MALLOC(9)

NAME
malloc, free, realloc, reallocf, MALLOC_DEFINE, MALLOC_DECLARE - kernel memory management routines

SYNOPSIS
#include <sys/types.h>
#include <sys/malloc.h>
void *
malloc(unsigned long size, struct malloc_type *type, int flags);
void
free(void *addr, struct malloc_type *type);
void *
realloc(void *addr, unsigned long size, struct malloc_type *type, int flags);
void *
reallocf(void *addr, unsigned long size, struct malloc_type *type, int flags);
MALLOC_DECLARE(type);
#include <sys/param.h>
#include <sys/malloc.h>
#include <sys/kernel.h>
MALLOC_DEFINE(type, shortdesc, longdesc);

DESCRIPTION
The malloc() function allocates uninitialized memory in kernel address space for an object 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.

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(), realloc(), and reallocf() functions cannot return NULL if M_WAITOK is specified.

M_USE_RESERVE
    Indicates that the system can dig into its reserve in order to obtain the requested memory. This option used to be called M_KERNEL but has been renamed to something more obvious. This option has been deprecated and is slowly being removed from the kernel, and so should not be used with any new programming.

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>.

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.

Programmers should be careful not to confuse the malloc flags M_NOWAIT and M_WAITOK with the mbuf(9) flags M_DONTWAIT and M_WAIT.

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.

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.

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
vmstat(8), contigmalloc(9), memguard(9), vnode(9)