man/man9/malloc.9

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.\" $NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
.\" $FreeBSD$
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.Dd June 13, 2018
.Dt MALLOC 9
.Os
.Sh NAME
.Nm malloc ,
.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 "size_t size" "struct malloc_type *type" "int flags"
.Ft void *
.Fn malloc_domain "size_t size" "struct malloc_type *type" "int domain" "int flags"
.Ft void *
.Fn mallocarray "size_t nmemb" "size_t size" "struct malloc_type *type" "int flags"
.Ft void
.Fn free "void *addr" "struct malloc_type *type"
.Ft void
.Fn free_domain "void *addr" "struct malloc_type *type"
.Ft void *
.Fn realloc "void *addr" "size_t size" "struct malloc_type *type" "int flags"
.Ft void *
.Fn reallocf "void *addr" "size_t 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 malloc_domain
variant allocates the object from the specified memory domain.  Memory allocated
with this function should be returned with
.Fn free_domain .
See
.Xr numa 9 for more details.
.Pp
The
.Fn mallocarray
function allocates uninitialized memory in kernel address space for an
array of
.Fa nmemb
entries 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
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_NODUMP
For allocations greater than page size, causes the allocated
memory to be excluded from kernel core dumps.
.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 mallocarray ,
.Fn realloc ,
and
.Fn reallocf
functions cannot return
.Dv NULL
if
.Dv M_WAITOK
is specified.
If the multiplication of
.Fa nmemb
and
.Fa size
would cause an integer overflow, the
.Fn mallocarray
function induces a panic.
.It Dv M_USE_RESERVE
Indicates that the system can use its reserve of memory to satisfy the
request.
This option should only be used in combination with
.Dv M_NOWAIT
when an allocation failure cannot be tolerated by the caller without
catastrophic effects on the system.
.It Dv M_EXEC
Indicates that the system should allocate executable memory.
If this flag is not set, the system will not allocate executable memory.
Not all platforms enforce a distinction between executable and
non-executable memory.
.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
.Vt "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 */

\&...
buf = malloc(sizeof(*buf), M_FOOBUF, M_NOWAIT);

.Ed
.Pp
In order to use
.Fn MALLOC_DEFINE ,
one must include
.In sys/param.h
(instead of
.In sys/types.h )
and
.In sys/kernel.h .
.Sh CONTEXT
.Fn malloc ,
.Fn realloc
and
.Fn reallocf
may not be called from fast interrupts handlers.
When called from threaded interrupts,
.Fa flags
must contain
.Dv M_NOWAIT .
.Pp
.Fn malloc ,
.Fn realloc
and
.Fn reallocf
may sleep when called with
.Dv M_WAITOK .
.Fn free
never sleeps.
However,
.Fn malloc ,
.Fn realloc ,
.Fn reallocf
and
.Fn free
may not be called in a critical section or while holding a spin lock.
.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
intertwining of VM Objects and Vnodes.
.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 contigmalloc 9 ,
.Xr memguard 9 ,
.Xr vnode 9