DDI_UMEM_ALLOC(9F) Kernel Functions for Drivers DDI_UMEM_ALLOC(9F)


ddi_umem_alloc, ddi_umem_free - allocate and free page-aligned kernel


#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);


illumos DDI specific (illumos 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

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.


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.


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

Writing Device Drivers


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:

o For synchronization objects such as locks and condition
variables. See mutex(9F), semaphore(9F), rwlock(9F), and

o For driver interrupt routines.

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

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

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.

March 19, 2002 DDI_UMEM_ALLOC(9F)