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 1.440 2008/05/06 18:13:26 danielk1977 Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"
#include <assert.h>
#include <string.h>

/*

** the same pager at the same time.  Other general-purpose mutexes in
** the btree layer handle that chore.
*/
#ifdef SQLITE_ENABLE_MEMORY_MANAGEMENT
  static void pagerEnter(Pager *p){
    p->iInUseDB++;
    if( p->iInUseMM && p->iInUseDB==1 ){
      sqlite3_mutex *mutex;
#ifndef SQLITE_MUTEX_NOOP

      mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_MEM2);
#endif
      p->iInUseDB = 0;
      sqlite3_mutex_enter(mutex);
      p->iInUseDB = 1;
      sqlite3_mutex_leave(mutex);
    }

**
** nReq is the number of bytes of memory required. Once this much has
** been released, the function returns. The return value is the total number 
** of bytes of memory released.
*/
int sqlite3PagerReleaseMemory(int nReq){
  int nReleased = 0;          /* Bytes of memory released so far */
  sqlite3_mutex *mutex;       /* The MEM2 mutex */
  Pager *pPager;              /* For looping over pagers */
  BusyHandler *savedBusy;     /* Saved copy of the busy handler */
  int rc = SQLITE_OK;

  /* Acquire the memory-management mutex
  */
#ifndef SQLITE_MUTEX_NOOP

  mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_MEM2);
#endif
  sqlite3_mutex_enter(mutex);

  /* Signal all database connections that memory management wants
  ** to have access to the pagers.
  */