Why does the first character of my string get cut off when I pass from swift to C? - c

Question

Here is the function in swift to convert from a swift string to a C string
func swiftStringToCString(swiftString: String) -> UnsafeMutablePointer<CString>?{
let convertedCString: [CChar]? = swiftString.cString(using: .utf8)
if let safeConvertedCString = convertedCString {
var cString = UnsafeMutablePointer<CString>.allocate(capacity: 1)
//defer {
// cString.deallocate()
//}
cString.pointee.count = UInt32(safeConvertedCString.count) - 1
cString.pointee.data = UnsafePointer<Int8>(safeConvertedCString)
return cString
}
else
{
return nil
}
}
The CString struct is defined in a C header file:
typedef struct {
const char *data;
uint32_t count;
} CString;
I also have an addition test function which simply prints out the string passed in:
extern void __cdecl testCString(CString *pCString);
When I call
testCString(swiftStringToCString(swiftString: "swiftString"))
This gets printed out:
wiftString
I also noticed that I get the warning
Initialization of 'UnsafePointer<Int8>' results in a dangling pointer
when I do
cString.pointee.data = UnsafePointer<Int8>(safeConvertedCString)

Answer 1

This approach is incorrect. You can't just hold onto pointers into a String's internal storage and expect it to stick around past the current line of code. That's what the warning is telling you.
In order to ensure that a pointer is valid, you need to use .withUnsafeBufferPointer. I would expect something along these lines:
"swiftString".utf8CString.withUnsafeBufferPointer { buffer in
var cString = CString(data: buffer.baseAddress!, count: UInt32(buffer.count))
testCString(&cString);
}
This ensures that the utf8 buffer exists until the end of the block. If you want to hold onto it beyond that, you're going to need to add code to copy those bytes into memory you allocate and release yourself.