realloc

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

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

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

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

     void *
     mallocarray(size_t nmemb, size_t size, struct malloc_type *type,
         int flags);

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

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

     void *
     reallocf(void *addr, size_t 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);

     #include <sys/param.h>
     #include <sys/domainset.h>

     void *
     malloc_domainset(size_t size, struct malloc_type *type,
         struct domainset *ds, int flags);

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

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

     The malloc_domainset() variant allocates memory from a specific numa(4)
     domain using the specified domain selection policy.  See domainset(9) for
     some example policies.  Memory allocated with this function should be
     returned with free_domain().

     The mallocarray() function allocates uninitialized memory in kernel address
     space for an array of nmemb entries 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(), mallocarray(), realloc(), and reallocf() functions cannot
             return NULL if M_WAITOK is specified.  If the multiplication of
             nmemb and size would cause an integer overflow, the mallocarray()
             function induces a panic.

     M_USE_RESERVE
             Indicates that the system can use its reserve of memory to satisfy
             the request.  This option should only be used in combination with
             M_NOWAIT when an allocation failure cannot be tolerated by the
             caller without catastrophic effects on the system.

     M_EXEC  Indicates that the system should allocate executable memory.  If
             this flag is not set, the system will not allocate executable
             memory.  Not all platforms enforce a distinction between executable
             and non-executable memory.

     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>.

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.  However, malloc(), realloc(), reallocf() and free()
     may not be called in a critical section or while holding a spin lock.

     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.

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.

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
     numa(4), vmstat(8), contigmalloc(9), domainset(9), memguard(9), vnode(9)

BSD                             October 30, 2018                             BSD