madvise, posix_madvise -- give advice about use of memory
Standard C Library (libc, -lc)
madvise(void *addr, size_t len, int behav);
posix_madvise(void *addr, size_t len, int behav);
The madvise() system call allows a process that has knowledge of its memory
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 behavior.
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
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 eliminating
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
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
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 daemon
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 performance
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
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
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
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.
mincore(2), mprotect(2), msync(2), munmap(2)
The posix_madvise() interface conforms to IEEE Std 1003.1-2001
The madvise() system call first appeared in 4.4BSD.
FreeBSD 5.2.1 July 19, 1996 FreeBSD 5.2.1 [ Back ]