Appending file

baard · #1 07-May-2007, 09:56

Hi,
What is wrong with this code.

CPP / C++ / C Code:

#include <cstdlib>
#include <iostream>
#include <fstream>
using namespace std;

ifstream::pos_type size;

char * memblock;


int main () {
	ofstream myfile("newfile.txt",ios::out|ios::binary|ios::app);
	
  ifstream file ("test.txt",ios::in|ios::binary|ios::ate);	
 
  if (file.is_open())
  {
    size = file.tellg();
   
    memblock = new char [size];
    file.seekg (0, ios::beg);
    
	file.read (memblock, size);
 file.close();

  myfile<<memblock<<"adding";

   myfile.close();
 
  cout << "appended \n";
  cout<<size;
    delete[] memblock;

  }
  else cout << "Unable to open file";
  system("pause");
  return 0;
}

There seems to be an array size problem .When I run the program with two text files, I also get some garbage added to the created newfile. When i run the program a second time, more garbage is seen.
How can I properly append two files?
Thanks in advance.

davekw7x · #2 08-May-2007, 08:36

Quote:

Originally Posted by baard

Hi,
What is wrong with this code.

CPP / C++ / C Code:

  char * memblock;
.
.
. 
  memblock = new char [size];
.
.
.   
  file.read (memblock, size);
.
.
.
  myfile<<memblock ...

You allocate an array with exactly as many bytes as the file and read the file into the array. (So far, so good, but you should test the state of the stream after attempting to read.)

Then, when you use the '<<' operator on a variable that is a pointer to char, it expects a null-terminated sequence of bytes. That is because the '<<' operator has been overloaded to treat pointers to char in a way that is consistent with C-style "strings". Since you didn't put a zero byte at the end, then there is no guarantee that there is a zero byte. Furthermore, since you may be reading a binary file, there may be embedded bytes in the file, and the '<<' operator would quit right there (without putting the zero byte in the output file and without going any further). The result might be that not all of the output will be processed.

SO: Even if it isn't a binary file with embedded zeros, if you are going to use file.read() to get the stuff in, why don't you use myfile.write() to write them out? (Or, just read and write a byte at a time with ; why go to the trouble of allocating storage for a buffer?)

Also: You should really test the state of the streams after each read and write operation, as well as after each attempt to open a file.

Regards,

Dave

baard · #3 08-May-2007, 10:33

Thank you very much for the info. Whew, never thought it has such complications. After using write() I was able to append properly. I was trying to create a kind of a resource file .
Thanks again.

davekw7x · #4 08-May-2007, 10:37

Quote:

Originally Posted by baard

T never thought it has such complications.

Doing it isn't complicated, as you have learned. Understanding what really happened and why your first effort didn't work is important. I wish I could go through it with fewer words.

Oh, well...

Regards,

Dave

Footnote: As I mentioned, it's not really necessary to read the entire file into a buffer; just write each byte as you read it. The main part of the program could be as simple as:

CPP / C++ / C Code:

    int size = 0;
    char inchar;
    do {
        inchar = infile.get();
        if (infile) {
            ++size;
            outfile.put(inchar);
        }
    } while (infile);
    cout << "Appended " << size << " bytes from " << inname
         <<  " to " << outname << endl;

baard · #5 09-May-2007, 06:23

. I understand. It is always better to know what is happening under the hood.
Will also try the snippet. Thanks again Mr.Dave.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Rate This Thread
Rate This Thread:

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Airport Log program using 3D linked List : problem reading from file	batrsau	C Programming Language	11	29-Feb-2008 07:44
Problem inserting (appending) in the middle of text file	rajeev nair	CPP / C++ Forum	3	17-Apr-2007 00:55
Download files in c for windows operating system	oozsakarya	C Programming Language	5	20-Jun-2006 03:33
CD burner wont burn!!	robertli55	Computer Hardware Forum	1	18-Jun-2004 10:53
Yet another CD burner problem: Lite-On LSC-24082K	Erwin	Computer Hardware Forum	1	22-May-2004 11:28

Appending file

Re: Appending file

Re: Appending file

Re: Appending file

Re: Appending file