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,
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.
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 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.
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()
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.
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
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, 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.