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)