Manual Pages  — MALLOC

The malloc_domainset() variant allocates memory from a specific numa(4) domain using the specified domain selection policy. See domainset(9) for some example policies.

The malloc_aligned() and malloc_domainset_aligned() variants return allocations aligned as specified by align, which must be non-zero, a power of two, and less than or equal to the page size.

Both malloc_exec() and malloc_domainset_exec() can be used to return executable memory. Not all platforms enforce a distinction between executable and non-executable memory.

The mallocarray() function allocates uninitialized memory in kernel address space for an array of nmemb entries whose size is specified by size.

The mallocarray_domainset() variant allocates memory from a specific numa(4) domain using the specified domain selection policy. See domainset(9) for some example policies.

The free() function releases memory at address addr that was previously allocated by malloc() for re-use. The memory is not zeroed. If addr is NULL, then free() does nothing.

Like free(), the zfree() function releases memory at address addr that was previously allocated by malloc() for re-use. However, zfree() will zero the memory before it is released.

The realloc() function changes the size of the previously allocated memory referenced by addr to size bytes. The contents of the memory are unchanged up to the lesser of the new and old sizes. Note that the returned value may differ from addr. If the requested memory cannot be allocated, NULL is returned and the memory referenced by addr is valid and unchanged. If addr is NULL, the realloc() function behaves identically to malloc() for the specified size.

The reallocf() function is identical to realloc() except that it will free the passed pointer when the requested memory cannot be allocated.

The malloc_usable_size() function returns the usable size of the allocation pointed to by addr. The return value may be larger than the size that was requested during allocation.

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_ZERO
	Causes the allocated memory to be set to all zeros.
M_NODUMP
	For allocations greater than page size, causes the allocated memory to be excluded from kernel core dumps.
M_NOWAIT
	Causes malloc(), realloc(), and reallocf() to return NULL if the request cannot be immediately fulfilled due to resource shortage. Note that M_NOWAIT is required when running in an interrupt context.
M_WAITOK
	Indicates that it is OK to wait for resources. If the request cannot be immediately fulfilled, the current process is put to sleep to wait for resources to be released by other processes. The malloc(), mallocarray(), realloc(), and reallocf() functions cannot return NULL if M_WAITOK is specified. If the multiplication of nmemb and size would cause an integer overflow, the mallocarray() function induces a panic.
M_USE_RESERVE
	Indicates that the system can use its reserve of memory to satisfy the request. This option should only be used in combination with M_NOWAIT when an allocation failure cannot be tolerated by the caller without catastrophic effects on the system.

Exactly one of either M_WAITOK or M_NOWAIT must be specified.

The type argument is used to perform statistics on memory usage, and for basic sanity checks. It can be used to identify multiple allocations. The statistics can be examined by 'vmstat -m'.

A type is defined using struct malloc_type via the MALLOC_DECLARE() and MALLOC_DEFINE() macros.

/* sys/something/foo_extern.h */

MALLOC_DECLARE(M_FOOBUF);

/* sys/something/foo_main.c */

MALLOC_DEFINE(M_FOOBUF, "foobuffers", "Buffers to foo data into the ether");

/* sys/something/foo_subr.c */

amp;...
buf = malloc(sizeof(*buf), M_FOOBUF, M_NOWAIT);

In order to use MALLOC_DEFINE(), one must include <sys/param.h> (instead of <sys/types.h>) and <sys/kernel.h>.

Any calls to malloc() (even with M_NOWAIT) or free() when holding a vnode(9) interlock, will cause a LOR (Lock Order Reversal) due to the intertwining of VM Objects and Vnodes.