man/man9/malloc.9

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.Dd November 7, 2004
.Dt MALLOC 9
.Os
.Sh NAME
.Nm malloc ,
.Nm MALLOC ,
.Nm realloc ,
.Nm free ,
.Nm FREE ,
.Nm malloc_roundup ,
.Nm malloc_type_attach ,
.Nm malloc_type_detach ,
.Nm malloc_type_setlimit ,
.Nm MALLOC_DEFINE_LIMIT ,
.Nm MALLOC_DEFINE ,
.Nm MALLOC_DECLARE
.Nd general-purpose kernel memory allocator
.Sh SYNOPSIS
.In sys/malloc.h
.Ft void *
.Fn malloc "unsigned long size" "struct malloc_type *type" "int flags"
.Fn MALLOC "space" "cast" "unsigned long size" "struct malloc_type *type" \
    "int flags"
.Ft void *
.Fn realloc "void *addr" "unsigned long newsize" "struct malloc_type *type" \
    "int flags"
.Ft void
.Fn free "void *addr" "struct malloc_type *type"
.Fn FREE "void *addr" "struct malloc_type *type"
.Ft unsigned long
.Fn malloc_roundup "unsigned long size"
.Ft void
.Fn malloc_type_attach "struct malloc_type *type"
.Ft void
.Fn malloc_type_detach "struct malloc_type *type"
.Ft void
.Fn malloc_type_setlimit "struct malloc_type *type" "unsigned long limit"
.In sys/mallocvar.h
.Fn MALLOC_DEFINE_LIMIT "type" "shortdesc" "longdesc" "limit"
.Fn MALLOC_DEFINE "type" "shortdesc" "longdesc"
.Fn MALLOC_DECLARE "type"
.Sh DESCRIPTION
The
.Fn malloc
function allocates uninitialized memory in kernel address space for an
object whose size is specified by
.Fa size .
.Fn malloc_roundup
returns the actual size of the allocation unit for the given value.
.Fn free
releases memory at address
.Fa addr
that was previously allocated by
.Fn malloc
for re-use.
Unlike
.Xr free 3 ,
.Fn free
does not accept an
.Fa addr
argument that is
.Dv NULL .
.Pp
The
.Fn realloc
function changes the size of the previously allocated memory referenced
by
.Fa addr
to
.Fa size
and returns a pointer to the
.Pq possibly moved
object.
The memory contents are unchanged up to the lesser of the new
and old sizes.
If the new size is larger, the newly allocated memory is
uninitialized.
If the requested memory cannot be allocated,
.Dv NULL
is returned and the memory referenced by
.Fa addr
is unchanged.
If
.Fa addr
is
.Dv NULL ,
then
.Fn realloc
behaves exactly as
.Fn malloc .
If the new size is 0, then
.Fn realloc
behaves exactly as
.Fn free .
.Pp
The
.Fn MALLOC
macro variant is functionally equivalent to
.Bd -literal -offset indent
(space) = (cast)malloc((u_long)(size), type, flags)
.Ed
.Pp
and the
.Fn FREE
macro variant is equivalent to
.Bd -literal -offset indent
free((caddr_t)(addr), type)
.Ed
.Pp
The
.Fn MALLOC
macro is intended to be used with a compile-time constant
.Fa size
so that the compiler can do constant folding.
In the comparison to
.Fn malloc
and
.Fn free
functions, the
.Fn MALLOC
and
.Fn FREE
macros may be faster, at the cost of increased code size.
There is no difference between the memory allocated with MALLOC and malloc.
i.e., no matter which MALLOC or malloc is used to allocate the memory,
either FREE or free can be used to free it.
.Pp
Unlike its standard C library counterpart
.Pq Xr malloc 3 ,
the kernel version takes two more arguments.
.Pp
The
.Fa flags
argument further qualifies
.Fn malloc
operational characteristics as follows:
.Bl -tag -offset indent -width M_CANFAIL
.It Dv M_NOWAIT
Causes
.Fn malloc
to return
.Dv NULL
if the request cannot be immediately fulfilled due to resource shortage.
If this flag is not set
(see
.Dv M_WAITOK ) ,
.Fn malloc
will never return
.Dv NULL .
.It Dv M_WAITOK
By default,
.Fn malloc
may call
.Xr tsleep 9
to wait for resources to be released by other processes, and this
flag represents this behaviour.
Note that
.Dv M_WAITOK
is conveniently defined to be 0, and hence may be or'ed into the
.Fa flags
argument to indicate that it's ok to wait for resources.
.It Dv M_ZERO
Causes the allocated memory to be set to all zeros.
