The following code snippet is a direct translation of the multiply-with-carry algorithm which can be found in various places (I took this one as a reference).
public class MultiplyWithCarryRandomGenerator
{
struct Static {
static var m_w:UInt = 521748629
static var m_z:UInt = 762436069
}
class var m_w:UInt{get{return Static.m_w } set{Static.m_w = newValue}};
class var m_z:UInt{get{return Static.m_z } set{Static.m_z = newValue}};
private class func GetUint()->UInt
{
m_z = 36969 * (m_z & 65535) + (m_z >> 16);
m_w = 18000 * (m_w & 65535) + (m_w >> 16);
return (m_z << 16) + m_w;
}
public class func GetUniform()->Double
{
return ((Double(GetUint()) + 1.0) * 2.32830643545449e-10);
}
}
Within XCode playground the uniform distribution remains somewhere between 0 and 40K while it should be in the interval (0,1).
Is there an obvious mistake in my code or an artifact of iOS, the playground...?
There are two differences between C# (the language of that example) and Swift that are causing this problem. The first is that a C# uint is a 32-bit unsigned integer, while a UInt in Swift is an unsigned integer matched to the architecture of the system it's executing on, which means in most cases today UInt is a 64-bit unsigned integer. Since all the constants in your code are geared toward 32 bits, simply change all the UInt declarations to UInt32 and you're halfway there.
The second difference is that addition operations overflow automatically in C# when working with unsigned integers, while in Swift an overflow crashes. You aren't seeing an issue yet, since you're using 32-bit constants with a 64-bit datatype, but after switching to UInt32 you'll start crashing on the overflow in this line:
return (m_z << 16) + m_w;
Swift provides an alternate set of operators, prefixed by &, that silently allow overflow—using &+ in that line solves the issue:
return (m_z << 16) &+ m_w;
And now you get the graph you were hoping for:
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I am trying to call a C function from Swift , but I do not know exactly how to define variables to pass parameters. This is the function declaration:
/* Function Declarations */
extern void compute_feature_set(const double input[11025],
double M_data[], int M_size[2],
double D_data[], int D_size[2],
double DD_data[],
int DD_size[2],
double VT[10], double *mp,
double r_42[42], double *fM);
The data is an array of floats. So I tried :
let s = data.compactMap{ Double($0)}
var mSize = Array<Int32>(repeating:Int32(0.0), count:2)
var dSize = Array<Int32>(repeating:Int32(0.0), count:2)
var dD_Size = Array<Int32>(repeating:Int32(0.0), count:2)
var mData = Array<Double>(repeating:0.0, count:48)
var dData = Array<Double>(repeating:0.0, count:48)
var dD_Data = Array<Double>(repeating:0.0, count:48)
var vt = Array<Double>(repeating:0.0, count:10)
var mp = Double(0.0)
var r = Array<Double>(repeating:0.0, count:42)
var fM = Double(0)
compute_feature_set(s, &cout, &mSize, &vx, &dSize, &dD_Data, &dD_Size, &vcta, &mp, &r, &fM)
When I run the code in Clion with the following function it works fine and the output matches the expected values:
static void main_compute_feature_set(void)
{
static double dv[11025];
double DD_data[48];
double D_data[48];
double M_data[48];
double r_42[42];
double VT[10];
double fM;
double mp;
int DD_size[2];
int D_size[2];
int M_size[2];
/* Initialize function 'compute_feature_set' input arguments. */
/* Initialize function input argument 'input'. */
/* Call the entry-point 'compute_feature_set'. */
argInit_11025x1_real_T(dv);
compute_feature_set(dv, M_data, M_size, D_data, D_size,
DD_data, Dd_size, VT,
&mp, r_42, &fM);
}
However, when I run my implementation in Swift, I get very different results.
You could try passing pointers of the Arrays, rather than the Arrays directly.
Using Imported C Functions in Swift | Apple Developer Documentation
Call Functions with Pointer Parameters
Whenever possible, Swift avoids giving you direct access to pointers. When importing C function parameters, however, Swift maps pointer parameters to standard library pointer types.
The following tables use Type as a placeholder type name to indicate syntax for the mappings.
For return types, variables, and arguments, the following mappings apply:
C Syntax
Swift Syntax
const Type *
UnsafePointer<Type>
Type *
UnsafeMutablePointer<Type>
double[] is pretty much equivalent to double * in this case.
Looks like the problem with your code is passing data to your function. You use compactMap to make an Array of Double and then pass the pointer of this array. But Array and Double are struct in Swift so you pass the pointer of struct with structs instead of array of double values.
