MADVISE(2)

NAME
madvise - give advice about use of memory

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
int
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 known behaviors are given in <sys/mman.h>:

    #define	MADV_NORMAL	0 /* no further special treatment */
    #define	MADV_RANDOM	1 /* expect random page references */
    #define	MADV_SEQUENTIAL	2 /* expect sequential references */
    #define	MADV_WILLNEED	3 /* will need these pages */
    #define	MADV_DONTNEED	4 /* don't need these pages */
    #define	MADV_FREE	5 /* data is now unimportant */
    #define MADV_NOSYNC	6 /* no explicit commit to physical backing store */
    #define MADV_AUTOSYNC	7 /* default commit method to physical backing store */
    #define MADV_NOCORE	8 /* do not include these pages in a core file */
    #define MADV_CORE	9 /* revert to including pages in a core file */

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 filesystem update daemon from gratuitously writing pages dirtied by the VM system to physical disk. Note that VM/filesystem 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 guarentee 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.

RETURN VALUES
Upon successful completion, madvise() returns 0. Otherwise a value of -1 is returned and errno is set to indicate the error.

ERRORS
The madvise() function will fail if:

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

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

HISTORY
The madvise() function first appeared in 4.4BSD.