SQLite

#else
# define unixShmMap     0
# define unixShmLock    0
# define unixShmBarrier 0
# define unixShmUnmap   0
#endif /* #ifndef SQLITE_OMIT_WAL */

/*
** Arguments x and y are both integers. Argument y must be a power of 2.
** Round x up to the nearest integer multiple of y. For example:
**
**     ROUNDUP(0,  8) ->  0
**     ROUNDUP(13, 8) -> 16
**     ROUNDUP(32, 8) -> 32
*/
#define ROUNDUP(x,y)     (((x)+y-1)&~(y-1))

/*
** Map, remap or unmap part of the database file.
*/
static int unixMremap(
  sqlite3_file *fd,               /* Main database file */
  int flags,                      /* Mask of SQLITE_MREMAP_XXX flags */
  sqlite3_int64 iOff,             /* Offset to start mapping at */
  sqlite3_int64 nOld,             /* Size of old mapping, or zero */
  sqlite3_int64 nNew,             /* Size of new mapping, or zero */
  void **ppMap                    /* IN/OUT: Old/new mappings */
){
  unixFile *p = (unixFile *)fd;   /* The underlying database file */
  int rc = SQLITE_OK;             /* Return code */
  void *pNew = 0;                 /* New mapping */
  i64 nNewRnd;                    /* nNew rounded up */
  i64 nOldRnd;                    /* nOld rounded up */

  assert( iOff==0 );


  /* If the SQLITE_MREMAP_EXTEND flag is set, then the size of the requested 
  ** mapping (nNew bytes) may be greater than the size of the database file.
  ** If this is the case, extend the file on disk using ftruncate().  */
  assert( nNew>0 || (flags & SQLITE_MREMAP_EXTEND)==0 );
  if( flags & SQLITE_MREMAP_EXTEND ){
    struct stat statbuf;          /* Low-level file information */
    rc = osFstat(p->h, &statbuf);
    if( rc==SQLITE_OK && nNew>statbuf.st_size ){
      rc = robust_ftruncate(p->h, nNew);
    }
    if( rc!=SQLITE_OK ) return rc;
  }

  /* According to some sources, the effect of changing the size of the
  ** underlying file on mapped regions that correspond to the added or
  ** removed pages is undefined. However, there is reason to believe that
  ** on modern platforms like Linux or OSX, things just work. For example,
  ** it is possible to create a mapping larger than the file on disk and
  ** extend the file on disk later on.
  **
  ** Exploit this on OSX to reduce the number of munmap()/mmap() calls
  ** if the file size is changing. In this case all mappings are rounded
  ** up to the nearest 4MB. And if a new mapping is requested that has the
  ** same rounded size as an old mapping, the old mapping can simply be
  ** reused as is.
  **
  ** It would be possible to do the above on Linux too. However, Linux has
  ** the non-standard mremap() call to resize existing mappings, which can
  ** be used instead.  */
#if defined(__APPLE__)
  nNewRnd = ROUNDUP(nNew, 4096*1024);
  nOldRnd = ROUNDUP(nOld, 4096*1024);
#else
  nNewRnd = ROUNDUP(nNew, 4096*1);
  nOldRnd = ROUNDUP(nOld, 4096*1);
#endif

  /* On OSX or Linux, reuse the old mapping if it is the right size. */
#if defined(__APPLE__) || defined(__linux__)
  if( nNewRnd==nOldRnd ){
    return SQLITE_OK;
  }
#endif

  /* On Linux, if there is both an old and new mapping, resize the old 
  ** mapping using the non-standard mremap() call.  */
#if defined(_GNU_SOURCE) && defined(__linux__)
  if( nNewRnd && nOldRnd ){
    void *pOld = *ppMap;
    *ppMap = pNew = mremap(pOld, nOldRnd, nNewRnd, MREMAP_MAYMOVE);
    if( pNew==MAP_FAILED ){
      *ppMap = 0;
      return SQLITE_IOERR_MREMAP;
    }
    return SQLITE_OK;
  }
#endif

  /* If we get this far, unmap any old mapping. */
  if( nOldRnd!=0 ){
    void *pOld = *ppMap;
    munmap(pOld, nOldRnd);
  }

  /* And, if required, use mmap() to create a new mapping. */
  if( nNewRnd>0 ){
    int flags = PROT_READ;
    if( (p->ctrlFlags & UNIXFILE_RDONLY)==0 ) flags |= PROT_WRITE;
    pNew = mmap(0, nNewRnd, flags, MAP_SHARED, p->h, iOff);
    if( pNew==MAP_FAILED ){
      pNew = 0;
      rc = SQLITE_IOERR_MREMAP;
    }
  }

  *ppMap = pNew;