man pages section 9: DDI and DKI Kernel Functions

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Updated: July 2014



kmem_alloc, kmem_zalloc, kmem_free - allocate kernel memory


#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).



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 available.


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 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 garbage.

kmem_alloc should never be called (regardless of KM_SLEEP or KM_NOSLEEP allocations) while holding a spin lock. Note that kmem_alloc can block even when the KM_NOSLEEP flag is provided.

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.


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 for Oracle Solaris 11.2


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.


kmem_alloc(0, flag) always returns NULL. kmem_free(NULL, 0) is legal.