Using string content into another program in CPP - c++

Question

I am trying to use the string content read from one program into another program. I have put some sample code to reproduce the example. I have a main function
//main.cpp
#include "read.h"
#include "read_fasta.h"
#include <iostream>
#include <string>
int main(int argc, char **argv)
{
index();
read_fasta_file(argc,argv);
std::cout << " : " << content << std::endl;
std::cout << "Length of fasta file: " << content.length() << std::endl;
return 0;
}
I read fasta file
//read_fasta.cpp
#include <iostream>
#include <fstream>
#include "read_fasta.h"
std::string content;
int read_fasta_file(int argc, char **argv)
{
std::ifstream input(argv[1]);
std::string line, name;
while( std::getline( input, line ).good() ){
if( line.empty() || line[0] == '>' ){ // Identifier marker
if( !name.empty() ){ // Print out what we read from the last entry
std::cout << name << " : " << content << std::endl;
name.clear();
}
if( !line.empty() ){
name = line.substr(1);
}
content.clear();
} else if( !name.empty() ){
if( line.find(' ') != std::string::npos ){ // Invalid sequence--no spaces allowed
name.clear();
content.clear();
} else {
content += line;
}
}
}
if( !name.empty() ){ // Print out what we read from the last entry
std::cout << name << " : " << content << std::endl;
std::cout << "Length of fasta file: " << content.length() << std::endl;
}
return 0;
}
Header file of read_fasta.cpp
//read_fasta.h
#ifndef read_fasta_h
#define read_fasta_h
#include <iostream>
#include <string>
#include <fstream>
int read_fasta_file(int argc, char **argv);
extern std::string content;
#endif
Till now every thing works fine. Now i try to use the std::string content in another cpp progam.
//read.cpp
#include <iostream>
#include <string>
#include "read.h"
void index()
{
std::cout<<"Do U see any printing :"<<content<<std::endl;
}
Header file for read.cpp
//read.h
#ifndef read_h
#define read_h
#include <iostream>
#include <string>
#include <vector>
void index();
extern std::string content;
#endif
But there seem to be nothing in std::string content in read.cpp program. I am missing something.