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 implementations 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 [EINVAL] 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.
- [EINVAL]
behav
contains an invalid value,
or
addr
is not a multiple of the page size as returned by the system call
sysconf(_SC_PAGE_SIZE).
- [EINVAL]
The address range specified by
addr
and
len
was not created by a successful call to
mmap()
or
shmat().
AUTHOR
madvise()
was developed by HP and OSF.
STANDARDS CONFORMANCE
madvise(): AES, SVID3
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