path: root/share/man/man9/malloc.9

.\"	$OpenBSD: malloc.9,v 1.25 2003/04/17 05:08:39 jmc Exp $
.\"	$NetBSD: malloc.9,v 1.2 1996/10/30 05:29:54 lukem Exp $
.\"
.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
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.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Paul Kranenburg.
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.Dd June 16, 1996
.Dt MALLOC 9
.Os
.Sh NAME
.Nm malloc
.Nd kernel memory allocator
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/malloc.h>
.Ft void *
.Fn malloc "unsigned long size" "int type" "int flags"
.Fn MALLOC "space" "cast" "unsigned long size" "int type" "int flags"
.Ft void
.Fn free "void *addr" "int type"
.Fn FREE "void *addr" "int 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 free
releases memory at address
.Fa addr
that was previously allocated by
.Fn malloc
for re-use.
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
These macros should only be used when the
.Fa size
argument is a constant.
.Pp
Unlike its standard C library counterpart
.Pq Xr malloc 3 ,
the kernel version takes two more arguments.
The
.Fa flags
argument further qualifies
.Fn malloc No Ns 's
operational characteristics as follows:
.Bl -tag -width xxx -offset indent
.It Dv M_NOWAIT
Causes
.Fn malloc
to return
.Dv NULL
if the request cannot be immediately fulfilled due to resource shortage.
Otherwise,
.Fn malloc
may call sleep to wait for resources to be released by other processes.
If this flag is not set,
.Fn malloc
will never return
.Dv NULL .
Note that
.Dv M_WAITOK
is conveniently defined to be 0, and hence maybe or'ed into the
.Fa flags
argument to indicate that it's OK to wait for resources.
.El
.Pp
Currently, only one flag is defined.
.Pp
The
.Fa type
argument broadly identifies the kernel subsystem for which the allocated
memory was needed, and is commonly used to maintain statistics about
kernel memory usage.
The following types are currently defined:
.Pp
.Bl -tag -offset indent -width XXXXXXXXXXXXXX -compact
.It Dv M_FREE
Should be on free list.
.It Dv M_MBUF
Mbuf memory.
.It Dv M_DEVBUF
Device driver memory.
.It Dv M_DEBUG
.Xr malloc 9
debug structures.
.It Dv M_PCB
Protocol control block.
.It Dv M_RTABLE
Routing tables.
.It Dv M_FTABLE
Fragment reassembly header.
.It Dv M_IFADDR
Interface address.
.It Dv M_SOOPTS
Socket options.
.It Dv M_SYSCTL
Sysctl persistent buffers.
.It Dv M_NAMEI
Namei path name buffer.
.It Dv M_IOCTLOPS
Ioctl data buffer.
.It Dv M_IOV
Large IOVs.
.It Dv M_MOUNT
Vfs mount struct.
.It Dv M_NFSREQ
NFS request header.
.It Dv M_NFSMNT
NFS mount structure.
.It Dv M_NFSNODE
NFS vnode private part.
.It Dv M_VNODE
Dynamically allocated vnodes.
.It Dv M_CACHE
Dynamically allocated cache entries.
.It Dv M_DQUOT
UFS quota entries.
.It Dv M_UFSMNT
UFS mount structure.
.It Dv M_SHM
SVID compatible shared memory segments.
.It Dv M_VMMAP
VM map structures.
.It Dv M_VMPMAP
VM pmap data.
.It Dv M_FILE
Open file structure.
.It Dv M_FILEDESC
Open file descriptor table.
.It Dv M_LOCKF
Byte-range locking structures.
.It Dv M_PROC
Proc structures.
.It Dv M_SUBPROC
Proc sub-structures.
.It Dv M_VCLUSTER
Cluster for VFS.
.It Dv M_MFSNODE
MFS vnode private part.
.It Dv M_NETADDR
Export host address structure.
.It Dv M_NFSSVC
NFS server structure.
.It Dv M_NFSUID
NFS uid mapping structure.
.It Dv M_NFSD
NFS server daemon structure.
.It Dv M_IPMOPTS
Internet multicast options.
.It Dv M_IPMADDR
Internet multicast address.
.It Dv M_IFMADDR
Link-level multicast address.
.It Dv M_MRTABLE
Multicast routing tables.
.It Dv M_ISOFSMNT
ISOFS mount structure.
