man/man9/kmalloc.9

.\"
.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Paul Kranenburg.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"        This product includes software developed by the NetBSD
.\"        Foundation, Inc. and its contributors.
.\" 4. Neither the name of The NetBSD Foundation nor the names of its
.\"    contributors may be used to endorse or promote products derived
.\"    from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE
.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.\" $NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
.\" $FreeBSD: src/share/man/man9/malloc.9,v 1.42 2005/02/22 17:20:20 brueffer Exp $
.\" $DragonFly: src/share/man/man9/kmalloc.9,v 1.5 2005/12/27 22:45:23 swildner Exp $
.\"
.Dd June 12, 2003
.Dt MALLOC 9
.Os
.Sh NAME
.Nm malloc ,
.Nm MALLOC ,
.Nm free ,
.Nm FREE ,
.Nm realloc ,
.Nm reallocf ,
.Nm MALLOC_DEFINE ,
.Nm MALLOC_DECLARE
.Nd kernel memory management routines
.Sh SYNOPSIS
.In sys/types.h
.In sys/malloc.h
.Ft void *
.Fn malloc "unsigned long size" "struct malloc_type *type" "int flags"
.Fn MALLOC space cast "unsigned long size" "struct malloc_type *type" "int flags"
.Ft void
.Fn free "void *addr" "struct malloc_type *type"
.Fn FREE "void *addr" "struct malloc_type *type"
.Ft void *
.Fn realloc "void *addr" "unsigned long size" "struct malloc_type *type" "int flags"
.Ft void *
.Fn reallocf "void *addr" "unsigned long size" "struct malloc_type *type" "int flags"
.Fn MALLOC_DECLARE type
.In sys/param.h
.In sys/malloc.h
.In sys/kernel.h
.Fn MALLOC_DEFINE type shortdesc longdesc
.Sh DESCRIPTION
The
.Fn malloc
function allocates uninitialized memory in kernel address space for an
object whose size is specified by
.Fa size .
.Pp
The
.Fn free
function releases memory at address
.Fa addr
that was previously allocated by
.Fn malloc
for re-use.
The memory is not zeroed.
The kernel implementation of
.Fn free
does not allow
.Fa addr
to be
.Dv NULL .
.Pp
The
.Fn realloc
function changes the size of the previously allocated memory referenced by
.Fa addr
to
.Fa size
bytes.
The contents of the memory are unchanged up to the lesser of the new and
old sizes.
Note that the returned value may differ from
.Fa addr .
If the requested memory cannot be allocated,
.Dv NULL
is returned and the memory referenced by
.Fa addr
is valid and unchanged.
If
.Fa addr
is
.Dv NULL ,
the
.Fn realloc
function behaves identically to
.Fn malloc
for the specified size.
.Pp
The
.Fn reallocf
function is identical to
.Fn realloc
except that it
will free the passed pointer when the requested memory cannot be allocated.
.Pp
The
.Fn MALLOC
macro variant is functionally equivalent to
.Bd -literal -offset indent
(space) = (cast)malloc((u_long)(size), type, flags)
.Ed
.Pp
and the
.Fn FREE
macro variant is equivalent to
.Bd -literal -offset indent
free((addr), type)
.Ed
.Pp
Unlike its standard C library counterpart
.Pq Xr malloc 3 ,
the kernel version takes two more arguments.
The
.Fa flags
argument further qualifies
.Fn malloc Ns 's
operational characteristics as follows:
.Bl -tag -width indent
.It Dv M_ZERO
Causes the allocated memory to be set to all zeros.
.It Dv M_NOWAIT
Causes
.Fn malloc ,
.Fn realloc ,
and
.Fn reallocf
to return
.Dv NULL
if the request cannot be immediately fulfilled due to resource shortage.
Note that
.Dv M_NOWAIT
is required when running in an interrupt context.
.It Dv M_WAITOK
Indicates that it is OK to wait for resources.
If the request cannot be immediately fulfilled, the current process is put
to sleep to wait for resources to be released by other processes.
The
.Fn malloc ,
.Fn realloc ,
and
.Fn reallocf
functions cannot return
.Dv NULL
if
.Dv M_WAITOK
is specified.
.It Dv M_USE_RESERVE
Indicates that the system can dig into its reserve in order to obtain the
requested memory.
This option used to be called
.Dv M_KERNEL
but has been renamed to something more obvious.
This option has been deprecated and is slowly being removed from the kernel,
and so should not be used with any new programming.
.El
.Pp
Exactly one of either
.Dv M_WAITOK
or
.Dv M_NOWAIT
must be specified.
.Pp
The
.Fa type
argument is used to perform statistics on memory usage, and for
basic sanity checks.
It can be used to identify multiple allocations.
The statistics can be examined by
.Sq vmstat -m .
.Pp
A
.Fa type
is defined using the
.Va malloc_type_t
typedef via the
.Fn MALLOC_DECLARE
and
.Fn MALLOC_DEFINE
macros.
.Bd -literal -offset indent
/* sys/something/foo_extern.h */

MALLOC_DECLARE(M_FOOBUF);

/* sys/something/foo_main.c */

MALLOC_DEFINE(M_FOOBUF, "foobuffers", "Buffers to foo data into the ether");

/* sys/something/foo_subr.c */

\&...
MALLOC(buf, struct foo_buf *, sizeof *buf, M_FOOBUF, M_NOWAIT);

.Ed
.Sh IMPLEMENTATION NOTES
The memory allocator allocates memory in chunks that have size a power
of two for requests up to the size of a page of memory.
For larger requests, one or more pages is allocated.
While it should not be relied upon, this information may be useful for
optimizing the efficiency of memory use.
.Sh RETURN VALUES
The
.Fn malloc ,
.Fn realloc ,
and
.Fn reallocf
functions return a kernel virtual address that is suitably aligned for
storage of any type of object, or
.Dv NULL
if the request could not be satisfied (implying that
.Dv M_NOWAIT
was set).
.Sh DIAGNOSTICS
A kernel compiled with the
.Dv INVARIANTS
configuration option attempts to detect memory corruption caused by
such things as writing outside the allocated area and imbalanced calls to the
.Fn malloc
and
.Fn free
functions.
Failing consistency checks will cause a panic or a system console
message.
.Sh SEE ALSO
.Xr vmstat 8 ,
.Xr vnode 9