To convert your data to array of bytes you should use withUnsafeBytes e.g.:
Swift:
let data = Data([0xaa, 0xbb, 0xcc, 0xdd])
data.withUnsafeBytes {
passData($0)
}
C/ObjC:
void passData(const double input[11025]) {
NSLog(#"%x", input[0]); // Prints: ddccbbaa
}
I am pretty new to Swift, and I don't have much exposure to C.
I am trying to write a function in C that will get a Swift string that I can then do something with. The problem is that I'm not 100% sure what the type should be in Swift to make C like what it sees.
So far, I have found several examples on Stack that seem like good starting points, but some examples seem dated for the current version of Swift.
I first started by using this example to get C and Swift talking to one another: Swift call C call Swift? I then took that and tried updating the Swift function to return a string of some kind. I understand that it needs to be a UTF-8 return type, but I'm not sure how to go about sending things properly. I've looked at How to pass a Swift string to a c function?, How to convert String to UnsafePointer<UInt8> and length, and How to convert string to unicode(UTF-8) string in Swift?, but none of them really work for a solution. Or I'm just typing it in incorrectly. So far, the closest I can get to returning something is as follows.
In Swift, my ViewController is:
import UIKit
class ViewController: UIViewController {
#_silgen_name("mySwiftFunc") // give the function a C name
public func mySwiftFunc(number: Int) -> [CChar]
{
print("Hello from Swift: \(number)")
let address: String = "hello there";
let newString = address.cString(using: String.Encoding.utf8)
return newString!
}
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
blah()
}
}
And in C, the header is like:
#ifndef cfile_h
#define cfile_h
#include <stdio.h>
const char * mySwiftFunc(int);
int blah(void);
#endif /* cfile_h */
And the source is like:
#include "cfile.h"
int blah() {
const char * retVal = mySwiftFunc(42); // call swift function
printf("Hello from C: %s", retVal);
return 0;
}
There is a bridging header file that just has #include "cfile.h". Obviously, there is still a lot of remnants from the first example, and these will be cleaned up later.
What needs to change to make this work? Right now, the console spits out
Hello from Swift: 42
Hello from C: (B\214
The Swift equivalent of const char * is UnsafePointer<CChar>?, so that's the correct return value. Then you have to think about memory management. One options is do allocate memory for the C string in the Swift function, and leave it to the caller to release the memory eventually:
public func mySwiftFunc(number: Int) -> UnsafePointer<CChar>? {
print("Hello from Swift: \(number)")
let address = "hello there"
let newString = strdup(address)
return UnsafePointer(newString)
}
passes a Swift string to strdup() so that a (temporary) C string representation is created automatically. This C string is then duplicated. The calling C function has to release that memory when it is no longer needed:
int blah() {
const char *retVal = mySwiftFunc(42);
printf("Hello from C: %s\n", retVal);
free((char *)retVal);
return 0;
}
⚠️ BUT: Please note that there are more problems in your code:
mySwiftFunc() is an instance method of a class, and therefore has an implicit self argument, which is ignored by the calling C function. That might work by chance, or cause strange failures.
#_silgen_name should not be used outside of the Swift standard library, see this discusssion in the Swift forum.
A slightly better alternative is #_cdecl but even that is not officially supported. #_cdecl can only be used with global functions.
Generally, calling Swift functions directly from C is not officially supported, see this discussion in the Swift forum for the reasons and possible alternatives.
I am using a 3rd party C library in my iOS application, which I am in the process of converting from Objective-C to Swift. I hit an obstacle when attempting to read one of the structs returned by the C library in Swift.
The struct looks similar to this:
typedef unsigned int LibUint;
typedef unsigned char LibUint8;
typedef struct RequestConfiguration_ {
LibUint8 names[30][128];
LibUint numberNames;
LibUint currentName;
} RequestConfiguration;
Which is imported into Swift as a Tuple containing 30 Tuples of 128 LibUint8 values. After a long time of trial and error using nested withUnsafePointer calls, I eventually began searching for solutions to iterating a Tuple in Swift.
