Architecture independent level 1 (DDI/DKI).

size

Number of bytes to allocate.

flag

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.

buf

Pointer to allocated memory.

kmem_alloc() 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_zalloc() is like kmem_alloc() but returns zero-filled memory.

kmem_free() 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.

kmem_alloc() and kmem_zalloc() 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. kmem_free() can be called from user or interrupt context.

copyout(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 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.