MALLOC(9r)MALLOC(9r)NAMEMALLOC - General: Allocates a variable-size section of kernel virtual
memory
SYNOPSIS
#include <sys/malloc.h>
MALLOC(
addr,
cast,
u_long size,
int type,
int flags );
ARGUMENTS
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 allo‐
cate. Typically, you pass the size as a constant to speed up the memory
allocation. Specifies the purpose for which the memory is being allo‐
cated. 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 mem‐
ory 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.
DESCRIPTION
The MALLOC routine (macro) allocates at least size bytes from the ker‐
nel 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 MALLOC routine.
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.
NOTES
A memory corruption can occur if a device driver continues to use the
memory after freeing it. The operating system provides a built-in mech‐
anism to debug such erroneous use of memory. You can enable this debug‐
ging 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 con‐
tent 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 structure.
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 mem‐
ory (stored in the last word of freed memory) may itself become cor‐
rupted.
CAUTIONS
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 preallo‐
cated pool. To be able to allocate memory from the virtual memory sub‐
system (which can page fault), the device driver must be in process
context.
RETURN VALUES
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 argu‐
ment.
SEE ALSO
Routines: FREE(9r)MALLOC(9r)
