Convert Swift string into CChar pointer

Question

In my swift app project, I have to interact with C APIs. One of the C function takes a char pointer as input, therefore I have to convert my swift string into char pointer. But I don't know how to do it properly.

Here's my sample code:

var user_id_ptr : UnsafeMutablePointer<CChar> = UnsafeMutablePointer.alloc(100)
var cusID = "123"  
user_id_ptr.memory = cusID.cStringUsingEncoding(NSUTF8StringEncoding)!

And I always got this error: Cannot assign a value of type '[CChar]' to a value of type 'CChar'

I also check the answer in: UnsafeMutablePointer<Int8> from String in Swift

Above answer is slightly different from mine. The example above is for only one character, but I have a string. Also, I have to pass multiple arguments to the function, I can't use something like let result = "N".withCString { dgeev_(UnsafeMutablePointer($0), UnsafeMutablePointer($0), &N, ...) }. Please show me some example for how to complete this.

Answer 1

If the C function does not mutate the strings passed as arguments, then the parameters should be declared as const char *, for example

int func1(const char *s1, const char *s2);

and in that case you can simply pass Swift strings which are automatically converted (compare String value to UnsafePointer<UInt8> function parameter behavior):

let str1 = "first string argument"
let str2 = "second string argument"

let result1 = func1(str1, str2)

If the function parameters are declared as char *

int func2(char *s1, char *s2);

then you have to use withCString() to obtain a temporary representation of the string as a NUL-terminated array of UTF-8 characters:

let str1 = "first string argument"
let str2 = "second string argument"

// Swift 2:
let result2 = str1.withCString { s1 in
    str2.withCString { s2 in
        func2(UnsafeMutablePointer(s1), UnsafeMutablePointer(s2))
    }
}

// Swift 3:
let result2 = str1.withCString { s1 in
    str2.withCString { s2 in
        func2(UnsafeMutablePointer(mutating: s1), UnsafeMutablePointer(mutating: s2))
    }
}

Note that this still assumes that the function does not mutate the passed strings, the UnsafeMutablePointer() conversion is only needed to make the compiler happy.