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(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 iden-
tically 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 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.
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 alloca-
tions. 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.
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
vmstat(8), contigmalloc(9), memguard(9), vnode(9)
BSD
January 16, 2014 BSD