FILE *tmp_copy = tmpfile();
file = fopen ( argv[2], "rw" );
if ((file != NULL )) {
    char line [ 128 ]; /* or other suitable maximum line size */
    while( ( fgets ( line, sizeof line, file ) != NULL ))  {
       fputs(line, tmp_copy);
    }
    fclose ( file );

        rewind(tmp);
        char *p;
      /* Reopen file now with write permissions */
        fopen(argv[2], "w");
        while (( p = fgets(line, 1024, tmp))!=NULL) {
            //line[strlen(line)-1] = ''; /* Clear away newline */
            //sprintf(line, "%s %sn", line, more);
            strcpy(line,"true");
            //fputs(line, file);
        }
fclose(file);
fclose(tmp);
            }

}

original_file = fopen(original_file_name, "r");
temporary_file_name = tmpnam("dummy");
temporary_file = fopen(temporary_file_name, "w");

while (fgets(line, original_file))
{
    remove_trailing_newline(line);
    strcat(line, " TRUEn");
    fputs(line, temporary_file);
}

fclose(temporary_file);
fclose(original_file);

rename(temporary_file_name, original_file_name);