MALLOC_DECLARE(9) kernel memory management routines

Other Alias

malloc, free, realloc, reallocf, MALLOC_DEFINE


In sys/types.h In sys/malloc.h Ft void * Fn malloc unsigned long size struct malloc_type *type int flags Ft void Fn free void *addr struct malloc_type *type Ft void * Fn realloc void *addr unsigned long size struct malloc_type *type int flags Ft void * Fn reallocf void *addr unsigned long size struct malloc_type *type int flags Fn MALLOC_DECLARE type In sys/param.h In sys/malloc.h In sys/kernel.h Fn MALLOC_DEFINE type shortdesc longdesc


The Fn malloc function allocates uninitialized memory in kernel address space for an object whose size is specified by Fa size .

The Fn free function releases memory at address Fa addr that was previously allocated by Fn malloc for re-use. The memory is not zeroed. If Fa addr is NULL then Fn free does nothing.

The Fn realloc function changes the size of the previously allocated memory referenced by Fa addr to Fa 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 Fa addr . If the requested memory cannot be allocated, NULL is returned and the memory referenced by Fa addr is valid and unchanged. If Fa addr is NULL the Fn realloc function behaves identically to Fn malloc for the specified size.

The Fn reallocf function is identical to Fn 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 Fa flags argument further qualifies Fn malloc Ns '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 Fn malloc , Fn realloc , and Fn 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 Fn malloc , Fn realloc , and Fn 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 Fa 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 Fa type is defined using Vt struct malloc_type via the Fn MALLOC_DECLARE and Fn 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 Fn 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


Fn malloc , Fn realloc and Fn reallocf may not be called from fast interrupts handlers. When called from threaded interrupts, Fa flags must contain M_NOWAIT

Fn malloc , Fn realloc and Fn reallocf may sleep when called with M_WAITOK Fn free never sleeps.

Any calls to Fn malloc (even with M_NOWAIT or Fn free when holding a vnode(9) interlock, will cause a LOR (Lock Order Reversal) due to the intertwining of VM Objects and Vnodes.


The Fn malloc , Fn realloc , and Fn 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 Fn malloc and Fn free functions. Failing consistency checks will cause a panic or a system console message.