NAME
malloc,
mallocarray, free —
kernel memory allocator
SYNOPSIS
#include
<sys/types.h>
#include <sys/malloc.h>
void *
malloc(size_t
size, int type,
int flags);
void *
mallocarray(size_t
nmemb, size_t size,
int type,
int flags);
void
free(void
*addr, int type,
size_t size);
DESCRIPTION
The
malloc()
function allocates uninitialized memory in kernel address space for an
object whose size is specified by size.
The
mallocarray()
function is the same as malloc(), but allocates
space for an array of nmemb objects and checks for
arithmetic overflow.
The
free()
function releases memory at address addr that was
previously allocated by malloc() or
mallocarray() for re-use. The same object size
originally provided to malloc() should be specified
by size, because free() will
operate faster knowing this. If tracking the size is difficult, specify
size as 0. If addr is a null
pointer, no action occurs.
The flags argument affects the
operational characteristics of
malloc()
and mallocarray() as follows:
M_WAITOK- If memory is currently unavailable,
malloc() may call sleep to wait for resources to be released by other processes. M_NOWAIT- Causes
malloc() to returnNULLif the request cannot be immediately fulfilled due to resource shortage. M_CANFAIL- In the
M_WAITOKcase, if not enough memory is available, returnNULLinstead of calling panic(9). Ifmallocarray() detects an overflow ormalloc() detects an excessive allocation, returnNULLinstead of calling panic(9). M_ZERO- Causes allocated memory to be zeroed.
One of M_NOWAIT or
M_WAITOK must be specified via the
flags argument.
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. These statistics can
be examined using vmstat(8) or systat(1) if either of the kernel
options(4) KMEMSTATS or
DEBUG are enabled.
The following types are currently defined:
M_DEVBUF- Device driver memory.
M_PCB- Protocol control blocks.
M_RTABLE- Routing tables.
M_PF- Packet filter structures.
M_IFADDR- Interface addresses.
M_IFGROUP- Interface groups.
M_SYSCTL- Sysctl persistent buffers.
M_COUNTERS- Per-CPU counters via counters_alloc(9).
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_LOG- Messages in kernel log stash.
M_VNODE- Dynamically allocated vnodes.
M_DQUOT- UFS quota entries.
M_UFSMNT- UFS mount structures.
M_SHM- SVID compatible shared memory segments.
M_VMMAP- VM map structures.
M_SEM- SVID compatible semaphores.
M_DIRHASH- UFS directory hash structures.
M_ACPI- ACPI structures.
M_VMPMAP- VM pmap data.
M_FILEDESC- Open file descriptor tables.
M_SIGIO- Sigio structures.
M_PROC- Proc structures.
M_SUBPROC- Proc sub-structures.
M_MFSNODE- MFS vnode private part.
M_NETADDR- Export host address structures.
M_NFSSVC- NFS server 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- Miscellaneous FS mount structures.
M_FUSEFS- FUSE FS mount structures.
M_PINSYSCALL- pinsyscalls(2) related data.
M_PFKEY- Pfkey data.
M_TDB- Transforms database.
M_XDATA- IPsec data.
M_PAGEDEP- File page dependencies.
M_INODEDEP- Inode dependencies.
M_NEWBLK- New block allocation.
M_INDIRDEP- Indirect block dependencies.
M_VMSWAP- VM swap structures.
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_WITNESS- witness(4) memory.
M_MEMDESC- Memory range.
M_CRYPTO_DATA- crypto(9) data buffers.
M_CREDENTIALS- ipsec(4) related credentials.
M_IP6OPT- IPv6 options.
M_IP6NDP- IPv6 Neighbor Discovery structures.
M_TEMP- Miscellaneous temporary data buffers.
M_NTFSMNT- NTFS mount structures.
M_NTFSNTNODE- NTFS ntnode information.
M_NTFSFNODE- NTFS fnode information.
M_NTFSDIR- NTFS directory buffers.
M_NTFSNTHASH- NTFS ntnode hash tables.
M_NTFSNTVATTR- NTFS file attribute information.
M_NTFSRDATA- NTFS resident data.
M_NTFSDECOMP- NTFS decompression temporary storage.
M_NTFSRUN- NTFS vrun storage.
M_KEVENT- kqueue(2) data structures.
M_SYNCACHE- SYN cache hash array.
M_UDFMOUNT- UDF mount structures.
M_UDFFENTRY- UDF file entries.
M_UDFFID- UDF file IDs.
M_AGP- AGP memory.
M_DRM- Direct Rendering Manager.
CONTEXT
malloc() and
mallocarray() can be called during autoconf, from
process context, or from interrupt context if
M_NOWAIT is passed via flags.
They can't be called from interrupt context if
M_WAITOK is passed via
flags.
free() can be called during autoconf, from
process context, or from interrupt context.
RETURN VALUES
malloc() and
mallocarray() return a kernel virtual address that
is suitably aligned for storage of any type of object.
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 unbalanced calls to
malloc() or mallocarray(),
and free(). Failing consistency checks will cause a
panic or a system console message:
- 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: “mallocarray: overflow”
- panic: “free: unaligned addr”
- panic: “free: duplicated free”
- panic: “free: multiple frees”
- panic: “free: non-malloced addr”
- panic: “free: size too large”
- panic: “free: size too small”
- panic: “kmeminit: minbucket too small/struct freelist too big”
- “multiply freed item ⟨addr⟩”
- “Data modified on freelist: ⟨data object description⟩”