[SOLVED] buffering

shamjs · 04-11-2012, 06:55 AM

Hi all,

i need to compute hash code for a file , currently im reading entire file at once and i perform hashing ,but i need to read a file chunk by chunk basis (say 512bytes) using a buffer and hash it and update it, im finding little difficult to read chunk by chunk basis ,please someone help me

Code:

long ComputeHashCode(string fname)
{
  int length,buflen;
  char * buffer;
  unsigned long adler = 1L;
 // checksum_calculator chk;
  unsigned int cs = 0;
  ifstream is;
 // chk.init();

  is.open (fname, ios::binary );

  // get length of file:
  is.seekg (0, ios::end);
  length = is.tellg();
  is.seekg (0, ios::beg);

  
  // allocate memory:
  buffer = new char [length];

  // read data as a block:
  is.read (buffer,length);
  is.close();

 // cs=chk.calculate(buffer);
  adler = update_adler32(adler,(unsigned char*) buffer, length);

//  cout.write (buffer,length);

  delete[] buffer;
  return adler;
}

MensaWater · 04-11-2012, 07:57 AM

Why is this marked Solved? Did you solve it? If so can you post your solution?

ta0kira · 04-11-2012, 08:53 AM

This would be a lot easier in C. Here's a starting point that's comparable to what you have:

Code:

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>

#define BUFFER_SIZE 512


unsigned long ComputeHashCode(const char *filename)
{
	//allocate a buffer (you can use any method)
	static char buffer[BUFFER_SIZE];

	unsigned long adler = 1L;

	//open the file without buffering
	int current_file = open(filename, O_RDONLY);
	if (current_file < 0)
	{
	fprintf(stderr, "could not open '%s': %s\n", filename, strerror(errno));
	return 0;
	}

	//keep reading 'BUFFER_SIZE' bytes until there is nothing left to read
	int read_size = 0;
	while ((read_size = read(current_file, buffer, BUFFER_SIZE)) > 0)
	{
// 	write(STDOUT_FILENO, buffer, read_size);
	adler = update_adler32(adler, (unsigned char*) buffer, read_size);
	}

	//close the current file
	close(current_file);

	return adler;
}


int main(int argc, char *argv[])
{
	//(this just loops through the filenames provided as arguments)
	for (int I = 1; I < argc; I++)
	{
	unsigned long adler = ComputeHashCode(argv[I]);
	fprintf(stdout, "%lu\n", adler);
	}

	return 0;
}