madvise.2 (2010 09)

m
madvise(2) madvise(2)
NAME
madvise() - advise the system of a process’s expected paging behavior
SYNOPSIS
#include <sys/mman.h>
int madvise(
caddr_t addr,
size_t len,
int behav );
DESCRIPTION
The madvise system call permits a process to advise the system about its expected future behavior in
referencing a mapped file, an anonymous memory region, or a shared memory region. Certain implemen-
tations can use this information to optimize the use of resources.
addr and len specify the address and length in bytes of the region to which the advice refers. For
MADV_DONTNEED
, the address and length must be contained within a successful call to
mmap() (see
mmap(2)) or a successful call to
shmat() (see shmat(2)); otherwise, madvise() fails with an [EIN-
VAL] error.
The behav argument is one the following flags defined in the header
<sys/mman.h>:
MADV_NORMAL Removes any previous advice and sets the default behavior. By default,
the kernel tracks access patterns on data objects and performs I/Os
based on process trends (that is, sequential versus random). Sequential
trends cause larger "read-ahead" I/Os, while random accesses reduce the
amount of I/O to avoid unnecessary I/O.
MADV_RANDOM Informs the kernel that any objects mapped in this range will be
accessed in a random matter. The kernel will read only the minimal
amount of data to satisfy the user fault.
MADV_SEQUENTIAL Informs the kernel that any objects mapped in this range will be
accessed in a sequential matter. The kernel will perform the maximum
read-ahead for every fault. The kernel does not pay attention to access
patterns and trends, but instead assumes sequentiality for every access
on the object.
MADV_DONTNEED Informs the kernel that the specified range is no longer needed by the
process. This allows the kernel to release the physical pages associated
with an address range back to the system for use by other processes.
MADV_DONTNEED is restricted to object ranges created with calls to
mmap() and shmat(). Attempting to use MADV_DONTNEED
on an
object that was not created using a call to
mmap() or shmat() will
result in [EINVAL] being returned to the caller.
MADV_WILLNEED Will need these pages.
MADV_SPACEAVAIL Ensure that resources are reserved.
WARNINGS
The current implementation of
madvise() defines MADV_SPACEAVAIL
and MADV_WILLNEED as null
operations.
RETURN VALUE
madvise() returns the following values:
0 Successful completion.
-1 Failure. errno is set to indicate the error.
ERRORS
If
madvise() fails, errno is set to one of the following values.
[EFAULT] The range specified by (addr , addr+len) is invalid for a process’s address space, or
permission was incorrect on the object for the behav specified.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1