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MALLOC(9) | Kernel Developer's Manual | MALLOC(9) |
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¶
Themalloc
() 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
(), andreallocf
() to returnNULL
if the request cannot be immediately fulfilled due to resource shortage. Note thatM_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
(), andreallocf
() functions cannot returnNULL
ifM_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.
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);
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 flagsM_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¶
Themalloc
(),
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 theINVARIANTS
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)November 15, 2012 | Debian |