man pages section 9: DDI and DKI Kernel Functions

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Updated: July 2014



ddi_umem_alloc, ddi_umem_free - allocate and free page-aligned kernel memory


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

void *ddi_umem_alloc(size_t size, int flag,
     ddi_umem_cookie_t *cookiep);
void ddi_umem_free(ddi_umem_cookie_t cookie);

Interface Level

Solaris DDI specific (Solaris DDI).




Number of bytes to allocate.


Used to determine the sleep and pageable conditions.

Possible sleep flags are DDI_UMEM_SLEEP, which allows sleeping until memory is available, and DDI_UMEM_NOSLEEP, which returns NULL immediately if memory is not available.

The default condition is to allocate locked memory; this can be changed to allocate pageable memory using the DDI_UMEM_PAGEABLE flag.


Pointer to a kernel memory cookie.



A kernel memory cookie allocated in ddi_umem_alloc().


ddi_umem_alloc() allocates page-aligned kernel memory and returns a pointer to the allocated memory. The number of bytes allocated is a multiple of the system page size (roundup of size). The allocated memory can be used in the kernel and can be exported to user space. See devmap(9E) and devmap_umem_setup(9F) for further information.

flag determines whether the caller can sleep for memory and whether the allocated memory is locked or not. DDI_UMEM_SLEEP allocations may sleep but are guaranteed to succeed. DDI_UMEM_NOSLEEP allocations do not sleep but may fail (return NULL) if memory is currently unavailable. If DDI_UMEM_PAGEABLE is set, pageable memory will be allocated. These pages can be swapped out to secondary memory devices. The initial contents of memory allocated using ddi_umem_alloc() is zero-filled.

*cookiep is a pointer to the kernel memory cookie that describes the kernel memory being allocated. A typical use of cookiep is in devmap_umem_setup(9F) when the drivers want to export the kernel memory to a user application.

To free the allocated memory, a driver calls ddi_umem_free() with the cookie obtained from ddi_umem_alloc(). ddi_umem_free() releases the entire buffer.

Return Values


Successful completion. ddi_umem_alloc() returns a pointer to the allocated memory.


Memory cannot be allocated by ddi_umem_alloc() because DDI_UMEM_NOSLEEP is set and the system is out of resources.


ddi_umem_alloc() can be called from any context if flag is set to DDI_UMEM_NOSLEEP. If DDI_UMEM_SLEEP is set, ddi_umem_alloc() can be called from user and kernel context only. ddi_umem_free () can be called from any context.

See also

devmap(9E), condvar(9F), devmap_umem_setup(9F), kmem_alloc(9F), mutex(9F), rwlock(9F), semaphore(9F)

Writing Device Drivers for Oracle Solaris 11.2


Setting the DDI_UMEM_PAGEABLE flag in ddi_umem_alloc() will result in an allocation of pageable memory. Because these pages can be swapped out to secondary memory devices, drivers should use this flag with care. This memory must not be used for the following purposes:

Memory allocated using ddi_umem_alloc() without setting DDI_UMEM_PAGEABLE flag cannot be 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 effect overall system performance. Over-commitment of kernel memory may cause unpredictable consequences.

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 an invalid pointer, will cause the system to corrupt data or panic.

Do not call ddi_umem_alloc() within DDI_SUSPEND and DDI_RESUME operations. Memory acquired at these times is not reliable. In some cases, such a call can cause a system to hang.


ddi_umem_alloc(0, flag, cookiep) always returns NULL. ddi_umem_free(NULL) has no effects on system.