Convert Swift String array to c char**

Question

I'm calling a C library from Swift 4 and I have troubles converting a [String] to const char *[].

The C API defines this method:

int getDREFs(const char* drefs[], unsigned char count);

which is exposed in Swift as

public func getDREFs(_ drefs: UnsafeMutablePointer<UnsafePointer<Int8>?>!, _ count: UInt8) -> Int32

The Swift wrapper I'm trying to write is the following

public func get(drefs: [String]) throws {

    var cDrefs = [UnsafePointer<Int8>]()
    for dref in drefs {
        cDrefs.append(dref.cString(using: .utf8)!)
    }
    let pDrefs = UnsafeMutablePointer<UnsafePointer<Int8>>(&cDrefs)
    getDREFFs(pDrefs, drefs.count)

}

but the error I get is

Cannot convert value of type 'UnsafeMutablePointer<UnsafePointer<Int8>>' to expected argument type 'UnsafeMutablePointer<UnsafePointer<Int8>?>!'

what am I missing?

Any feedback on the answer? Please let me know if you need more information. — Martin R
– Martin R, Commented Jan 5, 2018 at 10:15

Martin R · Accepted Answer · 2017-12-20 18:52:26Z

getDREFSs expects a pointer to an array of optional Int8 pointers. Also the second argument must be converted to UInt8.

So this would compile:

public func get(drefs: [String]) -> Int {
    var cDrefs = [UnsafePointer<Int8>?]()
    for dref in drefs {
        cDrefs.append(dref.cString(using: .utf8))
    }
    let result = getDREFs(&cDrefs, UInt8(drefs.count))
    return Int(result)
}

But a quick test shows that is does not work if called with multiple strings. The reason is that the arrays returned by dref.cString(using: .utf8) can already be deallocated (and the pointer invalid) when the C function is called.

Here is a working version, a slight modification of Convert a Swift Array of String to a to a C string array pointer for this particular case:

public func get(drefs: [String]) -> Int {
    var cargs = drefs.map { UnsafePointer<Int8>(strdup($0)) }
    let result = getDREFs(&cargs, UInt8(drefs.count))
    for ptr in cargs { free(UnsafeMutablePointer(mutating: ptr)) }
    return Int(result)
}

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