kmem_alloc, kmem_zalloc, kmem_free - allocate kernel memory
void *kmem_alloc(size_t size, int flag);
void *kmem_zalloc(size_t size, int flag);
void kmem_free(void*buf, size_t size);
Architecture independent level 1 (DDI/DKI).
Number of bytes to allocate.
Determines whether caller can sleep for memory. Possible flags
are KM_SLEEP to allow sleeping until memory is available, or
KM_NOSLEEP to return NULL immediately if memory is not avail‐
Pointer to allocated memory.
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 guaran‐
teed to succeed. KM_NOSLEEP allocations are guaranteed not to sleep
but may fail (return NULL) if no memory is currently available. The
initial contents of memory allocated using kmem_alloc() are random
The kmem_zalloc() function is like kmem_alloc() but returns zero-filled
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.
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.
The kmem_alloc() and kmem_zalloc() functions can be called from inter‐
rupt 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 ALSOcopyout(9F), freerbuf(9F), getrbuf(9F)
Writing Device Drivers
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 per‐
formance. 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.
kmem_alloc(0, flag) always returns NULL. kmem_free(NULL, 0) is legal.
Jan 16, 2006 KMEM_ALLOC(9F)