Swift 5 : how to deallocate memory, allocated by shared library - swift

Question

I have the following C function inside a shared library :
int GetRxDataBlock( char** data )
{
CHECK_FACADE_INITIALIZATION( "getRxDataBlock : Client facade not initialized", __LINE__ );
if ( data == nullptr ) {
printErrorMsg("getRxDataBlock : nullptr", __LINE__);
return -1;
}
int ret = 0;
try {
std::string rxData = g_facade.value()->getRxDataBlock();
*data = (char*) malloc(rxData.size() + 1);
memset(*data, 0, rxData.size() + 1 );
memcpy(*data, rxData.c_str(), rxData.size());
}catch(...) {
ret = -1;
}
return ret;
}
And this is the way I call it from Swift code :
var rxData: UnsafeMutablePointer? = nil
let apiResponse = GetRxDataBlock(&rxData)
print("Rx data : ret = \(apiResponse)")
if let dataStr = rxData {
let rxStr = String(cString: dataStr)
print("Rx data = \(rxStr)")
}
What is the correct way to deallocate memory returned by GetRxDataBlock function ?

Answer 1

malloc()ed memory must be released with free():
if let dataStr = rxData {
// do something with `dataStr`
// ...
free(dataStr)
}
This is also a good use-case for defer, which calls the closure just before the scope of the block is left:
if let dataStr = rxData {
defer { free(dataStr) }
// do something with `dataStr`
// ...
}
On Apple platforms, free() is imported from the standard C library as part of the Darwin module (which is imported by Foundation, AppKit, or UIKit). On Linux you would import Glibc.