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MALLOC(9r)
NAME
  MALLOC - 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
  addr
      Specifies the memory pointer that points to the allocated memory. You
      specify the addr argument's data type in the cast argument.

  cast
      Specifies the data type of the addr argument and the type of the memory
      pointer returned by MALLOC.

  size
      Specifies the size in bytes of the memory to allocate. Typically, you
      pass the size as a constant to speed up the memory allocation.

  type
      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).

  flags
      Specifies one of the following flag constants defined in
      /usr/sys/include/sys/malloc.h:

      M_WAITOK
	      Allocates memory from the virtual memory subsystem if there is
	      not enough memory in the preallocated pool. This constant
	      signifies that MALLOC can block.

      M_NOWAIT
	      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.

      M_ZERO  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 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 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 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 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
       memory (stored in the last word of freed memory) may itself become
       corrupted.

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 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.

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 argument.

SEE ALSO
  Routines: FREE(9r)

Index for Section 9r

Alphabetical listing for M

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