MADVISE(2) FreeBSD 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
memory behavior to describe it to the system. The posix_madvise()
interface is identical, except it returns an error number on error and
does not modify errno, 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
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
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 address range. Consequently,
future references to this address range are more likely
to 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
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
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), posix_fadvise(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.
FreeBSD 13.1-RELEASE-p6 July 12, 2015 FreeBSD 13.1-RELEASE-p6
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