Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
MALLOC(9)		 BSD Kernel Developer's	Manual		     MALLOC(9)

     malloc, MALLOC, free, FREE, realloc, reallocf, MALLOC_DEFINE,
     MALLOC_DECLARE -- kernel memory management	routines

     #include <sys/types.h>
     #include <sys/malloc.h>

     void *
     malloc(unsigned long size,	struct malloc_type *type, int flags);

     MALLOC(space, cast, unsigned long size, struct malloc_type	*type,
	 int flags);

     free(void *addr, struct malloc_type *type);

     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);


     #include <sys/param.h>
     #include <sys/malloc.h>
     #include <sys/kernel.h>

     MALLOC_DEFINE(type, shortdesc, longdesc);

     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 mem-
     ory referenced by addr to size bytes.  The	contents of the	memory are un-
     changed up	to the lesser of the new and old sizes.	 Note that the re-
     turned value may differ from addr.	 If the	requested memory cannot	be al-
     located, 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.

     The MALLOC() macro	variant	is functionally	equivalent to

	   (space) = (cast)malloc((u_long)(size), type,	flags)

     and the FREE() macro variant is equivalent	to

	   free((addr),	type)

     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.

	     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 con-

	     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.

	     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.

     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 */


	   /* 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);

     In	order to use MALLOC_DEFINE(), one must include <sys/param.h> (instead
     of	<sys/types.h>) and <sys/kernel.h>.

     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

     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 re-
     quests, 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_TRYWAIT.

     malloc(), realloc() and reallocf()	may not	be called from fast interrupts
     handlers.	When called from threaded interrupts, flags must contain

     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 in-
     tertwining	of VM Objects and Vnodes.

     vmstat(8),	contigmalloc(9), vnode(9)

     A kernel compiled with the	INVARIANTS configuration option	attempts to
     detect memory corruption caused by	such things as writing outside the al-
     located area and imbalanced calls to the malloc() and free() functions.
     Failing consistency checks	will cause a panic or a	system console mes-

BSD				 June 12, 2003				   BSD


Want to link to this manual page? Use this URL:

home | help