SQLite

** The pager is used to access a database disk file.  It implements
** atomic commit and rollback through the use of a journal file that
** is separate from the database file.  The pager also implements file
** locking to prevent two processes from writing the same database
** file simultaneously, or one process from reading the database while
** another is writing.
**
** @(#) $Id: pager.c,v 1.350 2007/07/19 16:35:17 drh Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"
#include "os.h"
#include "pager.h"
#include <assert.h>
#include <string.h>

** on success or an error code is something goes wrong.
*/
static int write32bits(OsFile *fd, u32 val){
  char ac[4];
  put32bits(ac, val);
  return sqlite3OsWrite(fd, ac, 4);
}

/*
** Read a 32-bit integer at offset 'offset' from the page identified by
** page header 'p'.
*/
static u32 retrieve32bits(PgHdr *p, int offset){
  unsigned char *ac;
  ac = &((unsigned char*)PGHDR_TO_DATA(p))[offset];
  return sqlite3Get4byte(ac);
}


/*
** This function should be called when an error occurs within the pager
** code. The first argument is a pointer to the pager structure, the
** second the error-code about to be returned by a pager API function. 
** The value returned is a copy of the second argument to this function. 
**

    /* Open page 1 of the file for writing. */
    rc = sqlite3PagerGet(pPager, 1, &pPgHdr);
    if( rc!=SQLITE_OK ) return rc;
    rc = sqlite3PagerWrite(pPgHdr);
    if( rc!=SQLITE_OK ) return rc;
  
    /* Increment the value just read and write it back to byte 24. */
    change_counter = sqlite3Get4byte(pPager->dbFileVers);
    change_counter++;
    put32bits(((char*)PGHDR_TO_DATA(pPgHdr))+24, change_counter);
    /* Release the page reference. */
    sqlite3PagerUnref(pPgHdr);
    pPager->changeCountDone = 1;
  }
  return SQLITE_OK;