.It Dv M_ISOFSNODE
ISOFS vnode private part.
.It Dv M_MSDOSFSMNT
MSDOS FS mount structure.
.It Dv M_MSDOSFSFAT
MSDOS FS fat table.
.It Dv M_MSDOSFSNODE
MSDOS FS vnode private part.
.It Dv M_TTYS
Allocated tty structures.
.It Dv M_EXEC
Argument lists & other mem used by exec.
.It Dv M_MISCFSMNT
Miscfs mount structures.
.It Dv M_ADOSFSMNT
Adosfs mount structures.
.It Dv M_ANODE
Adosfs anode structures and tables.
.It Dv M_ADOSFSBITMAP
Adosfs bitmap.
.It Dv M_EXT2FSNODE
EXT2FS vnode private part.
.It Dv M_PFKEY
Pfkey data.
.It Dv M_TDB
Transforms database.
.It Dv M_XDATA
IPsec data.
.It Dv M_VFS
VFS file systems.
.It Dv M_PAGEDEP
File page dependencies.
.It Dv M_INODEDEP
Inode dependencies.
.It Dv M_NEWBLK
New block allocation.
.It Dv M_VMSWAP
VM swap structures.
.It Dv M_RAIDFRAME
RAIDframe data.
.It Dv M_UVMAMAP
UVM amap and related.
.It Dv M_UVMAOBJ
UVM aobj and related.
.It Dv M_USB
USB general.
.It Dv M_USBDEV
USB device driver.
.It Dv M_USBHC
USB host controller.
.It Dv M_MEMDESC
Memory range.
.It Dv M_UFS_EXTATTR
UFS Extended Attributes.
.It Dv M_CREDENTIALS
.Xr ipsec 4
related credentials.
.It Dv M_PACKET_TAGS
Packet-attached information tags.
.It Dv M1394CTL
IEEE 1394 control structures.
.It Dv M1394DATA
IEEE 1394 data buffers.
.It Dv M_IP6OPT
IPv6 options.
.It Dv M_IP6NDP
IPv6 neighbour discovery structures.
.It Dv M_IP6RR
IPv6 router renumbering prefix.
.It Dv M_RR_ADDR
IPv6 router renumbering interface identifiers.
.It Dv M_TEMP
Miscellaneous temporary data buffers.
.El
.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\ \&
.Ox
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 SEE ALSO
.Xr vmstat 8
.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 unbalanced 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 kmeminit: 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 DEBUGGING
A kernel compiled with the
.Cm MALLOC_DEBUG
option allows for more extensive debugging of memory allocations.
The
.Va debug_malloc_type ,
.Va debug_malloc_size ,
.Va debug_malloc_size_lo
and
.Va debug_malloc_size_hi
variables choose which allocation to debug.
.Va debug_malloc_type
should be set to the memory type and
.Va debug_malloc_size
should be set to the memory size to debug.
0 can be used as a wildcard.
.Va debug_malloc_size_lo
and
.Va debug_malloc_size_hi
can be used to specify a range of sizes if the exact size to debug is not
known.
When those are used,
.Va debug_malloc_size
needs to be set to the wildcard.
.Dv M_DEBUG
can also be specified as an allocation type to force allocation with
debugging.
.Pp
Every call to
.Fn malloc
with a memory type and size that matches the debugged type and size will
allocate two virtual pages.
The pointer returned will be aligned so that
the requested area will end at the page boundary and the second virtual page
will be left unmapped.
This way we can catch reads and writes outside the allocated area.
.Pp
Every call to
.Fn free
with memory that was returned by the debugging malloc will cause the memory
area to become unmapped so that we can catch dangling reads and writes to
freed memory.
.Pp
There are no special diagnostics if any errors are caught by the debugging
malloc.
The errors will look like normal access to unmapped memory.
On a memory access error, the
.Ic show malloc
command in
.Xr ddb 4
can be invoked to see what memory areas are allocated and freed.
If the faulting address is within two pages from an address on the allocated
list, there was an access outside the allocated area.
If the faulting address is within two pages from an address on the free list,
there was an access to freed memory.
.Pp
Care needs to be taken when using the
.Cm MALLOC_DEBUG
option:  the memory consumption can run away pretty quickly and there is
a severe performance degradation when allocating and freeing debugged memory
types.