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NAME

       madvise - give advice about use of memory

SYNOPSIS

       #include <sys/mman.h>

       int madvise(void *start, size_t length, int advice);

DESCRIPTION

       The madvise() system call advises the kernel about how to handle paging
       input/output in the address range beginning at address start  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 parameter which can be

       MADV_NORMAL
              No special treatment. This is the default.

       MADV_RANDOM
              Expect page references in random order.  (Hence, read ahead  may
              be less useful than normally.)

       MADV_SEQUENTIAL
              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.)

       MADV_WILLNEED
              Expect  access  in  the near future.  (Hence, it might be a good
              idea to read some pages ahead.)

       MADV_DONTNEED
              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 re-
              loading of the memory contents from the underlying  mapped  file
              (see  mmap())  or zero-fill-on-demand pages for mappings without
              an underlying file.

RETURN VALUE

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

ERRORS

       EAGAIN A kernel resource was temporarily unavailable.

       EBADF  The map exists, but the area maps something that isn’t a file.

       EINVAL The  value len is negative, start is not page-aligned, advice is
              not a valid value, or the application is attempting  to  release
              locked or shared pages (with MADV_DONTNEED).

       EIO    (for  MADV_WILLNEED)  Paging  in  this  area  would  exceed  the
              process’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.

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 behaviour.

       The Linux implementation requires  that  the  address  start  be  page-
       aligned,  and  allows length to be zero. If there are some parts of the
       specified address range that are  not  mapped,  the  Linux  version  of
       madvise()  ignores  them  and applies the call to the rest (but returns
       ENOMEM from the system call, as it should).

HISTORY

       The madvise() function first appeared in 4.4BSD.

CONFORMING TO

       POSIX.1b (POSIX.4).  POSIX  1003.1-2001  describes  posix_madvise  with
       constants  POSIX_MADV_NORMAL,  etc.,  with  a  behaviour  close to that
       described here. There is a similar posix_fadvise() for file access.

SEE ALSO

       getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)