Quick Navigator

Search Site

Unix VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Contact Us
Online Help
Domain Status
Man Pages

Virtual Servers

Topology Map

Server Agreement
Year 2038

USA Flag



Man Pages

Manual Reference Pages  -  MALLOC (9)


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


Implementation Notes
Return Values
See Also


.In sys/types.h
.In 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
.In sys/param.h
.In sys/malloc.h
.In 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 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:
  Causes the allocated memory to be set to all zeros.
  For allocations greater than page size, causes the allocated memory to be excluded from kernel core dumps.
  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.
  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 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
.Vt 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 */

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

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


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.


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.


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).


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.


vmstat(8), contigmalloc(9), memguard(9), vnode(9)
Search for    or go to Top of page |  Section 9 |  Main Index

Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.