madvise(2) - NetBSD Manual Pages

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MADVISE(2)                NetBSD System Calls Manual                MADVISE(2)


NAME

     madvise, posix_madvise -- give advice about use of memory


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <sys/mman.h>

     int
     madvise(void *addr, size_t len, int behav);

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


DESCRIPTION

     The madvise() system call allows a process that has knowledge of its mem-
     ory behavior to describe it to the system.  The posix_madvise() interface
     is identical and is provided for standards conformance.

     The known behaviors are:

     MADV_NORMAL      Tells the system to revert to the default paging behav-
                      ior.

     MADV_RANDOM      Is a hint that pages will be accessed randomly, and
                      prefetching is likely not advantageous.

     MADV_SEQUENTIAL  Is a hint that pages will be accessed sequentially, from
                      the lower address to higher address.  It might cause the
                      VM system to depress the priority of pages immediately
                      preceding a given page when it is faulted in.

     MADV_WILLNEED    Is a hint that pages will be accessed in the near
                      future.  It might cause the VM system to make pages that
                      are in a given virtual address range to temporarily have
                      higher priority, and if they are in memory, decrease the
                      likelihood of them being freed.  It might immediately
                      map the pages that are already in memory into the
                      process, thereby eliminating unnecessary overhead of
                      going through the entire process of faulting the pages
                      in.  It might or might not fault pages in from backing
                      store.

     MADV_DONTNEED    Is a hint that pages will not be accessed in the near
                      future.  It might allow the VM system to decrease the
                      in-memory priority of pages in the specified range.

     MADV_FREE        Gives the VM system the freedom to free pages, and tells
                      the system that information in the specified page range
                      is no longer important.

     Portable programs that call the posix_madvise() interface should use the
     aliases POSIX_MADV_NORMAL, POSIX_MADV_SEQUENTIAL, POSIX_MADV_RANDOM,
     POSIX_MADV_WILLNEED, and POSIX_MADV_DONTNEED rather than the flags
     described above.


RETURN VALUES

     Upon successful completion, a value of 0 is returned.  Otherwise, a value
     of -1 is returned and errno is set to indicate the error.


ERRORS

     madvise() will fail if:

     [EINVAL]           Invalid parameters were provided.


SEE ALSO

     mincore(2), mprotect(2), msync(2), munmap(2), posix_fadvise(2)


STANDARDS

     The posix_madvise() system call is expected to conform to the IEEE Std
     1003.1-2001 (``POSIX.1'') standard.


HISTORY

     The madvise system call first appeared in 4.4BSD, but until NetBSD 1.5 it
     did not perform any of the requests on, or change any behavior of the
     address range given.  The posix_madvise() call was added in NetBSD 5.0.

NetBSD 8.2                      March 29, 2011                      NetBSD 8.2