NAME

       posix_madvise - give advice about patterns of memory usage

SYNOPSIS

       #include <sys/mman.h>

       int posix_madvise(void *addr, size_t len, int advice);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       posix_madvise():
           _POSIX_C_SOURCE >= 200112L

DESCRIPTION

       The posix_madvise() function allows an application to advise the system about its expected
       patterns of usage of memory in the address range starting at addr and continuing  for  len
       bytes.   The  system  is  free  to  use this advice in order to improve the performance of
       memory accesses (or to ignore the advice altogether), but  calling  posix_madvise()  shall
       not affect the semantics of access to memory in the specified range.

       The advice argument is one of the following:

       POSIX_MADV_NORMAL
              The  application  has no special advice regarding its memory usage patterns for the
              specified address range.  This is the default behavior.

       POSIX_MADV_SEQUENTIAL
              The application expects to access the specified address range sequentially, running
              from  lower  addresses  to  higher  addresses.   Hence, pages in this region can be
              aggressively read ahead, and may be freed soon after they are accessed.

       POSIX_MADV_RANDOM
              The application expects to access the specified address range randomly.  Thus, read
              ahead may be less useful than normally.

       POSIX_MADV_WILLNEED
              The  application  expects to access the specified address range in the near future.
              Thus, read ahead may be beneficial.

       POSIX_MADV_DONTNEED
              The application expects that it will not access the specified address range in  the
              near future.

RETURN VALUE

       On success, posix_madvise() returns 0.  On failure, it returns a positive error number.

ERRORS

       EINVAL addr is not a multiple of the system page size or len is negative.

       EINVAL advice is invalid.

       ENOMEM Addresses  in  the specified range are partially or completely outside the caller's
              address space.

VERSIONS

       Support for posix_madvise() first appeared in glibc version 2.2.

CONFORMING TO

       POSIX.1-2001.

NOTES

       POSIX.1 permits an implementation to generate an error if len is 0.  On Linux,  specifying
       len as 0 is permitted (as a successful no-op).

       In  glibc,  this  function  is  implemented  using  madvise(2).  However, since glibc 2.6,
       POSIX_MADV_DONTNEED is treated as a no-op, because  the  corresponding  madvise(2)  value,
       MADV_DONTNEED, has destructive semantics.

SEE ALSO

       madvise(2), posix_fadvise(2)

COLOPHON

       This  page  is  part of release 4.15 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.