What I ended up using is the following functions:
/**
* Perform iterator on every children of the type using reflection
*/
func iterateChildren<T>(reflectable: T, #noescape iterator: (String?, Any) -> Void) {
let mirror = Mirror(reflecting: reflectable)
for i in mirror.children {
iterator(i.label, i.value)
}
}
/**
* Returns a String containing the characters within the Tuple
*/
func libUint8TupleToString<T>(tuple: T) -> String {
var result = [CChar]()
let mirror = Mirror(reflecting: tuple)
for child in mirror.children {
let char = CChar(child.value as! LibUint8)
result.append(char)
// Null reached, skip the rest.
if char == 0 {
break;
}
}
// Always null terminate; faster than checking if last is null.
result.append(CChar(0))
return String.fromCString(result) ?? ""
}
/**
* Returns an array of Strings by decoding characters within the Tuple
*/
func libUint8StringsInTuple<T>(tuple: T, length: Int = 0) -> [String] {
var idx = 0
var strings = [String]()
iterateChildren(tuple) { (label, value) in
guard length > 0 && idx < length else { return }
let str = libUint8TupleToString(value)
strings.append(str)
idx++
}
return strings
}
Usage
func handleConfiguration(config: RequestConfiguration) {
// Declaration types are added for clarity
let names: [String] = libUint8StringsInTuple(config.names, config.numberNames)
let currentName: String = names[config.currentName]
}
My solution uses reflection to iterate the first Tuple, and reflection to iterate the second, because I was getting incorrect strings when using withUnsafePointer for the nested Tuples, which I assume is due to signage. Surely there must be a way to read the C strings in the array, using an UnsafePointer alike withUsafePointer(&struct.cstring) { String.fromCString(UnsafePointer($0)) }.
To be clear, I'm looking for the fastest way to read these C strings in Swift, even if that involves using Reflection.
Here is a possible solution:
func handleConfiguration(var config: RequestConfiguration) {
let numStrings = Int(config.numberNames)
let lenStrings = sizeofValue(config.names.0)
let names = (0 ..< numStrings).map { idx in
withUnsafePointer(&config.names) {
String.fromCString(UnsafePointer<CChar>($0) + idx * lenStrings) ?? ""
}
}
let currentName = names[Int(config.currentName)]
print(names, currentName)
}
It uses the fact that
LibUint8 names[30][128];
are 30*128 contiguous bytes in memory. withUnsafePointer(&config.names)
calls the closure with $0 as a pointer to the start of that
memory location, and
UnsafePointer<CChar>($0) + idx * lenStrings
is a pointer to the start of the idx-th subarray. The above code requires
that each subarray contains a NUL-terminated UTF-8 string.
The solution suggested by Martin R looks good to me and, as far as I can see from my limited testing, does work. However, as Martin pointed out, it requires that the strings be NUL-terminated UTF-8. Here are two more possible approaches. These follow the principle of handling the complexity of C data structures in C instead of dealing with it in Swift. Which of these approaches you choose depends on what specifically you are doing with RequestConfiguration in your app. If you are not comfortable programming in C, then a pure Swift approach, like the one suggested by Martin, might be a better choice.
For the purposes of this discussion, we will assume that the 3rd party C library has the following function for retrieving RequestConfiguration:
const RequestConfiguration * getConfig();
Approach 1: Make the RequestConfiguration object available to your Swift code, but extract names from it using the following C helper function:
const unsigned char * getNameFromConfig(const RequestConfiguration * rc, unsigned int nameIdx)
{
return rc->names[nameIdx];
}
Both this function's signature and the RequestConfiguration type must be available to the Swift code via the bridging header. You can then do something like this in Swift:
var cfg : UnsafePointer<RequestConfiguration> = getConfig()
if let s = String.fromCString(UnsafePointer<CChar>(getNameFromConfig(cfg, cfg.memory.currentName)))
{
print(s)
}
This approach is nice if you need the RequestConfiguration object available to Swift in order to check the number of names in multiple places, for example.
Approach 2: You just need to be able to get the name at a given position. In this case the RequestConfiguration type does not even need to be visible to Swift. You can write a helper C function like this:
const unsigned char * getNameFromConfig1(unsigned int idx)
{
const RequestConfiguration * p = getConfig();
return p->names[idx];
}
and use it in Swift as follows:
if let s = String.fromCString(UnsafePointer<CChar>(getNameFromConfig1(2)))
{
print(s)
}
This will print the name at position 2 (counting from 0). Of course, with this approach you might also want to have C helpers that return the count of names as well as the current name index.
Again, with these 2 approaches it is assumed the strings are NUL-terminated UTF-8. There are other approaches possible, these are just examples.
Also please note that the above assumes that you access RequestConfiguration as read-only. If you also want to modify it and make the changes visible to the 3rd party library C code, then it's a different ballgame.
var cpuInfo: processor_info_array_t = nil
var numCpuInfo: mach_msg_type_number_t = 0
var coresTotalUsage: Float = 0.0
var numCPUsU: natural_t = 0
let err = host_processor_info(mach_host_self(), PROCESSOR_CPU_LOAD_INFO, &numCPUsU, &cpuInfo, &numCpuInfo);
assert(err == KERN_SUCCESS, "Failed call to host_processor_info")
Hi, I am calling the above C API host_processor_info to get process load informations from swift, no problem there.
cpuInfo is a inout parameter (pointer) that, on return, will point to a structure containing the CPU information allocated by that API.
