NAME¶
malloc
,
free
,
realloc
,
reallocf
,
MALLOC_DEFINE
,
MALLOC_DECLARE
—
kernel memory management routines
SYNOPSIS¶
#include
<sys/types.h>
#include
<sys/malloc.h>
void *
malloc
(
unsigned
long size,
struct malloc_type
*type,
int
flags);
void
free
(
void
*addr,
struct
malloc_type *type);
void *
realloc
(
void
*addr,
unsigned
long size,
struct malloc_type
*type,
int
flags);
void *
reallocf
(
void
*addr,
unsigned
long size,
struct malloc_type
*type,
int
flags);
MALLOC_DECLARE
(
type);
#include
<sys/param.h>
#include
<sys/malloc.h>
#include
<sys/kernel.h>
MALLOC_DEFINE
(
type,
shortdesc,
longdesc);
DESCRIPTION¶
The
malloc
() function allocates uninitialized
memory in kernel address space for an object whose size is specified by
size.
The
free
() function releases memory at
address
addr that was previously allocated by
malloc
() for re-use. The memory is not
zeroed. If
addr is
NULL
, then
free
() does nothing.
The
realloc
() function changes the size of
the previously allocated memory referenced by
addr to
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
addr. If the requested memory cannot be
allocated,
NULL
is returned and the memory
referenced by
addr is valid and unchanged. If
addr is
NULL
, the
realloc
() function behaves identically to
malloc
() for the specified size.
The
reallocf
() function is identical to
realloc
() except that it will free the
passed pointer when the requested memory cannot be allocated.
Unlike its standard C library counterpart
(
malloc(3)), the kernel version takes two more
arguments. The
flags argument further
qualifies
malloc
()'s operational
characteristics as follows:
M_ZERO
- Causes the allocated memory to be set to all zeros.
M_NODUMP
- For allocations greater than page size, causes the allocated memory to be
excluded from kernel core dumps.
M_NOWAIT
- Causes
malloc
(),
realloc
(), and
reallocf
() to return
NULL
if the request cannot be
immediately fulfilled due to resource shortage. Note that
M_NOWAIT
is required when running in an
interrupt context.
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
malloc
(),
realloc
(), and
reallocf
() functions cannot return
NULL
if
M_WAITOK
is specified.
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
M_NOWAIT
when an allocation failure
cannot be tolerated by the caller without catastrophic effects on the
system.
Exactly one of either
M_WAITOK
or
M_NOWAIT
must be specified.
The
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 ‘vmstat
-m’.
A
type is defined using
struct malloc_type via the
MALLOC_DECLARE
() and
MALLOC_DEFINE
() macros.
/* 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);
In order to use
MALLOC_DEFINE
(), one must
include
<sys/param.h>
(instead of
<sys/types.h>
)
and
<sys/kernel.h>
.
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.
Programmers should be careful not to confuse the malloc flags
M_NOWAIT
and
M_WAITOK
with the
mbuf(9) flags
M_DONTWAIT
and
M_WAIT
.
CONTEXT¶
malloc
(),
realloc
() and
reallocf
() may not be called from fast
interrupts handlers. When called from threaded interrupts,
flags must contain
M_NOWAIT
.
malloc
(),
realloc
() and
reallocf
() may sleep when called with
M_WAITOK
.
free
() never sleeps.
Any calls to
malloc
() (even with
M_NOWAIT
) or
free
() when holding a
vnode(9) interlock, will cause a LOR (Lock Order
Reversal) due to the intertwining of VM Objects and Vnodes.
RETURN VALUES¶
The
malloc
(),
realloc
(), and
reallocf
() functions return a kernel
virtual address that is suitably aligned for storage of any type of object, or
NULL
if the request could not be satisfied
(implying that
M_NOWAIT
was set).
DIAGNOSTICS¶
A kernel compiled with the
INVARIANTS
configuration option attempts to detect memory corruption caused by such
things as writing outside the allocated area and imbalanced calls to the
malloc
() and
free
() functions. Failing consistency
checks will cause a panic or a system console message.
SEE ALSO¶
vmstat(8),
contigmalloc(9),
memguard(9),
vnode(9)