I'm working through Sheng Liang's "The JNI Programmer's Guide and
Specification", and am on chapter 3, working the "Prompt" example.  I'm
working through all the examples using gcc on GNU/Linux and VC++ on MS
Windows.  Here's the C code in Sheng's book:

  #include <stdio.h>
  #include "Prompt.h"

  JNIEXPORT jstring JNICALL
  Java_Prompt_getLine( JNIEnv *env, jobject obj, jstring prompt )
  {
     char buf[128];
     const jbyte *str;

     str = (*env)->GetStringUTFChars( env, prompt, NULL );
     if ( str == NULL ) return NULL;

     printf("%s", str);
     (*env)->ReleaseStringUTFChars(env, prompt, str);

     scanf("%s", buf);
     return (*env)->NewStringUTF(env, buf);
  }

When I compile this with gcc, I get the warnings:

  Prompt.c: In function Java_Prompt_getLine() Prompt.c:16: warning: pointer
     targets in assignment differ in signedness

  Prompt.c:20: warning: pointer targets in passing argument 3 of
     (*env)->ReleaseStringUTFChars() differ in signedness

The code to focus on here is:

     const jbyte *str;
     str = (*env)->GetStringUTFChars( env, prompt, NULL );
     (*env)->ReleaseStringUTFChars(env, prompt, str);

Looking into jni.h:

  const char* (JNICALL *GetStringUTFChars)
     (JNIEnv *env, jstring str, jboolean *isCopy);

  void (JNICALL *ReleaseStringUTFChars)
     (JNIEnv *env, jstring str, const char* chars);

And from linux/jni_md.h:

  typedef signed char jbyte;

Doing some Googling, it appears that the C standard allows char to be either
signed or unsigned by default.  Using a small test program where I set a char
variable equal to 255, it appears that with gcc on GNU/Linux, a char by
default is signed.

So I don't see the problem here.

  * GetStringUTRChars() returns a ptr to a signed char.
  * The 3rd arg of ReleaseStringUTFChars() takes a ptr to a signed char.
  * A jbyte is a signed char

So what exactly is gcc complaining about?

Thanks for reading down this far.  :)

Pete