man/man9/malloc.9

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.\" $NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
.\" $FreeBSD$
.\"
.Dd April 9, 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.
If
.Fa addr
is
.Dv NULL ,
then
.Fn free
does nothing.
.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 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.
The statistics can be examined by
.Sq vmstat -m .
.Pp
A
.Fa type
is defined using
.Va struct malloc_type
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
.Pp
In order to use
.Fn MALLOC_DEFINE
one must include
.Aq sys/param.h
(instead of
.Aq sys/types.h )
and
.Aq sys/kernel.h .
.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 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.
.Pp
Programmers should be careful not to confuse the malloc flags
.Dv M_NOWAIT
and
.Dv M_WAITOK
with the
.Xr mbuf 9
flags
.Dv M_DONTWAIT
and
.Dv M_TRYWAIT .
.Sh LOCKING CONSIDERATIONS
.Fn malloc ,
.Fn realloc
and
.Fn reallocf
may not be called from fast interrupts handlers.
When called from threaded interrupts
.Ar flag
must contain
.Dv M_NOWAIT .
.Pp
.Fn malloc ,
.Fn realloc
and
.Fn reallocf
must not be called with
.Dv M_WAITOK
while a mutex other than Giant is held.
Giant may or may not be held when
.Fn free
is called.
.Pp
Any calls to
.Fn malloc
(even with
.Dv M_NOWAIT )
or
.Fn free
when holding a
.Xr vnode 9
interlock, will cause a LOR (Lock Order Reversal) due to the
interwining of VM Objects and Vnodes.
.Sh SEE ALSO
.Xr vmstat 8 ,
.Xr vnode 9
.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.