gistfile1.c

static void
decide_maybe_mmap (_IO_FILE *fp)
{
  /* We use the file in read-only mode.  This could mean we can
     mmap the file and use it without any copying.  But not all
     file descriptors are for mmap-able objects and on 32-bit
     machines we don't want to map files which are too large since
     this would require too much virtual memory.  */
  struct _G_stat64 st;

  if (_IO_SYSSTAT (fp, &st) == 0
      && S_ISREG (st.st_mode) && st.st_size != 0
      /* Limit the file size to 1MB for 32-bit machines.  */
      && (sizeof (ptrdiff_t) > 4 || st.st_size < 1*1024*1024)
      /* Sanity check.  */
      && (fp->_offset == _IO_pos_BAD || fp->_offset <= st.st_size))
    {
      /* Try to map the file.  */
      void *p;

# ifdef _G_MMAP64
      p = _G_MMAP64 (NULL, st.st_size, PROT_READ, MAP_SHARED, fp->_fileno, 0);
# else
      p = __mmap (NULL, st.st_size, PROT_READ, MAP_SHARED, fp->_fileno, 0);
# endif
      if (p != MAP_FAILED)
        {
          /* OK, we managed to map the file.  Set the buffer up and use a
             special jump table with simplified underflow functions which
             never tries to read anything from the file.  */

          if (
# ifdef _G_LSEEK64
              _G_LSEEK64
# else
              __lseek
# endif
              (fp->_fileno, st.st_size, SEEK_SET) != st.st_size)
            {
              (void) __munmap (p, st.st_size);
              fp->_offset = _IO_pos_BAD;
            }
          else
            {
              _IO_setb (fp, p, (char *) p + st.st_size, 0);

              if (fp->_offset == _IO_pos_BAD)
                fp->_offset = 0;

              _IO_setg (fp, p, p + fp->_offset, p + st.st_size);
              fp->_offset = st.st_size;

              if (fp->_mode <= 0)
                _IO_JUMPS ((struct _IO_FILE_plus *)fp) = &_IO_file_jumps_mmap;
              else
                _IO_JUMPS ((struct _IO_FILE_plus *)fp) = &_IO_wfile_jumps_mmap;
              fp->_wide_data->_wide_vtable = &_IO_wfile_jumps_mmap;

              return;
            }
        }
    }

  /* We couldn't use mmap, so revert to the vanilla file operations.  */

  if (fp->_mode <= 0)
    _IO_JUMPS ((struct _IO_FILE_plus *) fp) = &_IO_file_jumps;
  else
    _IO_JUMPS ((struct _IO_FILE_plus *) fp) = &_IO_wfile_jumps;
  fp->_wide_data->_wide_vtable = &_IO_wfile_jumps;
}