Provided by: freebsd-manpages_12.2-1_all 
NAME
malloc, free, realloc, reallocf, MALLOC_DEFINE, MALLOC_DECLARE — kernel memory management routines
SYNOPSIS
#include <sys/types.h>
#include <sys/malloc.h>
void *
malloc(size_t size, struct malloc_type *type, int flags);
void *
mallocarray(size_t nmemb, size_t size, struct malloc_type *type, int flags);
void
free(void *addr, struct malloc_type *type);
void *
realloc(void *addr, size_t size, struct malloc_type *type, int flags);
void *
reallocf(void *addr, size_t size, struct malloc_type *type, int flags);
size_t
malloc_usable_size(const void *addr);
MALLOC_DECLARE(type);
#include <sys/param.h>
#include <sys/malloc.h>
#include <sys/kernel.h>
MALLOC_DEFINE(type, shortdesc, longdesc);
#include <sys/param.h>
#include <sys/domainset.h>
void *
malloc_domainset(size_t size, struct malloc_type *type, struct domainset *ds, int flags);
void
free_domain(void *addr, struct malloc_type *type);
DESCRIPTION
The malloc() function allocates uninitialized memory in kernel address space for an object whose size is
specified by size.
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. Memory allocated with this function
should be returned with free_domain().
The mallocarray() function allocates uninitialized memory in kernel address space for an array of nmemb
entries whose size is specified by size.
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.
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.
M_EXEC Indicates that the system should allocate executable memory. If this flag is not set, the system
will not allocate executable memory. Not all platforms enforce a distinction between executable
and non-executable memory.
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 */
...
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>.
CONTEXT
malloc(), realloc() and reallocf() may not be called from fast interrupts handlers. When called from
threaded interrupts, flags must contain M_NOWAIT.
malloc(), realloc() and reallocf() may sleep when called with M_WAITOK. free() never sleeps. However,
malloc(), realloc(), reallocf() and free() may not be called in a critical section or while holding a
spin lock.
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.
IMPLEMENTATION NOTES
The memory allocator allocates memory in chunks that have size a power of two for requests up to the size
of a page of memory. For larger requests, one or more pages is allocated. While it should not be relied
upon, this information may be useful for optimizing the efficiency of memory use.
RETURN VALUES
The malloc(), realloc(), and reallocf() functions return a kernel virtual address that is suitably
aligned for storage of any type of object, or NULL if the request could not be satisfied (implying that
M_NOWAIT was set).
DIAGNOSTICS
A kernel compiled with the INVARIANTS configuration option attempts to detect memory corruption caused by
such things as writing outside the allocated area and imbalanced calls to the malloc() and free()
functions. Failing consistency checks will cause a panic or a system console message.
SEE ALSO
numa(4), vmstat(8), contigmalloc(9), domainset(9), memguard(9), vnode(9)
Debian August 28, 2020 MALLOC(9)