NAME

       malloc, free, realloc, reallocf, MALLOC_DEFINE, MALLOC_DECLARE — kernel memory management routines

SYNOPSIS

       #include <sys/types.h>
       #include <sys/malloc.h>

       void *
       malloc(unsigned long size, struct malloc_type *type, int flags);

       void
       free(void *addr, struct malloc_type *type);

       void *
       realloc(void *addr, unsigned long size, struct malloc_type *type, int flags);

       void *
       reallocf(void *addr, unsigned long size, struct malloc_type *type, int flags);

       MALLOC_DECLARE(type);

       #include <sys/param.h>
       #include <sys/malloc.h>
       #include <sys/kernel.h>

       MALLOC_DEFINE(type, shortdesc, longdesc);

DESCRIPTION

       The  malloc() function allocates uninitialized memory in kernel address space for an object whose size is
       specified by size.

       The free() function releases memory at address addr that was previously allocated by malloc() for re-use.
       The memory is not zeroed.  If addr is NULL, then free() does nothing.

       The realloc() function changes the size of the previously allocated memory referenced  by  addr  to  size
       bytes.   The  contents  of the memory are unchanged up to the lesser of the new and old sizes.  Note that
       the returned value may differ from addr.  If the requested memory cannot be allocated, NULL  is  returned
       and  the  memory  referenced  by  addr  is  valid and unchanged.  If addr is NULL, the realloc() function
       behaves identically to malloc() for the specified size.

       The reallocf() function is identical to realloc() except that it will free the passed  pointer  when  the
       requested memory cannot be allocated.

       Unlike  its standard C library counterpart (malloc(3)), the kernel version takes two more arguments.  The
       flags argument further qualifies malloc()'s operational characteristics as follows:

       M_ZERO  Causes the allocated memory to be set to all zeros.

       M_NODUMP
               For allocations greater than page size, causes the allocated memory to be  excluded  from  kernel
               core dumps.

       M_NOWAIT
               Causes  malloc(),  realloc(),  and reallocf() to return NULL if the request cannot be immediately
               fulfilled due to resource shortage.  Note that M_NOWAIT is required when running in an  interrupt
               context.

       M_WAITOK
               Indicates  that  it is OK to wait for resources.  If the request cannot be immediately fulfilled,
               the current process is put to sleep to wait for resources to be released by other processes.  The
               malloc(), realloc(), and reallocf() functions cannot return NULL if M_WAITOK is specified.

       M_USE_RESERVE
               Indicates that the system can dig into its reserve in order to obtain the requested memory.  This
               option used to be called M_KERNEL but has been renamed to something more  obvious.   This  option
               has  been  deprecated and is slowly being removed from the kernel, and so should not be used with
               any new programming.

       Exactly one of either M_WAITOK or M_NOWAIT must be specified.

       The type argument is used to perform statistics on memory usage, and for basic sanity checks.  It can  be
       used to identify multiple allocations.  The statistics can be examined by ‘vmstat -m’.

       A type is defined using struct malloc_type via the MALLOC_DECLARE() and MALLOC_DEFINE() macros.

             /* sys/something/foo_extern.h */

             MALLOC_DECLARE(M_FOOBUF);

             /* sys/something/foo_main.c */

             MALLOC_DEFINE(M_FOOBUF, "foobuffers", "Buffers to foo data into the ether");

             /* sys/something/foo_subr.c */

             ...
             buf = malloc(sizeof *buf, M_FOOBUF, M_NOWAIT);

       In  order  to  use  MALLOC_DEFINE(),  one  must  include  <sys/param.h>  (instead  of  <sys/types.h>) and
       <sys/kernel.h>.

IMPLEMENTATION NOTES

       The memory allocator allocates memory in chunks that have size a power of two for requests up to the size
       of a page of memory.  For larger requests, one or more pages is allocated.  While it should not be relied
       upon, this information may be useful for optimizing the efficiency of memory use.

       Programmers should be careful not to confuse the malloc flags M_NOWAIT  and  M_WAITOK  with  the  mbuf(9)
       flags M_DONTWAIT and M_WAIT.

CONTEXT

       malloc(),  realloc()  and  reallocf()  may not be called from fast interrupts handlers.  When called from
       threaded interrupts, flags must contain M_NOWAIT.

       malloc(), realloc() and reallocf() may sleep when called with M_WAITOK.  free() never sleeps.

       Any calls to malloc() (even with M_NOWAIT) or free() when holding a vnode(9) interlock, will cause a  LOR
       (Lock Order Reversal) due to the intertwining of VM Objects and Vnodes.

RETURN VALUES

       The  malloc(),  realloc(),  and  reallocf()  functions  return  a kernel virtual address that is suitably
       aligned for storage of any type of object, or NULL if the request could not be satisfied  (implying  that
       M_NOWAIT was set).

DIAGNOSTICS

       A kernel compiled with the INVARIANTS configuration option attempts to detect memory corruption caused by
       such  things  as  writing  outside  the  allocated  area  and imbalanced calls to the malloc() and free()
       functions.  Failing consistency checks will cause a panic or a system console message.

SEE ALSO

       vmstat(8), contigmalloc(9), memguard(9), vnode(9)

Debian                                          January 28, 2012                                       MALLOC(9)