32>kern_return_t - Int32
There's one tricky Swift bit along with a bug in your code standing in your way here. First, the task_info_out parameter needs to be a UnsafeMutablePointer<UInt32>, but needs to actually point to an instance of mach_task_basic_info. We can get around this by creating a UnsafeMutablePointer<mach_task_basic_info> and wrapping it in another UnsafeMutablePointer at call time - the compiler will use type inference to know we want that wrapping pointer to be sub-typed as UInt32.
Second, you're calling sizeof(mach_task_basic_info_t) (the pointer to mach_task_basic_info) when you should be calling sizeinfo(mach_task_basic_info), so your byte count ends up too low to hold the data structure.
On further research, this got a little more complicated. The original code for this was incorrect, in that size should be initialized to the constant MACH_TASK_BASIC_INFO_COUNT. Unfortunately, that's a macro, not a simple constant:
#define MACH_TASK_BASIC_INFO_COUNT (sizeof(mach_task_basic_info_data_t) / sizeof(natural_t))
Swift doesn't import those, so we'll need to redefine it ourselves. Here's working code for all this:
// constant
let MACH_TASK_BASIC_INFO_COUNT = (sizeof(mach_task_basic_info_data_t) / sizeof(natural_t))
// prepare parameters
let name = mach_task_self_
let flavor = task_flavor_t(MACH_TASK_BASIC_INFO)
var size = mach_msg_type_number_t(MACH_TASK_BASIC_INFO_COUNT)
// allocate pointer to mach_task_basic_info
var infoPointer = UnsafeMutablePointer<mach_task_basic_info>.alloc(1)
// call task_info - note extra UnsafeMutablePointer(...) call
let kerr = task_info(name, flavor, UnsafeMutablePointer(infoPointer), &size)
// get mach_task_basic_info struct out of pointer
let info = infoPointer.move()
// deallocate pointer
infoPointer.dealloc(1)
// check return value for success / failure
if kerr == KERN_SUCCESS {
println("Memory in use (in bytes): \(info.resident_size)")
} else {
let errorString = String(CString: mach_error_string(kerr), encoding: NSASCIIStringEncoding)
println(errorString ?? "Error: couldn't parse error string")
}
Took me a bit to update Airspeed Velocity's answer to the latest swift syntax (Swift 3, beta 6), but here is what I got:
func report_memory() {
var info = mach_task_basic_info()
let MACH_TASK_BASIC_INFO_COUNT = MemoryLayout<mach_task_basic_info>.stride/MemoryLayout<natural_t>.stride
var count = mach_msg_type_number_t(MACH_TASK_BASIC_INFO_COUNT)
let kerr: kern_return_t = withUnsafeMutablePointer(to: &info) {
$0.withMemoryRebound(to: integer_t.self, capacity: MACH_TASK_BASIC_INFO_COUNT) {
task_info(mach_task_self_,
task_flavor_t(MACH_TASK_BASIC_INFO),
$0,
&count)
}
}
if kerr == KERN_SUCCESS {
print("Memory in use (in bytes): \(info.resident_size)")
}
else {
print("Error with task_info(): " +
(String(cString: mach_error_string(kerr), encoding: String.Encoding.ascii) ?? "unknown error"))
}
}
Hope that's helpful.
Nate’s answer is excellent but there’s a tweak you can make to simplify it.
First, rather than allocating/deallocating the task_basic_info pointer, you can create the struct on the stack, then use withUnsafeMutablePointer to get a pointer directly to it which you can pass in.
func report_memory() {
var info = mach_task_basic_info()
var count = mach_msg_type_number_t(sizeofValue(info))/4
let kerr: kern_return_t = withUnsafeMutablePointer(&info) {
task_info(mach_task_self_,
task_flavor_t(MACH_TASK_BASIC_INFO),
task_info_t($0),
&count)
}
if kerr == KERN_SUCCESS {
println("Memory in use (in bytes): \(info.resident_size)")
}
else {
println("Error with task_info(): " +
(String.fromCString(mach_error_string(kerr)) ?? "unknown error"))
}
}
Airspeed Velocity's answer in Swift 3...
func GetMemory()
{
var info = mach_task_basic_info()
var count = mach_msg_type_number_t(MemoryLayout.size(ofValue: info))/4
let kerr: kern_return_t = withUnsafeMutablePointer(to: &info)
{
task_info(mach_task_self_,
task_flavor_t(MACH_TASK_BASIC_INFO),
$0.withMemoryRebound(to: Int32.self, capacity: 1) { zeroPtr in
task_info_t(zeroPtr)
},
&count)
}
if kerr == KERN_SUCCESS {
print("Memory in use (in bytes): \(info.resident_size)")
}
else {
print("Error with task_info(): " +
(String.init(validatingUTF8: mach_error_string(kerr)) ?? "unknown error"))
}
}
