MALLOC(9) BSD Kernel Manual MALLOC(9)
malloc - kernel memory allocator
#include <sys/types.h>
#include <sys/malloc.h>
void *
malloc(unsigned long size, int type, int flags);
MALLOC(space, cast, unsigned long size, int type, int flags);
void
free(void *addr, int type);
FREE(void *addr, int type);
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 MALLOC() macro variant is functionally equivalent to
(space) = (cast)malloc((u_long)(size), type, flags)
and the FREE() macro variant is equivalent to
free((caddr_t)(addr), type)
These macros should only be used when the size argument is a constant.
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() to return NULL if the request cannot be im-
mediately fulfilled due to resource shortage. Otherwise, mal-
loc() may call sleep to wait for resources to be released by
other processes. If this flag is not set, malloc() will never
return NULL. Note that M_WAITOK is conveniently defined to be
0, and hence maybe or'ed into the flags argument to indicate
that it's OK to wait for resources.
Currently, only one flag is defined.
The 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:
M_FREE Should be on free list.
M_MBUF Mbuf memory.
M_DEVBUF Device driver memory.
M_DEBUG malloc debug structures.
M_PCB Protocol control blocks.
M_RTABLE Routing tables.
M_FTABLE Fragment reassembly headers.
M_IFADDR Interface addresses.
M_SOOPTS Socket options.
M_SYSCTL Sysctl persistent buffers.
M_NAMEI Namei path name buffers.
M_IOCTLOPS Ioctl data buffers.
M_IOV Large IOVs.
M_MOUNT VFS mount structs.
M_NFSREQ NFS request headers.
M_NFSMNT NFS mount structures.
M_NFSNODE NFS vnode private part.
M_VNODE Dynamically allocated vnodes.
M_CACHE Dynamically allocated cache entries.
M_DQUOT UFS quota entries.
M_UFSMNT UFS mount structures.
M_SHM SVID compatible shared memory segments.
M_VMMAP VM map structures.
M_VMPMAP VM pmap data.
M_FILE Open file structures.
M_FILEDESC Open file descriptor tables.
M_PROC Proc structures.
M_SUBPROC Proc sub-structures.
M_VCLUSTER Cluster for VFS.
M_MFSNODE MFS vnode private part.
M_NETADDR Export host address structures.
M_NFSSVC NFS server structures.
M_NFSUID NFS uid mapping structures.
M_NFSD NFS server daemon structures.
M_IPMOPTS Internet multicast options.
M_IPMADDR Internet multicast addresses.
M_IFMADDR Link-level multicast addresses.
M_MRTABLE Multicast routing tables.
M_ISOFSMNT ISOFS mount structures.
M_ISOFSNODE ISOFS vnode private part.
M_MSDOSFSMNT MSDOS FS mount structures.
M_MSDOSFSFAT MSDOS FS FAT tables.
M_MSDOSFSNODE MSDOS FS vnode private part.
M_TTYS Allocated tty structures.
M_EXEC Argument lists & other mem used by exec.
M_MISCFSMNT Misc. FS mount structures.
M_ADOSFSMNT ADOSFS mount structures.
M_ANODE ADOSFS anode structures and tables.
M_ADOSFSBITMAP ADOSFS bitmap.
M_EXT2FSNODE EXT2FS vnode private part.
M_PFKEY Pfkey data.
M_TDB Transforms database.
M_XDATA IPsec data.
M_VFS VFS filesystems.
M_PAGEDEP File page dependencies.
M_INODEDEP Inode dependencies.
M_NEWBLK New block allocation.
M_VMSWAP VM swap structures.
M_RAIDFRAME RAIDframe data.
M_UVMAMAP UVM amap and related.
M_UVMAOBJ UVM aobj and related.
M_USB USB general.
M_USBDEV USB device driver.
M_USBHC USB host controller.
M_MEMDESC Memory range.
M_UFS_EXTATTR UFS Extended Attributes.
M_CREDENTIALS ipsec(4) related credentials.
M_PACKET_TAGS Packet-attached information tags.
M1394CTL IEEE 1394 control structures.
M1394DATA IEEE 1394 data buffers.
M_EMULDATA Per process emulation data.
M_IP6OPT IPv6 options.
M_IP6NDP IPv6 neighbour discovery structures.
M_IP6RR IPv6 router renumbering prefix.
M_RR_ADDR IPv6 router renumbering interface identifiers.
M_TEMP Miscellaneous temporary data buffers.
M_NTFSMNT NTFS mount structures.
M_NTFSNTNODE NTFS ntnode information.
M_NTFSNODE NTFS fnode information.
M_NTFSDIR NTFS directory buffers.
M_NTFSHASH NTFS ntnode hash tables.
M_NTFSVATTR NTFS file attribute information.
M_NTFSRDATA NTFS resident data.
M_NTFSDECOMP NTFS decompression temporary storage.
M_NTFSRUN NTFS vrun storage.
Statistics based on the type argument are maintained only if the kernel
option KMEMSTATS is used when compiling the kernel (the default in
current OpenBSD kernels) and can be examined by using 'vmstat -m'.
malloc() returns a kernel virtual address that is suitably aligned for
storage of any type of object.
A kernel compiled with the DIAGNOSTIC configuration option attempts to
detect memory corruption caused by such things as writing outside the al-
located area and unbalanced calls to the malloc() and free() functions.
Failing consistency checks will cause a panic or a system console mes-
sage:
• panic: "malloc - bogus type"
• panic: "malloc: out of space in kmem_map"
• panic: "malloc: allocation too large"
• panic: "malloc: wrong bucket"
• panic: "malloc: lost data"
• panic: "free: unaligned addr"
• panic: "free: duplicated free"
• panic: "free: multiple frees"
• panic: "kmeminit: minbucket too small/struct freelist too big"
• "multiply freed item <addr>"
• "Data modified on freelist: <data object description>"
A kernel compiled with the MALLOC_DEBUG option allows for more extensive
debugging of memory allocations. The debug_malloc_type,
debug_malloc_size, debug_malloc_size_lo and debug_malloc_size_hi vari-
ables choose which allocation to debug. debug_malloc_type should be set
to the memory type and debug_malloc_size should be set to the memory size
to debug. 0 can be used as a wildcard. debug_malloc_size_lo and
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, debug_malloc_size needs
to be set to the wildcard. M_DEBUG can also be specified as an allocation
type to force allocation with debugging.
Every call to malloc() with a memory type and size that matches the de-
bugged type and size will allocate two virtual pages. The pointer re-
turned 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.
Every call to free() with memory that was returned by the debugging mal-
loc will cause the memory area to become unmapped so that we can catch
dangling reads and writes to freed memory.
There are no special diagnostics if any errors are caught by the debug-
ging malloc. The errors will look like normal access to unmapped memory.
On a memory access error, the show malloc command in ddb(4) can be in-
voked 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.
Care needs to be taken when using the 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.
vmstat(8)
MirBSD #10-current June 16, 1996 3