FreeBSD man pages : MALLOC (9)
MALLOC(9) FreeBSD Kernel Developer's Manual MALLOC(9)
NAME
malloc, MALLOC, free, FREE - kernel memory management routines
SYNOPSIS
#include <sys/types.h>
#include <sys/malloc.h>
void *
malloc(unsigned long size, struct malloc_type *type, int flags);
MALLOC(space, cast, unsigned long size, struct malloc_type *type,
int flags);
void
free(void *addr, struct malloc_type *type);
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);
DESCRIPTION
The malloc() function allocates uninitialized memory in kernel address
space for an object whose size is specified by size.
free() 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 mem-
ory 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 identi-
cally to malloc() for the specified size.
The reallocf() function call is identical to the realloc function call,
except that it will free the passed pointer when the requested memory
cannot be allocated.
The MALLOC() macro variant is functionally equivalent to
(space) = (cast)malloc((u_long)(size), type, flags)
and the FREE() macro variant is equivalent to
free((addr), type)
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_NOWAIT
Causes malloc(), realloc(), or reallocf() to return NULL if the
request cannot be immediately fulfilled due to resource shortage.
Otherwise, the current process may be put to sleep to wait for
resources to be released by other processes. If this flag is
set, malloc() will return NULL rather then block. Note that
M_WAITOK is defined to be 0, meaning that blocking operation is
the default. Also note that M_NOWAIT is required when running in
an interrupt context.
M_ASLEEP
Causes malloc(), realloc(), or reallocf() to call asleep() if the
request cannot be immediately fulfilled due to a resource short-
age. M_ASLEEP is not useful alone and should always be or'd with
M_NOWAIT to allow the function to call asleep() and return NULL
immediately. It is expected that the caller will at some point
call await() and then retry the allocation. Depending on the
routine in question, the caller may decide to propagate the tem-
porary failure up the call chain and actually have some other
higher level routine block on the async wait that the function
queued.
M_WAITOK
Indicates that it is Ok to wait for resources. It is unconve-
niently defined as 0 so care should be taken never to compare
against this value directly or try to AND it as a flag. The
default operation is to block until the memory allocation suc-
ceeds. malloc(), realloc(), and reallocf() can only return NULL
if M_NOWAIT 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.
The type argument is used to perform statistics on memory usage, and for
basic sanity checks. The statistics can be examined by `vmstat -m'.
A type is defined using the malloc_type_t typedef 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 */
...
MALLOC(buf, struct foo_buf *, sizeof *buf, M_FOOBUF, M_NOWAIT);
RETURN VALUES
malloc(), realloc(), and reallocf() 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). If
M_ASLEEP was set and the function returns NULL, it will call asleep() as
a side effect.
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.
SEE ALSO
vmstat(8)
DIAGNOSTICS
A kernel compiled with the DIAGNOSTIC 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 mes-
sage:
o panic: ``malloc: bogus type''
o panic: ``malloc: allocation too large''
o panic: ``malloc: wrong bucket''
o panic: ``malloc: lost data''
o panic: ``free: address 0x%x out of range''
o panic: ``free: type %d out of range''
o panic: ``free: unaligned addr <description of object>''
o panic: ``free: item modified''
o panic: ``free: multiple free[s]''
o ``Data modified on freelist: <description of object>''
FreeBSD 4.8 June 16, 1996 FreeBSD 4.8
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