KMEM_ALLOC(9F) Kernel Functions for Drivers KMEM_ALLOC(9F)

NAME

kmem_alloc, kmem_zalloc, kmem_free - allocate kernel memory

SYNOPSIS

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


void *kmem_alloc(size_t size, int flag);


void *kmem_zalloc(size_t size, int flag);


void kmem_free(void *buf, size_t size);

INTERFACE LEVEL

Architecture independent level 1 (DDI/DKI).

PARAMETERS

size
Number of bytes to allocate.


flag
Determines whether caller can sleep for memory. Possible
flags are KM_SLEEP to allow sleeping until memory is
available, KM_NOSLEEP to return NULL if memory is not
available even after some reclamation attempts, and
KM_NOSLEEP_LAZY to return NULL without reclamation attempts.
KM_NOSLEEP_LAZY is actually two flags combined: (KM_NOSLEEP |
KM_NORMALPRI), the latter flag indicating not to attempt
reclamation before giving up and returning NULL. If any
mention of KM_NOSLEEP appears in this man page by itself, it
applies equally to KM_NOSLEEP_LAZY as well.


buf
Pointer to allocated memory.

DESCRIPTION

The kmem_alloc() function allocates size bytes of kernel memory and
returns a pointer to the allocated memory. The allocated memory is at
least double-word aligned, so it can hold any C data structure. No
greater alignment can be assumed. flag determines whether the caller
can sleep for memory. KM_SLEEP allocations may sleep but are
guaranteed to succeed. KM_NOSLEEP and KM_NOSLEEP_LAZY allocations
are guaranteed not to sleep but may fail (return NULL) if no memory
is currently available. KM_NOSLEEP will first attempt to aggressively
reclaim memory from otherwise unused blocks, while KM_NOSLEEP_LAZY
will not attempt any reclamation. The initial contents of memory
allocated using kmem_alloc() are random garbage.


The kmem_zalloc() function is like kmem_alloc() but returns zero-
filled memory.


The kmem_free() function frees previously allocated kernel memory.
The buffer address and size must exactly match the original
allocation. Memory cannot be returned piecemeal.

RETURN VALUES

If successful, kmem_alloc() and kmem_zalloc() return a pointer to the
allocated memory. If KM_NOSLEEP is set and memory cannot be allocated
without sleeping, kmem_alloc() and kmem_zalloc() return NULL.

CONTEXT

The kmem_alloc() and kmem_zalloc() functions can be called from
interrupt context only if the KM_NOSLEEP flag is set. They can be
called from user context with any valid flag. The kmem_free()
function can be called from from user, interrupt, or kernel context.

SEE ALSO

copyout(9F), freerbuf(9F), getrbuf(9F)


Writing Device Drivers

WARNINGS

Memory allocated using kmem_alloc() is not paged. Available memory is
therefore limited by the total physical memory on the system. It is
also limited by the available kernel virtual address space, which is
often the more restrictive constraint on large-memory configurations.


Excessive use of kernel memory is likely to affect overall system
performance. Overcommitment of kernel memory will cause the system
to hang or panic.


Misuse of the kernel memory allocator, such as writing past the end
of a buffer, using a buffer after freeing it, freeing a buffer twice,
or freeing a null or invalid pointer, will corrupt the kernel heap
and may cause the system to corrupt data or panic.


The initial contents of memory allocated using kmem_alloc() are
random garbage. This random garbage may include secure kernel data.
Therefore, uninitialized kernel memory should be handled carefully.
For example, never copyout(9F) a potentially uninitialized buffer.

NOTES

kmem_alloc(0, flag) always returns NULL, but if KM_SLEEP is set, this
behavior is considered to be deprecated; the system may be configured
to explicitly panic in this case in lieu of returning NULL.
kmem_free(NULL, 0) is legal, however.

November 20, 2019 KMEM_ALLOC(9F)