path: root/usr/src/man/man3c/madvise.3c

'\" te
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.TH MADVISE 3C "Mar 28, 2016"
.SH NAME
madvise \- provide advice to VM system
.SH SYNOPSIS
.LP
.nf
#include <sys/types.h>
#include <sys/mman.h>

\fBint\fR \fBmadvise\fR(\fBcaddr_t\fR \fIaddr\fR, \fBsize_t\fR \fIlen\fR, \fBint\fR \fIadvice\fR);
.fi

.SH DESCRIPTION
.LP
The \fBmadvise()\fR function advises the kernel that a region of user mapped
memory in the range [\fIaddr\fR, \fIaddr\fR + \fIlen\fR) will be accessed
following 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. In general (and true to the name of the function),
the advice is merely advisory, and the only user-visible ramifications
are in terms of performance, not semantics. Note that
\fBMADV_PURGE\fR is an exception to this; see below for details.
.sp
.LP
Values for \fIadvice\fR are defined in <\fBsys/mman.h\fR> as:
.sp
.in +2
.nf
#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_PURGE            0x9  /* contents will be purged */
.fi
.in -2

.sp
.ne 2
.na
\fB\fBMADV_NORMAL\fR\fR
.ad
.RS 23n
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.
.RE

.sp
.ne 2
.na
\fB\fBMADV_RANDOM\fR\fR
.ad
.RS 23n
Tell the kernel to read in a minimum amount of data from a mapped file on any
single particular access. If  \fBMADV_NORMAL\fR 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.
.RE

.sp
.ne 2
.na
\fB\fBMADV_SEQUENTIAL\fR\fR
.ad
.RS 23n
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.
.RE

.sp
.ne 2
.na
\fB\fBMADV_WILLNEED\fR\fR
.ad
.RS 23n
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.
.RE

.sp
.ne 2
.na
\fB\fBMADV_DONTNEED\fR\fR
.ad
.RS 23n
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.
While the semantics of \fBMADV_DONTNEED\fR are similar to other systems,
they differ significantly from the semantics on Linux, where
\fBMADV_DONTNEED\fR will actually synchronously purge the address range,
and subsequent faults will load from either backing store or be
zero-filled on demand. If the peculiar Linux semantics are
desired, \fBMADV_PURGE\fR should be used in lieu of \fBMADV_DONTNEED\fR.
.RE

.sp
.ne 2
.na
\fB\fBMADV_FREE\fR\fR
.ad
.RS 23n
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.  Otherwise, it will contain the data that was there prior
to the \fBMADV_FREE\fR call. References made to the address range will not make
the system read from backing store (swap space) until the page is modified
again.
.sp
This value cannot be used on mappings that have underlying file objects.
.RE

.sp
.ne 2
.na
\fB\fBMADV_PURGE\fR\fR
.ad
.RS 23n
Tell the kernel to purge the specified address range.  The mapping will
be retained, but the pages themselves will be destroyed; subsequent
faults on the range will result in the page being read from backing
store (if file-backed) or being zero-filled on demand (if anonymous).  Note
that these semantics are generally inferior to \fBMADV_FREE\fR, which gives the
system more flexibility and results in better performance
when pages are, in fact, reused by the caller. Indeed, \fBMADV_PURGE\fR only
exists to provide an equivalent to the unfortunate
\fBMADV_DONTNEED\fR semantics found in Linux, upon which some programs
have (regrettably) come to depend. In de novo applications,
\fBMADV_PURGE\fR should be avoided; \fBMADV_FREE\fR should always be
preferred.
.RE

.sp
.ne 2
.na
\fB\fBMADV_ACCESS_LWP\fR\fR
.ad
.RS 23n
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.
.RE

.sp
.ne 2
.na
\fB\fBMADV_ACCESS_MANY\fR\fR
.ad
.RS 23n
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.
.RE

.sp
.ne 2
.na
\fB\fBMADV_ACCESS_DEFAULT\fR\fR
.ad
.RS 23n
Reset the kernel's expectation for how the specified range will be accessed to
the default.
.RE

.sp
.LP
The \fBmadvise()\fR 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.
.SH RETURN VALUES
.LP
Upon successful completion, \fBmadvise()\fR returns \fB0\fR; otherwise, it
returns \fB\(mi1\fR and sets \fBerrno\fR to indicate the error.
.SH ERRORS
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
Some or all mappings  in the  address  range [\fIaddr\fR,  \fIaddr\fR  +
\fIlen\fR) are locked for I/O.
.RE

.sp
.ne 2
.na
\fB\fBEBUSY\fR\fR
.ad
.RS 10n
Some or all of the addresses in the range [\fIaddr\fR, \fIaddr\fR + \fIlen\fR)
are locked and \fBMS_SYNC\fR with the \fBMS_INVALIDATE\fR option is specified.
.RE

.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
Some or all of the addresses in the specified range could not be read into
memory from the underlying object when performing \fBMADV_WILLNEED\fR. The
\fBmadvise()\fR function could return prior to this condition being detected,
in which case \fBerrno\fR will not be set to \fBEFAULT\fR.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIaddr\fR argument is not a multiple of the page size as returned by
\fBsysconf\fR(3C), the length of the specified address range is equal to 0, or
the \fIadvice\fR argument was invalid.
.RE

.sp
.ne 2
.na
\fB\fBEIO\fR\fR
.ad
.RS 10n
An I/O error occurred while reading from or writing to the file system.
.RE

.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 10n
Addresses in the range [\fIaddr\fR, \fIaddr\fR + \fIlen\fR) are outside the
valid range for the address space of a process, or specify one or more pages
that are not mapped.
.RE

.sp
.ne 2
.na
\fB\fBESTALE\fR\fR
.ad
.RS 10n
Stale \fBNFS\fR file handle.
.RE

.SH ATTRIBUTES
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Stable
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.LP
\fBmeminfo\fR(2), \fBmmap\fR(2), \fBsysconf\fR(3C), \fBattributes\fR(5)