How can I make Swig correctly wrap a char* buffer that is modified in C as a Java Something-or-other?

Question

I am trying to wrap some legacy code for use in Java and I was quite happy to see that Swig was able to handle the header file and it generate a great wrapper that almost works. Now I am looking for the deep magic that will make it really work.

In C I have a function that looks like this

DLL_IMPORT int DustyVoodoo(char *buff, int len,  char *curse);

This integer returned by this function is an error code in case it fails. The arguments are

• buff is a character buffer
• len is the length of the data in the buffer
• curse the another character buffer that contains the result of calling DustyVoodoo

So, you can see where this is going, the result is actually coming back via the third argument. Also len is confusing since it may be the length of both buffers, they are always allocated as being the same size in calling code but given what DustyVoodoo does I don't think that they need be the same. To be safe both buffers should be the same size in practice, say 512 chars.

The C code generated for the binding is as follows:

SWIGEXPORT jint JNICALL Java_pemapiJNI_DustyVoodoo(JNIEnv *jenv, jclass jcls, jstring 

jarg1, jint jarg2, jstring jarg3) {
  jint jresult = 0 ;
  char *arg1 = (char *) 0 ;
  int arg2 ;
  char *arg3 = (char *) 0 ;
  int result;

  (void)jenv;
  (void)jcls;
  arg1 = 0;
  if (jarg1) {
    arg1 = (char *)(*jenv)->GetStringUTFChars(jenv, jarg1, 0);
    if (!arg1) return 0;
  }
  arg2 = (int)jarg2; 
  arg3 = 0;
  if (jarg3) {
    arg3 = (char *)(*jenv)->GetStringUTFChars(jenv, jarg3, 0);
    if (!arg3) return 0;
  }
  result = (int)PemnEncrypt(arg1,arg2,arg3);
  jresult = (jint)result; 
  if (arg1) (*jenv)->ReleaseStringUTFChars(jenv, jarg1, (const char *)arg1);
  if (arg3) (*jenv)->ReleaseStringUTFChars(jenv, jarg3, (const char *)arg3);
  return jresult;
}

It is correct for what it does; however, it misses the fact that cursed is not just an input, it is altered by the function and should be returned as an output. It also does not know that the java Strings are really buffers and should be backed by a suitably sized array.

I think that Swig can do the right thing here, I just can't figure out from the documentation how to tell Swig what it needs to know. Any typemap masers in the house?

Answer 1

Thanks to Thomas for the nudge in the correct direction. The solution to this was to create a custom typemap that uses a StringBuffer to get the result back. I found the code in the examples/java/typemap directory of the SWIG install. I must have overlooked that before while I was searching.

I have attached the example code below, I am presently using the alternative approach suggested. However, the first approach of using the BYTE typemap wil require some changes in my Java code but might actually make more sense in the long run.

Thanks for the help, and now I get to see if I can accept my own answer...

/* File : example.i */
%module example
%{
/*
   example of a function that returns a value in the char * argument
   normally used like:

   char buf[bigenough];
   f1(buf);
*/

void f1(char *s) {
  if(s != NULL) {
    sprintf(s, "hello world");
  }
}

void f2(char *s) {
  f1(s);
}

void f3(char *s) {
  f1(s);
}

%}

/* default behaviour is that of input arg, Java cannot return a value in a 
 * string argument, so any changes made by f1(char*) will not be seen in the Java
 * string passed to the f1 function.
*/
void f1(char *s);

%include various.i

/* use the BYTE argout typemap to get around this. Changes in the string by 
 * f2 can be seen in Java. */
void f2(char *BYTE);



/* Alternative approach uses a StringBuffer typemap for argout */

/* Define the types to use in the generated JNI C code and Java code */
%typemap(jni) char *SBUF "jobject"
%typemap(jtype) char *SBUF "StringBuffer"
%typemap(jstype) char *SBUF "StringBuffer"

/* How to convert Java(JNI) type to requested C type */
%typemap(in) char *SBUF {

  $1 = NULL;
  if($input != NULL) {
    /* Get the String from the StringBuffer */
    jmethodID setLengthID;
    jclass sbufClass = (*jenv)->GetObjectClass(jenv, $input);
    jmethodID toStringID = (*jenv)->GetMethodID(jenv, sbufClass, "toString", "()Ljava/lang/String;");
    jstring js = (jstring) (*jenv)->CallObjectMethod(jenv, $input, toStringID);

    /* Convert the String to a C string */
    const char *pCharStr = (*jenv)->GetStringUTFChars(jenv, js, 0);

    /* Take a copy of the C string as the typemap is for a non const C string */
    jmethodID capacityID = (*jenv)->GetMethodID(jenv, sbufClass, "capacity", "()I");
    jint capacity = (*jenv)->CallIntMethod(jenv, $input, capacityID);
    $1 = (char *) malloc(capacity+1);
    strcpy($1, pCharStr);

    /* Release the UTF string we obtained with GetStringUTFChars */
    (*jenv)->ReleaseStringUTFChars(jenv,  js, pCharStr);

    /* Zero the original StringBuffer, so we can replace it with the result */
    setLengthID = (*jenv)->GetMethodID(jenv, sbufClass, "setLength", "(I)V");
    (*jenv)->CallVoidMethod(jenv, $input, setLengthID, (jint) 0);
  }
}

/* How to convert the C type to the Java(JNI) type */
%typemap(argout) char *SBUF {

  if($1 != NULL) {
    /* Append the result to the empty StringBuffer */
    jstring newString = (*jenv)->NewStringUTF(jenv, $1);
    jclass sbufClass = (*jenv)->GetObjectClass(jenv, $input);
    jmethodID appendStringID = (*jenv)->GetMethodID(jenv, sbufClass, "append", "(Ljava/lang/String;)Ljava/lang/StringBuffer;");
    (*jenv)->CallObjectMethod(jenv, $input, appendStringID, newString);

    /* Clean up the string object, no longer needed */
    free($1);
    $1 = NULL;
  }  
}
/* Prevent the default freearg typemap from being used */
%typemap(freearg) char *SBUF ""

/* Convert the jstype to jtype typemap type */
%typemap(javain) char *SBUF "$javainput"

/* apply the new typemap to our function */
void f3(char *SBUF);

Answer 2

Perhaps this part of the SWIG documentation is helpful:

A common problem in some C programs is handling parameters passed as simple pointers or references. For example:

void add(int x, int y, int *result) {
    *result = x + y;
}

[...]

The typemaps.i library file will help in these situations. For example:

%module example
%include "typemaps.i"

void add(int, int, int *OUTPUT);

There's also a section on wrapping arrays.

I'm sorry this isn't a ready-made, complete answer. SWIG is mind-bending at times.