A function to convert a C char array to a String

Question

The statfs struct in C has these char array members:

char f_mntonname[MNAMELEN];    /* directory on which mounted */
char f_mntfromname[MNAMELEN];  /* mounted file system */

The Swift type for these arrays in the Darwin.sys.mount module is a tuple with 90 elements:

public var f_mntonname: (Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8,
                         ...
                         Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8)

Another question about Converting a C char array to a String has an answer with code that I use twice in this example:

import Darwin.sys.mount
var vols: UnsafeMutablePointer<statfs>?
let count = getmntinfo(&vols, 0)    
if let volsarray = vols, count > 0 {
    for i in 0 ..< count {
        var vol = volsarray[Int(i)]
        let mntOnName = withUnsafePointer(to: &vol.f_mntonname) {
            $0.withMemoryRebound(to: UInt8.self,
                                 capacity: MemoryLayout.size(ofValue: vol.f_mntonname)) {
                String(cString: $0)
            }
        }
        let mntFromName = withUnsafePointer(to: &vol.f_mntfromname) {
            $0.withMemoryRebound(to: UInt8.self,
                                 capacity: MemoryLayout.size(ofValue: vol.f_mntfromname)) {
                String(cString: $0)
            }
        }
        print("on \(mntOnName)  from \(mntFromName)")
    }
}

To avoid repeating the code twice, I refactored the conversion code into the function below, but passing &vol.f_mntonname to it won't compile, and I can't see a way to fix the problem by using a different type for the first argument.

func charArrayToString(_ array: UnsafePointer<Int8>, capacity: Int) -> String {
    return array.withMemoryRebound(to: UInt8.self, capacity: capacity) {
        String(cString: $0)
    }
}

var a = (Int8(65), Int8(66), Int8(67))
print(charArrayToString(&a, capacity: 3)) // doesn't compile

The compiler complains about call to charArrayToString:
Cannot convert value of type '(Int8, Int8, Int8)' to expected argument type 'Int8'

It also complains when I pass a instead of &a:
Cannot convert value of type '(Int8, Int8, Int8)' to expected argument type 'UnsafePointer<Int8>'

Community · Accepted Answer · 2020-06-20 09:12:55Z

You are trying to pass a pointer to a tuple as a point to a Int8. Instead you need to pass a pointer to the first element of the tuple. For example define the function:

func charPointerToString(_ pointer: UnsafePointer<Int8>) -> String
{
   return String(cString: UnsafeRawPointer(pointer).assumingMemoryBound(to: CChar.self))
}

(This function is adapted from and example in Apple's UnsafeRawPointer Migration). Use this in your code to shorten your two let's passing a pointer to the first tuple element:

let mntOnName = charPointerToString(&vol.f_mntonname.0)
let mntFromName = charPointerToString(&vol.f_mntfromname.0)

However, despite that following the code in Apple's document it seems a bit overkill, you can also just:

let mntOnName = String(cString: &vol.f_mntonname.0)
let mntFromName = String(cString: &vol.f_mntfromname.0)

(All code tested using Xcode 8.2.1.)

HTH

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