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MALLOC(9) Kernel Developer's Manual MALLOC(9)

malloc, mallocarray, free — kernel memory allocator

#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);

The () function allocates uninitialized memory in kernel address space for an object whose size is specified by size.

The () function is the same as malloc(), but allocates space for an array of nmemb objects and checks for arithmetic overflow.

The () 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 () and mallocarray() as follows:

If memory is currently unavailable, malloc() may call sleep to wait for resources to be released by other processes.
Causes malloc() to return NULL if the request cannot be immediately fulfilled due to resource shortage.
In the M_WAITOK case, if not enough memory is available, return NULL instead of calling panic(9). If mallocarray() detects an overflow or malloc() detects an excessive allocation, return NULL instead of calling panic(9).
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:

Should be on free list.
Device driver memory.
Protocol control blocks.
Routing tables.
Interface addresses.
Socket options.
Sysctl persistent buffers.
Per-CPU Counters for use via counters_alloc(9).
Ioctl data buffers.
Large IOVs.
VFS mount structs.
NFS request headers.
NFS mount structures.
Dynamically allocated vnodes.
Dynamically allocated cache entries.
UFS quota entries.
UFS mount structures.
SVID compatible shared memory segments.
VM map structures.
SVID compatible semaphores.
UFS directory hash structures.
ACPI structures.
VM pmap data.
Open file structures.
Open file descriptor tables.
Sigio structures.
Proc structures.
Proc sub-structures.
Cluster for VFS.
MFS vnode private part.
Export host address structures.
NFS server structures.
NFS server daemon structures.
Internet multicast options.
Internet multicast addresses.
Link-level multicast addresses.
Multicast routing tables.
ISOFS mount structures.
ISOFS vnode private part.
MSDOS FS mount structures.
MSDOS FS FAT tables.
MSDOS FS vnode private part.
Allocated tty structures.
Argument lists & other mem used by exec.
Miscellaneous FS mount structures.
FUSE FS mount structures.
Pfkey data.
Transforms database.
IPsec data.
File page dependencies.
Inode dependencies.
New block allocation.
Indirect block dependencies.
VM swap structures.
UVM amap and related.
UVM aobj and related.
USB general.
USB device driver.
USB host controller.
witness(4) memory.
Memory range.
crypto(9) data buffers.
ipsec(4) related credentials.
Per process emulation data.
IPv6 options.
IPv6 neighbour discovery structures.
Miscellaneous temporary data buffers.
NTFS mount structures.
NTFS ntnode information.
NTFS fnode information.
NTFS directory buffers.
NTFS ntnode hash tables.
NTFS file attribute information.
NTFS resident data.
NTFS decompression temporary storage.
NTFS vrun storage.
kqueue(2) data structures.
SYN cache hash array.
UDF mount structures.
UDF file entries.
UDF file ID.
AGP memory.
Direct Rendering Manager.

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.

malloc() and mallocarray() return 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 allocated area and unbalanced calls to malloc() or mallocarray(), and free(). Failing consistency checks will cause a panic or a system console message:

systat(1), vmstat(8)

February 7, 2019 OpenBSD-6.7