How to take screenshots in opengl

bool save_screenshot(string filename, int w, int h)
{	
  //This prevents the images getting padded 
 // when the width multiplied by 3 is not a multiple of 4
  glPixelStorei(GL_PACK_ALIGNMENT, 1);

  int nSize = w*h*3;
  // First let's create our buffer, 3 channels per Pixel
  char* dataBuffer = (char*)malloc(nSize*sizeof(char));

  if (!dataBuffer) return false;
   
   // Let's fetch them from the backbuffer	
   // We request the pixels in GL_BGR format, thanks to Berzeger for the tip
   glReadPixels((GLint)0, (GLint)0,
		(GLint)w, (GLint)h,
		 GL_BGR, GL_UNSIGNED_BYTE, dataBuffer);
	
   //Now the file creation
   FILE *filePtr = fopen(filename.c_str(), "wb");
   if (!filePtr) return false;
	
   
   unsigned char TGAheader[12]={0,0,2,0,0,0,0,0,0,0,0,0};
   unsigned char header[6] = { w%256,w/256,
			       h%256,h/256,
			       24,0};
   // We write the headers
   fwrite(TGAheader,	sizeof(unsigned char),	12,	filePtr);
   fwrite(header,	sizeof(unsigned char),	6,	filePtr);
   // And finally our image data
   fwrite(dataBuffer,	sizeof(GLubyte),	nSize,	filePtr);
   fclose(filePtr);

   free(dataBuffer);

  return true;
}