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- .Dd July 12, 2015
- .Dt MADVISE 2
- .Os
- .Sh NAME
- .Nm madvise , posix_madvise
- .Nd give advice about use of memory
- .Sh LIBRARY
- .Lb libc
- .Sh SYNOPSIS
- .In sys/mman.h
- .Ft int
- .Fn madvise "void *addr" "size_t len" "int behav"
- .Ft int
- .Fn posix_madvise "void *addr" "size_t len" "int behav"
- .Sh DESCRIPTION
- The
- .Fn madvise
- system call
- allows a process that has knowledge of its memory behavior
- to describe it to the system.
- The
- .Fn posix_madvise
- interface is identical, except it returns an error number on error and does
- not modify
- .Va errno ,
- and is provided for standards conformance.
- .Pp
- The known behaviors are:
- .Bl -tag -width MADV_SEQUENTIAL
- .It Dv MADV_NORMAL
- Tells the system to revert to the default paging
- behavior.
- .It Dv MADV_RANDOM
- Is a hint that pages will be accessed randomly, and prefetching
- is likely not advantageous.
- .It Dv MADV_SEQUENTIAL
- Causes the VM system to depress the priority of
- pages immediately preceding a given page when it is faulted in.
- .It Dv 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.
- .It Dv 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.
- .It Dv 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
- .Xr 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
- .Dv 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.
- .It Dv 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.
- .Pp
- 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
- .Xr mmap 2
- .It Dv 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
- .Xr msync 2
- or
- .Xr fsync 2
- system calls.
- .It Dv MADV_NOCORE
- Region is not included in a core file.
- .It Dv MADV_CORE
- Include region in a core file.
- .It Dv 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.
- .El
- .Pp
- Portable programs that call the
- .Fn posix_madvise
- interface should use the aliases
- .Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
- .Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
- and
- .Dv POSIX_MADV_DONTNEED
- rather than the flags described above.
- .Sh RETURN VALUES
- .Rv -std madvise
- .Sh ERRORS
- The
- .Fn madvise
- system call will fail if:
- .Bl -tag -width Er
- .It Bq Er EINVAL
- The
- .Fa behav
- argument is not valid.
- .It Bq Er ENOMEM
- The virtual address range specified by the
- .Fa addr
- and
- .Fa len
- arguments is not valid.
- .It Bq Er EPERM
- .Dv MADV_PROTECT
- was specified and the process does not have superuser privileges.
- .El
- .Sh SEE ALSO
- .Xr mincore 2 ,
- .Xr mprotect 2 ,
- .Xr msync 2 ,
- .Xr munmap 2 ,
- .Xr posix_fadvise 2
- .Sh STANDARDS
- The
- .Fn posix_madvise
- interface conforms to
- .St -p1003.1-2001 .
- .Sh HISTORY
- The
- .Fn madvise
- system call first appeared in
- .Bx 4.4 .
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