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           #include <sys/mman.h>
           int madvise(void *addr, size_t length, int advice);
       Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
           madvise(): _BSD_SOURCE


           The madvise() system call advises the kernel about how to handle paging
           input/output in the address range beginning at address  addr  and  with
           size  length bytes.  It allows an application to tell the kernel how it
           expects to use some mapped or shared memory areas, so that  the  kernel
           can  choose  appropriate  read-ahead and caching techniques.  This call
           does not influence the semantics of the application (except in the case
           of  MADV_DONTNEED),  but  may influence its performance.  The kernel is
           free to ignore the advice.
           The advice is indicated in the advice argument which can be
                  No special treatment.  This is the default.
                  Expect page references in random order.  (Hence, read ahead  may
                  be less useful than normally.)
                  Expect  page  references  in sequential order.  (Hence, pages in
                  the given range can be aggressively read ahead, and may be freed
                  soon after they are accessed.)
                  Expect  access  in  the near future.  (Hence, it might be a good
                  idea to read some pages ahead.)
                  Do not expect access in the near future.  (For the  time  being,
                  the  application is finished with the given range, so the kernel
                  can free resources associated with it.)  Subsequent accesses  of
                  pages  in  this  range  will  succeed, but will result either in
                  reloading of the memory contents from the underlying mapped file
                  (see  mmap(2)) or zero-fill-on-demand pages for mappings without
                  an underlying file.
           MADV_REMOVE (since Linux 2.6.16)
                  Free up a given range of pages and its associated backing store.
                  Currently,  only  shmfs/tmpfs  supports  this; other filesystems
                  return with the error ENOSYS.
           MADV_DONTFORK (since Linux 2.6.16)
                  receiving a SIGBUS and the page being unmapped.  This feature is
                  intended for testing of memory error-handling code; it is avail-
                  able only if the kernel was configured with  CONFIG_MEMORY_FAIL-
           MADV_SOFT_OFFLINE (since Linux 2.6.33)
                  Soft  offline  the  pages  in  the  range  specified by addr and
                  length.  The memory of each page in the specified range is  pre-
                  served (i.e., when next accessed, the same content will be visi-
                  ble, but in a new physical page frame), and the original page is
                  offlined  (i.e.,  no longer used, and taken out of normal memory
                  management).  The effect of the MADV_SOFT_OFFLINE  operation  is
                  invisible  to (i.e., does not change the semantics of) the call-
                  ing process.  This feature is intended  for  testing  of  memory
                  error-handling code; it is available only if the kernel was con-
                  figured with CONFIG_MEMORY_FAILURE.
           MADV_MERGEABLE (since Linux 2.6.32)
                  Enable Kernel Samepage Merging (KSM) for the pages in the  range
                  specified  by addr and length.  The kernel regularly scans those
                  areas of user memory that have been marked as mergeable, looking
                  for  pages with identical content.  These are replaced by a sin-
                  gle write-protected page (which is  automatically  copied  if  a
                  process  later  wants  to  update the content of the page).  KSM
                  merges only private anonymous pages (see mmap(2)).  The KSM fea-
                  ture  is  intended for applications that generate many instances
                  of the same data (e.g., virtualization systems such as KVM).  It
                  can  consume  a lot of processing power; use with care.  See the
                  Linux  kernel  source  file  Documentation/vm/ksm.txt  for  more
                  details.  The MADV_MERGEABLE and MADV_UNMERGEABLE operations are
                  available only if the kernel was configured with CONFIG_KSM.
           MADV_UNMERGEABLE (since Linux 2.6.32)
                  Undo the effect of an earlier MADV_MERGEABLE  operation  on  the
                  specified  address  range;  KSM  unmerges  whatever pages it had
                  merged in the address range specified by addr and length.
           MADV_HUGEPAGE (since Linux 2.6.38)
                  Enables Transparent Huge Pages (THP)  for  pages  in  the  range
                  specified by addr and length.  Currently, Transparent Huge Pages
                  work only with private anonymous pages (see mmap(2)).  The  ker-
                  nel will regularly scan the areas marked as huge page candidates
                  to replace them with huge pages.  The kernel will also  allocate
                  huge  pages directly when the region is naturally aligned to the
                  huge page size (see posix_memalign(2)).  This feature is primar-
                  ily  aimed  at  applications that use large mappings of data and
                  access large regions of that memory at a time (e.g., virtualiza-
                  tion  systems  such  as  QEMU).  It can very easily waste memory
                  (e.g., a 2MB mapping that only ever accesses 1 byte will  result
                  in  2MB of wired memory instead of one 4KB page).  See the Linux
                  kernel  source  file  Documentation/vm/transhuge.txt  for   more
                  details.   The  MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are
           MADV_DODUMP (since Linux 3.4)
                  Undo the effect of an earlier MADV_DONTDUMP.


           On  success  madvise() returns zero.  On error, it returns -1 and errno
           is set appropriately.


           EAGAIN A kernel resource was temporarily unavailable.
           EBADF  The map exists, but the area maps something that isn't a file.
           EINVAL This error can occur for the following reasons:
                  *  The value len is negative.
                  *  addr is not page-aligned.
                  *  advice is not a valid value
                  *  The application is attempting to  release  locked  or  shared
                     pages (with MADV_DONTNEED).
                  *  MADV_MERGEABLE  or  MADV_UNMERGEABLE was specified in advice,
                     but the kernel was not configured with CONFIG_KSM.
           EIO    (for MADV_WILLNEED) Paging in this area would  exceed  the  pro-
                  cess's maximum resident set size.
           ENOMEM (for MADV_WILLNEED) Not enough memory: paging in failed.
           ENOMEM Addresses  in  the  specified range are not currently mapped, or
                  are outside the address space of the process.


           POSIX.1b.   POSIX.1-2001  describes  posix_madvise(3)  with   constants
           POSIX_MADV_NORMAL,  POSIX_MADV_RANDOM, and so on, with a behavior close
           to that described here.  There is a similar posix_fadvise(2)  for  file
           and MADV_UNMERGEABLE are Linux-specific.


       Linux notes
           The current Linux implementation (2.4.0) views this system call more as
           a  command  than as advice and hence may return an error when it cannot
           do what it usually would do in  response  to  this  advice.   (See  the
           ERRORS description above.)  This is nonstandard behavior.
           The  Linux  implementation  requires  that  the  address  addr be page-

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