The caller is reponsible for deallocating the memory; I can do that easily from objective C but haven't had any luck in swift. I know I could wrap that call into an objective C extension but I'm trying to learn swift and would like, if possible, avoid the obj-c solution.
in obj-c I would deallocate with:
size_t cpuInfoSize = sizeof(integer_t) * numCpuInfo;
vm_deallocate(mach_task_self(), (vm_address_t) cpuInfo, cpuInfoSize)
cpuInfo in swift is an UnsafeMutablePointer not convertible into a vm_address_t.
Any help appreciated, thanks.
processor_info_array_t is a pointer type, and vm_address_t is an integer type
(ultimately an alias for UInt). (Judging from the comments in <i386/vm_types.h>
this might to be for historical reasons.)
The only way to convert a pointer to an integer (of the same size) in Swift is unsafeBitCast.
mach_init.h defines
extern mach_port_t mach_task_self_;
#define mach_task_self() mach_task_self_
Only the extern variable is visible in Swift, not the macro.
This gives:
let cpuInfoSize = vm_size_t(sizeof(integer_t)) * vm_size_t(numCpuInfo)
vm_deallocate(mach_task_self_, unsafeBitCast(cpuInfo, vm_address_t.self), cpuInfoSize)
In Swift 4, the equivalent code appears to be:
let cpuInfoSize = vm_size_t(MemoryLayout<integer_t>.stride * Int(numCpuInfo))
vm_deallocate(mach_task_self_, vm_address_t(bitPattern: cpuInfo), cpuInfoSize)
In particular, the initializer UInt(bitPattern:) is now apparently preferred to unsafeBitCast() to initialize an unsigned integer with the bit pattern of a pointer (I guess this usage is not longer considered "unsafe"). It correctly handles a nil pointer, returning 0 in this case.
I'm more of an Android developer, but i'm beginning to see the light at the end of the tunnel on iOS development.
There is, however, one coding pattern I can't seem to find an equivalent for.
The use of static fields as flags.
Android :
public final static int ERROR_EMPTY = 1;
public final static int ERROR_NO_CONNECTION = 2;
public final static int ERROR_WRONG_USER = 4;
...
if (error == MyClass.ERROR_EMPTY) {//do things}
What would be the proper way to achieve this on iOS ?
Thanks.
Using Objective-C and C
i often use prefixes:
typedef enum MyClass_Error {
// never use MyClass_Error_Undefined
// or you may favor MyClass_Error_None for a valid error code
MyClass_Error_Undefined = 0,
MyClass_Error_Empty = 1,
MyClass_Error_NoConnection = 2,
MyClass_Error_WrongUser = 4
// ...
} MyClass_Error;
for these value collections. then you get benefits such as typesafety and switch value checking.
for non-type constants:
enum { MyClass_ConstantName = 4 };
and feel free to hide these in the *.m when private.
also note that C enums may have gaps in their defined values (unlike Java's).
Update: there's an even better way to declare an enum, as demonstrated in Abizern's answer -- if you're sticking with the most recent toolchains. the big reason to use this extension is for binary compatibility and encoding (although i favor fixed-width types for these purposes).
There are a few other variations, for the cases when you want to use existing types:
Private Constant
MyClass.m
static const NSRange MyClass_InputRange = {1,1};
Public Constant
MyClass.h
extern const NSRange MyClass_InputRange;
MyClass.m
const NSRange MyClass_InputRange = {1,1};
Using C++
You would likely favor introducing a new scope for these values -- either in a class or a namespace, rather than simulating the scope using prefixes.
Common Mistakes
Use of #define for constants (unless definition is mandatory when preprocessing)
Use of short identifiers, and identifiers which are not prefixed
Use of static values in headers
Not using const when possible
Declaring them in the header, when they could be in the *.m source.
Just to add to Justin's excellent answer - the Modern Objective-C definition for the enum would be:
typedef enum MyClass_Error : NSUInteger {
// never use MyClass_Error_Undefined
// or you may favor MyClass_Error_None for a valid error code
MyClass_Error_Undefined = 0,
MyClass_Error_Empty = 1,
MyClass_Error_NoConnection = 2
// ...
} MyClass_Error;