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 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 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.