.It Dv M_CANFAIL
Changes behaviour for
.Dv M_WAITOK
case - if the requested memory size is bigger than
.Fn malloc
can ever allocate, return failure, rather than calling
.Xr panic 9 .
This is different to M_NOWAIT, since
the call can still wait for resources.
.Pp
Rather than depending on
.Dv M_CANFAIL ,
kernel code should do proper bound checking itself.
This flag should only be used in cases where this is not feasible.
Since it can hide real kernel bugs, its usage is
.Em strongly discouraged .
.El
.Pp
The
.Fa type
argument describes the subsystem and/or use within a subsystem for which
the allocated memory was needed, and is commonly used to maintain statistics
about kernel memory usage and, optionally, enforce limits on this usage for
certain memory types.
.Pp
In addition to some built-in generic types defined by the kernel
memory allocator, subsystems may define their own types.
.Pp
The
.Fn MALLOC_DEFINE_LIMIT
macro defines a malloc type named
.Fa type
with the short description
.Fa shortdesc ,
which must be a constant string; this description will be used for
kernel memory statistics reporting.
The
.Fa longdesc
argument, also a constant string, is intended as way to place a
comment in the actual type definition, and is not currently stored
in the type structure.
The
.Fa limit
argument specifies the maximum amount of memory, in bytes, that this
malloc type can consume.
.Pp
The
.Fn MALLOC_DEFINE
macro is equivalent to the
.Fn MALLOC_DEFINE_LIMIT
macro with a
.Fa limit
argument of 0.
If kernel memory statistics are being gathered, the system will
choose a reasonable default limit for the malloc type.
.Pp
The
.Fn MALLOC_DECLARE
macro is intended for use in header files which are included by
code which needs to use the malloc type, providing the necessary
extern declaration.
.Pp
Code which includes
\*[Lt]sys/malloc.h\*[Gt]
does not need to include
\*[Lt]sys/mallocvar.h\*[Gt]
to get these macro definitions.
The
\*[Lt]sys/mallocvar.h\*[Gt]
header file is intended for other header files which need to use the
.Fn MALLOC_DECLARE
macro.
.Pp
The
.Fn malloc_type_attach
function attaches the malloc type
.Fa type
to the kernel memory allocator.
This is intended for use by LKMs; malloc types included in modules
statically-linked into the kernel are automatically registered with
the kernel memory allocator.
.Pp
The
.Fn malloc_type_detach
function detaches the malloc type
.Fa type
previously attached with
.Fn malloc_type_attach .
.Pp
The
.Fn malloc_type_setlimit
function sets the memory limit of the malloc type
.Fa type
to
.Fa limit
bytes.
The type must already be registered with the kernel memory allocator.
.Pp
The following generic malloc types are currently defined:
.Pp
.Bl -tag -offset indent -width XXXXXXXXXXXXXX -compact
.It Dv M_DEVBUF
Device driver memory.
.It Dv M_DMAMAP
.Xr bus_dma 9
structures.
.It Dv M_FREE
Should be on free list.
.It Dv M_PCB
Protocol control block.
.It Dv M_SOFTINTR
Softinterrupt structures.
.It Dv M_TEMP
Misc temporary data buffers.
.El
.Pp
Other malloc types are defined by the corresponding subsystem; see the
documentation for that subsystem for information its available malloc
types.
.Pp
Statistics based on the
.Fa type
argument are maintained only if the kernel option
.Dv KMEMSTATS
is used when compiling the kernel
.Po
the default in current
.Nx
kernels
.Pc
and can be examined by using
.Sq vmstat -m .
.Sh RETURN VALUES
.Fn malloc
returns a kernel virtual address that is suitably aligned for storage of
any type of object.
.Sh DIAGNOSTICS
A kernel compiled with the
.Dv DIAGNOSTIC
configuration option attempts to detect memory corruption caused by
such things as writing outside the allocated area and imbalanced calls to the
.Fn malloc
and
.Fn free
functions.
Failing consistency checks will cause a panic or a system console message:
.Bl -bullet -offset indent -compact
.Pp
.It
panic:
.Dq malloc - bogus type
.It
panic:
.Dq malloc: out of space in kmem_map
.It
panic:
.Dq malloc: allocation too large
.It
panic:
.Dq malloc: wrong bucket
.It
panic:
.Dq malloc: lost data
.It
panic:
.Dq free: unaligned addr
.It
panic:
.Dq free: duplicated free
.It
panic:
.Dq free: multiple frees
.It
panic:
.Dq init: minbucket too small/struct freelist too big
.It
.Dq multiply freed item Aq addr
.It
.Dq Data modified on freelist: Aq data object description
.El
.Sh SEE ALSO
.Xr vmstat 1