Mapping C# string to std::string in native plugin causes crash

Alright I found out after a day's work that my Android app was crashing because I was mapping c# string to std::string in my native plugin.
So how can we actually pass strings between NDK and Unity?

 

Code (csharp):

1.  
2. char *myString;
3.  

?

 

Thanks for the reply Tseng
Here is the function I've been calling

Code (csharp):

1.  
2.     EXPORT_API const char* getProperty(char* name){
3.         const char* result= FDMManager::getInstance()->getProperty(name);
4.         LOGD("INTERFACE - GETPROPERTY - %s", result);
5.         return result;
6.     }
7.  

From C# side, I can successfully pass name. result is also received fine from FDMManager and printed from the LOGD command. But things don't go as expected when the function returns.
When I declare this function as returning string as follows

Code (csharp):

1. private static extern string getProperty(string name);

my Android app crashes giving a crash dump.

When I try to marshal it by using

Code (csharp):

1.  
2.     [DllImport ("jinterface", CharSet=CharSet.Ansi, EntryPoint="getProperty", ExactSpelling=true)]
3.     private static extern IntPtr getProperty(string name);
4.  

and

Code (csharp):

1.     public static string GetProperty(string name){
2.         Debug.Log("GetProperty -"+getProperty(name));
3.         string val=Marshal.PtrToStringAnsi(getProperty(name));
4.         Debug.Log("GetProperty -"+val);
5.         return val;
6.     }

Then I just receive empty string.

 

When returning/out'ing strings, the strings are not suitable for it, since strings are immutable.

I remember reading that in out cases (public extern void something(out string someString)) one should user StringBuilder as the marshaling of it works well with char* outs/results.

Isn't Marshal.PtrToStringAnsi unsafe (unmanaged) code and needs to run in either a unsafe block or unsafe declared method?

Though, it should be noted here that Unity's disables unsafe code, due to some "security issues" which iirc allowed to enable/access some of the pro features with basic license. So no unsafe code possible at the moment

 

Thanks a lot Tseng
It's working now. I changed my native function to the following

Code (csharp):

1.  
2.     EXPORT_API void getProperty(char* name, char* val){
3.         const char* result= FDMManager::getInstance()->getProperty(name);
4.         LOGD("INTERFACE - GETPROPERTY - %s", result);
5.         strcpy(val,result);
6.     }
7.  

Is this code alright taking memory leaks into consideration?