Manual browser: kmem(9)

KMEM(9) Kernel Developer's Manual KMEM(9)


kmemkernel wired memory allocator


#include <sys/kmem.h>

void *
kmem_alloc(size_t size, km_flag_t kmflags);

void *
kmem_zalloc(size_t size, km_flag_t kmflags);

kmem_free(void *p, size_t size);

void *
kmem_intr_alloc(size_t size, km_flag_t kmflags);

void *
kmem_intr_zalloc(size_t size, km_flag_t kmflags);

kmem_intr_free(void *p, size_t size);

char *
kmem_asprintf(const char *fmt, ...);

options DEBUG


kmem_alloc() allocates kernel wired memory. It takes the following arguments.
Specify the size of allocation in bytes.
Either of the following:
If the allocation cannot be satisfied immediately, sleep until enough memory is available.
Don't sleep. Immediately return NULL if there is not enough memory available. It should only be used when failure to allocate will not have harmful, user-visible effects.

Use of KM_NOSLEEP is strongly discouraged as it can create transient, hard to debug failures that occur when the system is under memory pressure.

In situations where it is not possible to sleep, for example because locks are held by the caller, the code path should be restructured to allow the allocation to be made in another place.

The contents of allocated memory are uninitialized.

Unlike Solaris, kmem_alloc(0, flags) is illegal.

kmem_zalloc() is the equivalent of kmem_alloc(), except that it initializes the memory to zero.

kmem_asprintf() functions as the well known asprintf() function, but allocates memory using kmem_alloc(). This routine can sleep during allocation. The size of the allocated area is the length of the returned character string, plus one (for the NUL terminator). This must be taken into consideration when freeing the returned area with kmem_free().

kmem_free() frees kernel wired memory allocated by kmem_alloc() or kmem_zalloc() so that it can be used for other purposes. It takes the following arguments.

The pointer to the memory being freed. It must be the one returned by kmem_alloc() or kmem_zalloc().
The size of the memory being freed, in bytes. It must be the same as the size argument used for kmem_alloc() or kmem_zalloc() when the memory was allocated.

Freeing NULL is illegal.

kmem_intr_alloc(), kmem_intr_zalloc() and kmem_intr_free() are the equivalents of the above kmem routines which can be called from the interrupt context. These routines are for the special cases. Normally, pool_cache(9) should be used for memory allocation from interrupt context.


Making KM_SLEEP allocations while holding mutexes or reader/writer locks is discouraged, as the caller can sleep for an unbounded amount of time in order to satisfy the allocation. This can in turn block other threads that wish to acquire locks held by the caller. It should be noted that kmem_free() may also block.

For some locks this is permissible or even unavoidable. For others, particularly locks that may be taken from soft interrupt context, it is a serious problem. As a general rule it is better not to allow this type of situation to develop. One way to circumvent the problem is to make allocations speculative and part of a retryable sequence. For example:

        /* speculative unlocked check */ 
        if (need to allocate) { 
                new_item = kmem_alloc(sizeof(*new_item), KM_SLEEP); 
        } else { 
                new_item = NULL; 
        /* check while holding lock for true status */ 
        if (need to allocate) { 
                if (new_item == NULL) { 
                        goto retry; 
                new_item = NULL; 
        if (new_item != NULL) { 
                /* did not use it after all */ 
                kmem_free(new_item, sizeof(*new_item)); 


Kernels compiled with the DEBUG option perform CPU intensive sanity checks on kmem operations, and include the kmguard facility which can be enabled at runtime.

kmguard adds additional, very high overhead runtime verification to kmem operations. To enable it, boot the system with the -d option, which causes the debugger to be entered early during the kernel boot process. Issue commands such as the following:

db> w kmem_guard_depth 0t30000 
db> c

This instructs kmguard to queue up to 60000 (30000*2) pages of unmapped KVA to catch use-after-free type errors. When kmem_free() is called, memory backing a freed item is unmapped and the kernel VA space pushed onto a FIFO. The VA space will not be reused until another 30k items have been freed. Until reused the kernel will catch invalid accesses and panic with a page fault. Limitations:

  • It has a severe impact on performance.
  • It is best used on a 64-bit machine with lots of RAM.
  • Allocations larger than PAGE_SIZE bypass the kmguard facility.

kmguard tries to catch the following types of bugs:

  • Overflow at time of occurrence, by means of a guard page.
  • Underflow at kmem_free(), by using a canary value.
  • Invalid pointer or size passed, at kmem_free().


On success, kmem_alloc() and kmem_zalloc() return a pointer to allocated memory. Otherwise, NULL is returned.


The kmem subsystem is implemented within the file sys/kern/subr_kmem.c.


Neither kmem_alloc() nor kmem_free() can be used from interrupt context, from a soft interrupt, or from a callout. Use pool_cache(9) in these situations.


As the memory allocated by kmem_alloc() is uninitialized, it can contain security-sensitive data left by its previous user. It is the caller's responsibility not to expose it to the world.
November 26, 2013 NetBSD 7.0