How can you assign a variable a value from another class? - c++

Question

I am writing a program that needs a pathname to eventually create a global string.
I currently have this pathname hard coded, but would like to use a global variable to replace this.
The problem I am having is that my global variable is undefined. I am also using JNI and the error I am getting is:
#
# A fatal error has been detected by the Java Runtime Environment:
#
# SIGSEGV (0xb) at pc=0x00007f65f981ddd0, pid=11660, tid=140075985102592
#
# JRE version: 7.0_21-b02
# Java VM: OpenJDK 64-Bit Server VM (23.7-b01 mixed mode linux-amd64 compressed oops)
# Problematic frame:
# C [libstdc++.so.6+0x9ddd0] std::string::size() const+0x0
#
# Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# An error report file with more information is saved as:
# /home/cbil/Desktop/SIGH_Project/July_9/src/hs_err_pid11660.log
#
# If you would like to submit a bug report, please include
# instructions on how to reproduce the bug and visit:
# https://bugs.launchpad.net/ubuntu/+source/openjdk-7/
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.
#
and here is relevant code that is reformatted for simplicity:
file1.cpp
#include <jni.h>
#include "file1.h"
#include <iostream>
#include <fstream>
#include <string.h>
using namespace std;
const char* myPath;
JNIEXPORT jint JNICALL Java_foo_foo(JNIEnv *env, jobject thisobj, jbyteArray passedPath){
/*Some conversion, don't think it is relevant, but it might be*/
jboolean isCopy;
jbyte* path = env->GetByteArrayElements(passedPath,&isCopy);
int length1 = strlen((const char*)path);
char* convertedVar = (char*)malloc(length1+1);
memcpy(convertedVar,path,length1);
convertedVar[length1] = '\0';
convertedVar = (char*)path;
/*End Conversion*/
globalVariable = convertedVar;
... //Some code to use the variable and releases
}
file2.h (This is where I declare the global variable)
#include <vector>
#include <fstream>
#include <string.h>
extern const char* globalVariable;
extern int someNum;
extern int someLength;
class file2{
public:
static std::vector<std::string> getSomeString(int &someNum, int &someLength);
private:
static const std::vector<std::string> anotherVar;
...//some other variables and methods
}
and finally the code that calls getSomeString
file3.cpp
#include "file2.h"
#include <string.h>
#include <vector>
#include <fstream>
using namespace std;
int someNum = 0;
int someLength = 0;
const char* globalVariable;
vector<string> file2::getSomeString(int &someNum, iint &someLength){
vector<string> data;
ifstream infile(globalVariable); //I think the problem is here
string line;
...// some code that goes through the data file specified by the path name
someNum = data.size();
someLength = data[0].size();
return data;
}
const vector<string> file2::anotherVar = getSomeString(someNum,someLength); //variable that uses the getSomeString method
}
I'll say it here again, I am using JNI. I have gotten errors saying the lib.so file I create has undefined variables, I don't know if this is helpful information.
Any help would be appreciated. Sorry about the long post, I am quite lost.

Answer 1

char* convertedVar = (char*)malloc(length1+1);
memcpy(convertedVar,path,length1);
convertedVar[length1] = '\0';
convertedVar = (char*)path;
I'm pretty sure this last line shouldn't be there. You are going through this malloc/memcpy dance, then promptly drop this allocated-and-initialized block of memory on the floor, and make convertedVar point to something else instead.
Further, I'm pretty sure path pointer will become invalid as soon as the function returns.

Answer 2

extern const char* globalVariable;
This declares the variable, but doesn't define it. You also need to provide a definition, in exactly one source file (not in a header file):
const char* globalVariable;
Curiously, you know enough to do just that with someNum and someLength, but not for globalVariable.