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madvise(3c)
Standard C Library Functions madvise(3C)
NAME
madvise - provide advice to VM system
SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
int madvise(void *addr, size_t len, int advice);
DESCRIPTION
The madvise() function advises the kernel that a region of user mapped
memory in the range [addr, addr + len) will be accessed following a
type of pattern. The kernel uses this information to optimize the pro‐
cedure for manipulating and maintaining the resources associated with
the specified mapping range.
Values for advice 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 */
#define MADV_FREE 0x5 /* Contents can be freed */
#define MADV_ACCESS_DEFAULT 0x6 /* default access */
#define MADV_ACCESS_LWP 0x7 /* next LWP to access heavily */
#define MADV_ACCESS_MANY 0x8 /* many processes to access heavily */
#define MADV_ACCESS_MANY_PSET 0x9 /* many processes in pset to access */
/* heavily */
MADV_NORMAL
This is 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
Tell 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
Tell 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.
MADV_WILLNEED
Tell the system that a certain address range is 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
Tell the kernel that the specified address range is no longer
needed, so the system starts to free the resources associated with
the address range.
MADV_DONTNEED and MADV_FREE perform related but distinct opera‐
tions. MADV_DONTNEED tries to move any data from the specified
address range out of memory, but it ensures that the contents of
that range will be recovered when they are next referenced.
MADV_FREE does not attempt to preserve the contents of the address
range. As a result, subsequent references to an address range that
received madvise (MADV_DONTNEED) are likely to be slower than ref‐
erences to a range that received madvise (MADV_FREE).
MADV_FREE
Tell the kernel that contents in the specified address range are no
longer important and the range will be overwritten. When there is
demand for memory, the system will free pages associated with the
specified address range. In this instance, the next time a page in
the address range is referenced, it will contain all zeroes. Other‐
wise, it will contain the data that was there prior to the
MADV_FREE call. References made to the address range will not make
the system read from backing store (swap space) until the page is
modified again.
This value cannot be used on mappings that have underlying file
objects.
MADV_DONTDUMP
Exclude the specified address range from the coredump of the
process. This is alias of the memcntl MC_CORE_PRUNE_OUT command.
MADV_DODUMP
Include the specified address range in a coredump of the process.
This is an alias of the memcntl MC_CORE_PRUNE_IN command.
MADV_ACCESS_LWP
Tell the kernel that the next LWP to touch the specified address
range will access it most heavily, so the kernel should try to
allocate the memory and other resources for this range and the LWP
accordingly.
MADV_ACCESS_MANY
Tell the kernel that many processes and/or LWPs will access the
specified address range randomly across the machine, so the kernel
should try to allocate the memory and other resources for this
range accordingly.
MADV_ACCESS_DEFAULT
Reset the kernel's expectation for how the specified range will be
accessed to the default.
MADV_ACCESS_MANY_PSET
Tell the kernel that many processes and/or LWPs in a processor set
will access the specified address range randomly, so the kernel
should try to allocate the memory and other resources for this
range accordingly.
The madvise() function should be used by applications with specific
knowledge of their access patterns over a memory object, such as a
mapped file, to increase system performance.
When applied to shared memory segments created by shmget_osm(2), the
MADV_ACCESS_LWP, MADV_ACCESS_MANY, MADV_ACCESS_DEFAULT, and
MADV_ACCESS_MANY_PSET options affect whole granules, and only work
through non-readonly mappings. The other madvise operations have no
effect on shared memory segments created by shmget_osm(2).
RETURN VALUES
Upon successful completion, madvise() returns 0; otherwise, it returns
−1 and sets errno to indicate the error.
ERRORS
EAGAIN Some or all mappings in the address range [addr, addr + len)
are locked for I/O.
EACCES A MADV_ACCESS_* operation targets a range [addr, addr + len)
which includes a shmget_osm(2) created segment with
SHM_RDONLY.
EBUSY Some or all of the addresses in the range [addr, addr + len)
are locked and MS_SYNC with the MS_INVALIDATE option is spec‐
ified.
EFAULT Some or all of the addresses in the specified range could not
be read into memory from the underlying object when perform‐
ing MADV_WILLNEED. The madvise() function could return prior
to this condition being detected, in which case errno will
not be set to EFAULT.
EINVAL The addr argument is not a multiple of the page size as
returned by sysconf(3C), the length of the specified address
range is equal to 0, or the advice argument was invalid.
The range described by addr and len span part of a segment
created by shmget_osm(2), and the spanned range is not
aligned to that segment's granule size.
EIO An I/O error occurred while reading from or writing to the
file system.
ENOMEM Addresses in the range [addr, addr + len) 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(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe
SEE ALSO
meminfo(2), memcntl(2), mmap(2),shmget_osm(2),sysconf(3C),attributes(7)
Oracle Solaris 11.4 12 June 2019 madvise(3C)