C Library Functions madvise(3)
NAME
madvise - provide advice to VM system
SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
int madvise(caddr_t _a_d_d_r, size_t _l_e_n, int _a_d_v_i_c_e);
DESCRIPTION
madvise() advises the kernel that a region of user mapped
memory in the range [_a_d_d_r, _a_d_d_r + _l_e_n) will be accessed fol-
lowing a type of pattern. The kernel uses this information
to optimize the procedure for manipulating and maintaining
the resources associated with the specified mapping range.
Values for _a_d_v_i_c_e are defined in <sys/mman.h> as:
#define MADV_NORMAL 0x0 /* No further special treatment */
#define MADV_RANDOM 0x1 /* Expect random page references */
#define MADV_SEQUENTIAL 0x2 /* Expect sequential page references */
#define MADV_WILLNEED 0x3 /* Will need these pages */
#define MADV_DONTNEED 0x4 /* Don't need these pages */
MADV_NORMAL
The default system characteristic where accessing
memory within the address range causes the system to
read data from the mapped file. The kernel reads all
data from files into pages which are retained for a
period of time as a "cache." System pages can be a
scarce resource, so the kernel steals pages from other
mappings when needed. This is a likely occurrence, but
adversely affects system performance only if a large
amount of memory is accessed.
MADV_RANDOM
Tells the kernel to read in a minimum amount of data
from a mapped file on any single particular access. If
MADV_NORMAL is in effect when an address of a mapped
file is accessed, the system tries to read in as much
data from the file as reasonable, in anticipation of
other accesses within a certain locality.
MADV_SEQUENTIAL
Tells the system that addresses in this range are
likely to be accessed only once, so the system will
free the resources mapping the address range as quickly
as possible. This is used in the cat(1) and cp(1)
utilities.
MADV_WILLNEED
Tells the system that a certain address range is
SunOS 5.6 Last change: 29 Dec 1996 1
C Library Functions madvise(3)
definitely needed so the kernel will start reading the
specified range into memory. This can benefit programs
wanting to minimize the time needed to access memory
the first time, as the kernel would need to read in
from the file.
MADV_DONTNEED
Tells the kernel that the specified address range is no
longer needed, so the system starts to free the
resources associated with the address range.
madvise() should be used by programs with specific knowledge
of their access patterns over a memory object, such as a
mapped file, to increase system performance.
RETURN VALUES
madvise() returns:
0 on success.
-1 on failure and sets errno to indicate the error.
ERRORS
EINVAL _a_d_d_r is not a multiple of the page size as
returned by sysconf(3C).
The length of the specified address range is
less than or equal to 0, or the advice was
invalid.
EIO An I/O error occurred while reading from or
writing to the file system.
ENOMEM Addresses in the range [_a_d_d_r, _a_d_d_r + _l_e_n) are
outside the valid range for the address space
of a process, or specify one or more pages
that are not mapped.
ESTALE Stale nfs file handle.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
__________________________________
| ATTRIBUTE TYPE| ATTRIBUTE VALUE|
|_______________________________|____________________________________|_
| MT-Level | MT-Safe |
|________________|__________________|
SEE ALSO
cat(1), cp(1), mmap(2), sysconf(3C), attributes(5)
SunOS 5.6 Last change: 29 Dec 1996 2
No mention of conforming to any standard her. HP-UX 10.20's manpage
claims conformance with AES and SVID3. It defines a MADV_SPACEAVAIL
behavior too, but notes that it isn't implemented.
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