DragonFly On-Line Manual Pages
KMALLOC(9) DragonFly Kernel Developer's Manual KMALLOC(9)
kmalloc, kfree, krealloc, MALLOC_DEFINE, MALLOC_DECLARE -- kernel memory
kmalloc(unsigned long size, struct malloc_type *type, int flags);
kfree(void *addr, struct malloc_type *type);
krealloc(void *addr, unsigned long size, struct malloc_type *type,
MALLOC_DEFINE(type, shortdesc, longdesc);
The kmalloc() function allocates uninitialized memory in kernel address
space for an object whose size is specified by size.
The kfree() function releases memory at address addr that was previously
allocated by kmalloc() for re-use. The memory is not zeroed. The kernel
implementation of kfree() does not allow addr to be NULL.
The krealloc() function changes the size of the previously allocated mem-
ory 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 krealloc() function behaves identi-
cally to kmalloc() for the specified size.
Unlike its standard C library counterpart (malloc(3)), the kernel version
takes two more arguments. The flags argument further qualifies
kmalloc()'s operational characteristics as follows:
M_ZERO Causes the allocated memory to be set to all zeros.
Causes kmalloc() and krealloc(), 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.
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 kmalloc() and krealloc(), functions cannot return NULL if
M_WAITOK is specified.
Indicates kmalloc() to dig into the system's reserved free pages
looking for enough room to perform the allocation. This is typi-
cally used in interrupts where you cannot afford kmalloc() to
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 code.
Rounds up the size to the nearest power of 2.
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 the malloc_type_t typedef via the
MALLOC_DECLARE() and MALLOC_DEFINE() macros.
/* sys/something/foo_extern.h */
/* sys/something/foo_main.c */
MALLOC_DEFINE(M_FOOBUF, "foobuffers", "Buffers to foo data into the ether");
/* sys/something/foo_subr.c */
buf = kmalloc(sizeof *buf, M_FOOBUF, M_NOWAIT);
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
The kmalloc() and krealloc(), 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).
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 kmalloc() and kfree() func-
tions. Failing consistency checks will cause a panic or a system console
vmstat(8), contigmalloc(9), memory(9), vnode(9)
DragonFly 3.5 October 8, 2012 DragonFly 3.5