MADVISE(3C)MADVISE(3C)NAMEmadvise - provide advice to VM system
int madvise(caddr_t addr, size_t len, int advice);
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 */
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.
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.
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.
Tell the system that a certain address range is
definitely needed so the kernel will start read‐
ing 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.
Tell the kernel that the specified address range
is no longer needed, so the system starts to
free the resources associated with the address
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 associ‐
ated with the specified address range. In this
instance, the next time a page in the address
range is referenced, it will contain all zeroes.
Otherwise, it will contain the data that was
there prior to the MADV_FREE call. References
made to the address range will not make the sys‐
tem read from backing store (swap space) until
the page is modified again.
This value cannot be used on mappings that have
underlying file objects.
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.
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
Reset the kernel's expectation for how the spec‐
ified range will be accessed to the default.
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.
Upon successful completion, madvise() returns 0; otherwise, it returns
−1 and sets errno to indicate the error.
Some or all mappings in the address range [addr, addr +
len) are locked for I/O.
Some or all of the addresses in the range [addr, addr + len)
are locked and MS_SYNC with the MS_INVALIDATE option is spec‐
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.
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.
An I/O error occurred while reading from or writing to the
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.
Stale NFS file handle.
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ Stable │
│MT-Level │ MT-Safe │
SEE ALSOmeminfo(2), mmap(2), sysconf(3C), attributes(5)
Feb 23, 2005 MADVISE(3C)