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MADVISE(2)		    BSD 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 behav);

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  Causes the VM system to depress the priority of pages
		      immediately preceding a given page when it is faulted
		      in.

     MADV_WILLNEED    Causes 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.
		      Additionally, the pages that are already in memory will
		      be immediately mapped into the process, thereby elimi‐
		      nating unnecessary overhead of going through the entire
		      process of faulting the pages in.	 This WILL NOT fault
		      pages in from backing store, but quickly map the pages
		      already in memory into the calling process.

     MADV_DONTNEED    Allows the VM system to decrease the in-memory priority
		      of pages in the specified range.	Additionally future
		      references to this address range will incur a page
		      fault.

     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.  This is an efficient way of
		      allowing malloc(3) to free pages anywhere in the address
		      space, while keeping the address space valid.  The next
		      time that the page is referenced, the page might be
		      demand zeroed, or might contain the data that was there
		      before the MADV_FREE call.  References made to that
		      address space range will not make the VM system page the
		      information back in from backing store until the page is
		      modified again.

     MADV_NOSYNC      Request that the system not flush the data associated
		      with this map to physical backing store unless it needs
		      to.  Typically this prevents the file system update dae‐
		      mon from gratuitously writing pages dirtied by the VM
		      system to physical disk.	Note that VM/file system
		      coherency is always maintained, this feature simply
		      ensures that the mapped data is only flush when it needs
		      to be, usually by the system pager.

		      This feature is typically used when you want to use a
		      file-backed shared memory area to communicate between
		      processes (IPC) and do not particularly need the data
		      being stored in that area to be physically written to
		      disk.  With this feature you get the equivalent perfor‐
		      mance with mmap that you would expect to get with SysV
		      shared memory calls, but in a more controllable and less
		      restrictive manner.  However, note that this feature is
		      not portable across UNIX platforms (though some may do
		      the right thing by default).  For more information see
		      the MAP_NOSYNC section of mmap(2)

     MADV_AUTOSYNC    Undoes the effects of MADV_NOSYNC for any future pages
		      dirtied within the address range.	 The effect on pages
		      already dirtied is indeterminate - they may or may not
		      be reverted.  You can guarantee reversion by using the
		      msync(2) or fsync(2) system calls.

     MADV_NOCORE      Region is not included in a core file.

     MADV_CORE	      Include region in a core file.

     MADV_PROTECT     Informs the VM system this process should not be killed
		      when the swap space is exhausted.	 The process must have
		      superuser privileges.  This should be used judiciously
		      in processes that must remain running for the system to
		      properly function.

     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
     The madvise() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     The madvise() system call will fail if:

     [EINVAL]		The behav argument is not valid.

     [ENOMEM]		The virtual address range specified by the addr and
			len arguments is not valid.

     [EPERM]		MADV_PROTECT was specified and the process does not
			have superuser privileges.

SEE ALSO
     mincore(2), mprotect(2), msync(2), munmap(2)

STANDARDS
     The posix_madvise() interface conforms to IEEE Std 1003.1-2001
     (“POSIX.1”).

HISTORY
     The madvise() system call first appeared in 4.4BSD.

BSD				 July 19, 1996				   BSD
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