Use OpenSSL BIO chain to read a base64-encoded file and write the decoded binary to a file

example_base64_decode_using_BIO.cpp

#include <string>                                                                                   
#include <cstring>                                                                                  
                                                                                                    
                                                                                                    
#include <stdint.h>                                                                                 
#include <stdio.h>                                                                                  
#include <stdlib.h>                                                                                 
#include <openssl/bio.h>                                                                            
//#include <openssl/bytestring.h>                                                                   
#include <openssl/crypto.h>                                                                         
//#include <openssl/digest.h>                                                                       
#include <openssl/err.h>                                                                            
#include <openssl/evp.h>                                                                            
#include <openssl/rsa.h>                                                                            
#include <openssl/x509.h>                                                                           
#include <fstream>                                                                                  
                                                                                                    
                                                                                                    
#include <sstream>                                                                                  
using namespace std;                                                                                
                                                                                                    
//##################################################################################################
void go()                                                                                           
{                                                                                                   
   BIO * infile = BIO_new_file( "signature.base64", "r" );                                          
   BIO * base64 = BIO_new( BIO_f_base64() );                                                        
   BIO * in_chain = infile;                                                                         
   in_chain = BIO_push( base64, in_chain );                                                         
                                                                                                    
   BIO * outfile = BIO_new_file( "signature.binary", "w" );                                         
   BIO * out_chain = outfile;                                                                       
                                                                                                    
   int const buffer_len = 1024;                                                                     
   char buffer[ buffer_len ];                                                                       
   int bytes_read = BIO_read( in_chain,  buffer, buffer_len );                                      
   while( bytes_read > 0 )                                                                          
   {                                                                                                
      int const bytes_written = BIO_write( out_chain, buffer, bytes_read );                         
      if( bytes_written <= 0 )                                                                      
      {                                                                                             
         stringstream msg;                                                                          
         msg << "Error writing to output chain. BIO_write returned " << bytes_written;              
         throw runtime_error( msg.str() );                                                          
      }                                                                                             
      bytes_read = BIO_read( in_chain, buffer, buffer_len );                                        
   }                                                                                                
                                                                                                    
   cout << "Final result: " << bytes_read << "\n";                                                  
}