#include <stdio.h>
#include <stdlib.h>

//argc holds how many parameters the program received when called
//argv holds their values
int main( int argc , char const *argv[] )
{  
  
  /* ENSURE THE FILE NAME WAS PASSED */
  if( argc != 2 )                     //make sure they submitted the file name we are going to copy
  {
    printf( "Please list the file name as a parameter to this program.\n" );
    exit(1);                          //exit with an error
  }
  
  
  /* ENSURE THE FILE EXISTS, AND WE CAN OPEN A TEMP FILE */
  char const* file_name = argv[1];                                //use the filename passed in from the command line
  char tmp_name[L_tmpnam];                                        //stores the name of the temporary file, L_tmpnam holds the size it needs to be
  tmpnam(tmp_name);                                               //get a name for our temporary output file, see http://www.gnu.org/s/libc/manual/html_node/Temporary-Files.html
  FILE *fin  = fopen( file_name , "r" );                          //fin is the file input pointer
  FILE *fout = fopen( tmp_name  , "w" );                          //we will copy the desired contents into the output file
  if( !fin  ) printf( "%s could not be opened.\n" , file_name );  //make sure the file we are reading from exists
  if( !fout ) printf( "There was an error opening a file.\n" );   //make sure the file we are writing to exists
  if( !fin || !fout ) exit(1);                                    //exit with an error
  
  
  /* COPY THE FILE */
  int c;                                            //we will store the file's character in c
  for( c=getc(fin)  ;  c != EOF  ;  c=getc(fin) )   //count characters and new lines in the file
  {
    fputc( c , fout );                              //copy it to the new file
    if( c == '\n' )                                 //if it is a newline
      fputc( '\n' , fout );                         //then copy it agian
  }
  
  
  /* REPLACE THE ORIGINAL FILE WITH THE NEW ONE */
  fclose(fin);                      //close the files
  fclose(fout);
  remove(file_name);                //remove the single-lined file
  rename(tmp_name,file_name);       //rename the double-lined file to hold it's value
  
  return 0;
}