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MALLOC(9)              FreeBSD Kernel Developer's Manual             MALLOC(9)

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

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

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

     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);


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

     MALLOC_DEFINE(type, shortdesc, longdesc);

     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.

             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

             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.

             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 */


           /* 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>.

     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.

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

     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.

     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

     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

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

FreeBSD 11.0-PRERELEASE        October 23, 2008        FreeBSD 11.0-PRERELEASE


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