[Swig-user] string parameters produce unsafe JNI code

[Steve R. <sm...@su...>]

Howdy,

I've been looking at the JNI code produced by SWIG 1.3.29 when wrapping
functions that take a char* or const char* parameter.  The result doesn't
look safe to me.

Here's the interface file:

%module test

%inline{
void func1( const char* x ) {};
void func2( char* x ) {};
}

Running "swig -java" produces the following code in the "_wrap.c" file =20
for func1().

JNIEXPORT void JNICALL Java_testJNI_func1(JNIEnv *jenv, jclass jcls, =20
jstring jarg1) {
   char *arg1 =3D (char *) 0 ;

   (void)jenv;
   (void)jcls;
   arg1 =3D 0;
   if (jarg1) {
     arg1 =3D (char *)(*jenv)->GetStringUTFChars(jenv, jarg1, 0);
     if (!arg1) return ;
   }
   func1((char const *)arg1);
   if (arg1) (*jenv)->ReleaseStringUTFChars(jenv, jarg1, arg1);
}

Why is arg1 a char* rather than const char*?  It's a bit odd to see
casting away const from the GetStringUTFChars() call, and then casting
back to const to call func1().

I was alarmed at first to see that there's no exception thrown here
if GetStringUTFChars() returns NULL.  The JNI specifications document
http://java.sun.com/j2se/1.5.0/docs/guide/jni/spec/functions.html#wp17265
simply says it returns "NULL if the operation fails".  However, the older
"JNI Programmer's Guide and Specification"
http://java.sun.com/docs/books/jni/html/objtypes.html#4001
says that an exception is thrown.  Presumably we can trust the older
spec?


Here's what func2() becomes.

JNIEXPORT void JNICALL Java_testJNI_func2(JNIEnv *jenv, jclass jcls, =20
jstring jarg1) {
   char *arg1 =3D (char *) 0 ;

   (void)jenv;
   (void)jcls;
   arg1 =3D 0;
   if (jarg1) {
     arg1 =3D (char *)(*jenv)->GetStringUTFChars(jenv, jarg1, 0);
     if (!arg1) return ;
   }
   func2(arg1);
   if (arg1) (*jenv)->ReleaseStringUTFChars(jenv, jarg1, arg1);
}

What alarms me about this is that func2() could try to mutate the string,
which is presumably a very bad idea.  Shouldn't the default typemaps
be making a copy of the string in order to pass a non-const pointer to
func2() rather than casting away constness and praying?

Any insight appreciated.

-Steve