MALLOC - General: Allocates a variable-size section of
kernel virtual memory
int flags );
Specifies the memory pointer that points to the allocated
memory. You specify the addr argument's data type in the
cast argument. Specifies the data type of the addr argument
and the type of the memory pointer returned by MALLOC.
Specifies the size in bytes of the memory to allocate.
Typically, you pass the size as a constant to speed
up the memory allocation. Specifies the purpose for which
the memory is being allocated. The memory types are
defined in the file <malloc.h>. Typically, kernel modules
use the constant M_DEVBUF to indicate that kernel module
memory is being allocated (or freed). Specifies one of
the following flag constants defined in
/usr/sys/include/sys/malloc.h: Allocates memory from the
virtual memory subsystem if there is not enough memory in
the preallocated pool. This constant signifies that MALLOC
can block. Does not allocate memory from the virtual memory
subsystem if there is not enough memory in the preallocated
pool. This constant signifies that MALLOC cannot
block. Allocates zero-filled memory. You pass this bit
value by ORing it to M_WAITOK or M_NOWAIT.
The MALLOC routine (macro) allocates at least size bytes
from the kernel memory and returns the address of the
allocated memory. A kernel module can allocate the memory
in interrupt and process contexts.
The MALLOC routine (macro) maintains a pool of preallocated
memory for quick allocation. If there is not enough
memory in the pool, MALLOC allocates memory from the virtual
memory subsystem by calling kmem_alloc, which can
potentially block (sleep). A kernel thread that allocates
and frees memory to and from the preallocated pool.
The MALLOC routine (macro) is actually a wrapper that
calls malloc. A kernel module should not directly call the
The type argument allows the memory allocator to keep
track of memory usage by a subsystem.
If the allocation size is greater than 16K, you must pass
M_WAITOK to the flags argument. You cannot allocate more
than 16K bytes of memory in interrupt context.
A memory corruption can occur if a device driver continues
to use the memory after freeing it. The operating system
provides a built-in mechanism to debug such erroneous use
of memory. You can enable this debugging feature at boot
time by providing the following boot parameter:
kmem_debug=1. When you enable this debugging feature, the
FREE routine stores the following in the last word of
freed memory: The program counter (pc) of the module that
last freed the memory The checksum of the memory content
The MALLOC routine checks the checksum of the memory content
before reallocating this corrupted memory. If the
checksum of the memory content does not match the corrupted
memory, MALLOC stores the debug information and
then causes the kernel to panic. The MALLOC routine stores
the address and size of the corrupted memory and the pc of
the routine that last freed it in a kmem_corrupt_data
You should consider the following when using this debugging
feature: This debugging feature does not detect cases
where the corruption occurs after MALLOC reallocates the
freed memory to some other module. There is a small
chance that the pc of the routine that freed the memory
(stored in the last word of freed memory) may itself
A device driver must not call MALLOC in interrupt context
with the flags argument set to M_WAITOK. If flags is set
to M_WAITOK, MALLOC checks if the kernel thread is in
interrupt context. If so, MALLOC returns a null pointer
and displays a message on the console terminal.
The M_WAITOK flag implies that it is valid to allocate
memory from the virtual memory subsystem if there is not
enough memory in the preallocated pool. To be able to
allocate memory from the virtual memory subsystem (which
can page fault), the device driver must be in process context.
Upon successful completion, MALLOC returns the address of
the allocated memory. The return type associated with this
address is the same as that specified for the addr argument.
If the memory allocation request cannot be fulfilled,
MALLOC returns a null pointer in the addr argument.
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