Google App Engine: Sessions

With the advent of Google App Engine (Python 2.7) and WebApp2, there have been many changes in the way people code on Google App Engine. WebApp2 includes a Session Management script in the module ‘webapp2_extras‘

This is a simple sessions example with Google App Engine with Python 2.7 and WebApp2. The code is self explanatory. I haven’t implemented exceptions and errors as I won’t be using this snippet of code anymore. But figuring out something is the fun part isn’t it?

Session Module:

#Import sessions for session handling
import webapp2
from webapp2_extras import sessions

#This is needed to configure the session secret key
#Runs first in the whole application
myconfig_dict = {}
myconfig_dict['webapp2_extras.sessions'] = {
    'secret_key': 'my-super-secret-key-somemorearbitarythingstosay',
}

#Session Handling class, gets the store, dispatches the request
class BaseSessionHandler(webapp2.RequestHandler):
    def dispatch(self):
        # Get a session store for this request.
        self.session_store = sessions.get_store(request=self.request)

        try:
            # Dispatch the request.
            webapp2.RequestHandler.dispatch(self)
        finally:
            # Save all sessions.
            self.session_store.save_sessions(self.response)

    @webapp2.cached_property
    def session(self):
        # Returns a session using the default cookie key.
        return self.session_store.get_session()
#End of BaseSessionHandler Class

Main Module:

import webapp2
from webapp2_extras import sessions
import session_module

#MainHandler class where we write code for ourselves
class MainHandler(session_module.BaseSessionHandler):
 def get(self):

  if self.session.get('counter'):
   self.response.out.write('Session is in place')
   counter = self.session.get('counter')
   self.session['counter'] = counter + 1
   self.response.out.write('Counter = ' + str(self.session.get('counter')))
  else:
   self.response.out.write('Fresh Session')
   self.session['counter'] = 1
   self.response.out.write('Counter = ' + str(self.session.get('counter')))
#End of MainHandler Class

#The application starts running after this is interpreted
app = webapp2.WSGIApplication([('/', MainHandler),], config = session_module.myconfig_dict)
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Replacing characters in a file (C)

The lesser known topic when it comes to C is FILE I/O. Even I’ve learnt a great deal in the past few days than what I’d learnt during my C classes. Today I sat down to solve the problem I’ve been facing on how to replace characters of a file in C. I searched through ‘stack-overflow’, dream in code and googled for about a half an hour without avail, but in the end the faithful GNU GCC Reference Manual helped me. It is related to my steganography project, where I try to replace the last bit of a byte and hide information using that. Let’s get down to business.

Please note that I’m talking in the context of replacing characters in a file. For example, say you want to replace all the ‘a’ characters with ‘b’.

These are the key points that led to my downfall: (even though I wiggled my way out of it!)
  • Opening a file for I/O – You need to open a file with “r+” if you are dealing with ASCII files and “rb+” if you are dealing with Binary files. The most common mistake one does when opening a file to read/write is that we forget and open the file using the “r” mode.
  • File pointer moves forward after every read or write. I had totally missed this point even though I knew it. I wasted about fifteen minutes on this!
  • Between a fgetc() and a fputc() call you need to call fflush() so that the changes you have made to take effect (in short it will transfer the buffer content to the file).

So with these things covered I’ll write an example which will help you understand the problem and the solution.

#include <stdio.h>

void read_file(FILE *fp)
{
    char c;
    fseek(fp,0,SEEK_SET)
    while(1)
    {
        c = (char) getc(fp);
        if(c == EOF)
            break;
        else
            printf("%c",c);
    }
    printf("\n\n");
}

void replace(FILE *fp, char find_c, char replace_c)
{
    char c;
    fseek(fp,0,SEEK_SET);
    while(1)
    {
        c = (char) getc(fp);
        if(c == EOF)
            break;
        else if(c == find_c)
        {
            printf("*");
            if(fseek(fp,-1,SEEK_CUR) != 0)
            {
                printf("Function: replace() :: Error: fseek() not working.\n");
                return -1;
            }
            if(fputc((int)replace_c,fp) == EOF)
            {
                printf("Function: replace() :: Error: fputc() not working.\n");
                return -1;
            }
            if(fflush(fp) == EOF)
            {
                printf("Function: replace() :: Error: fflush() not working.\n");
            }
        }
    }
    printf("\n\n");
}

int main()
{
    char find_c = 'l';
    char replace_c = 'm';
    FILE *fp;  

    printf("Before Replacement:\n");
    fp = fopen("message.txt","r");
    if(fp == NULL)
    {
        printf("Function: main() :: Error: fopen() not working for \"r\" mode.\n");
        return -1;
    }
    read_file(fp);
    fclose(fp);

    printf("During Replacement:\n");
    fp = fopen("message.txt","r+");
    if(fp == NULL)
    {
        printf("Function: main() :: Error: fopen() not working for \"r+\" mode.\n");
        return -1;
    }
    replace(fp,find_c,replace_c);
    fclose(fp);

    printf("After Replacement:\n");
    fp = fopen("message.txt","r");
    if(fp == NULL)
    {
        printf("Function: main() :: Error: fopen() not working for \"r\" mode.\n");
        return -1;
    }
    read_file(fp);
    fclose(fp);
    return 0;
}

One more thing I want to tell you is to have a copy of GCC Reference Manual with you, it really helps you a lot!

Processing pixels in a Bitmap Image file

I was meddling with some image processing, I had to process some pixels from a BMP image, if you’re looking for the same then this is the blog post for you. The Algorithm is pretty simple, I’ve used the Wikipedia article BMP File format as my reference, the source is available in a GitHub repository.

//Program to analyze a Bitmap File.
//Please read the Wikipedia article follow this algorithm
//even more clearly.</pre>
//Moderate familiarity with C required to understand the
//the functions in the source code, but not the algorithm.

Start Program

Openthe BMP file

Get the BMP Magic
Print the BMP Magic
**Nothing of Importance, just
the letters 'B' and 'M'

Get the BMP Header
Print the BMP Header
**Contains the Pixel Data Offset (address)
**Other Information (Not important):
>File Size
>Creator 1
>Creator 2

Get the DIB` Header
Print the DIB Header
**Contains Width in Pixels (in pixels)
**Contains Height in Pixels (in pixels)
**Contains Pixel Depth (in bits)
**Other Information (not important):
>Header Size (in bytes)
>Compression Type (ENUM)
>Horizontal, Vertical (in pixels/m)
>No of color planes (usually 1)
>Pixel Data size (don't confuse with filesize)
>No of colors in palette (0 - default)
>No of important colors (0 - all are important)

No. of bytes per pixel = Pixel Depth / 8

Go the Pixel Data Offset
foreach x in Height (No. of rows)
{
  foreach y in Width (No. of columns)
  {
    foreach b in [No. of bytes per pixel]
    {
      print [BYTE DATA]
      //You can perform any other operation
      //once you can print the information.
    }
  }
  print NEWLINE
}

Close the BMP File
End Program

DIB: Device Independent Bitmap
NOTE: I’ve printed out all the information in the bitmap file though

The source code is available here in my GitHub repository.