The FreeBSD kernel provides four functions for non-pageable memory allocation and release: malloc, free, realloc, and reallocf. These functions can handle requests of arbitrary size or alignment, and they are the preferred way to allocate kernel memory.

#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);

The malloc function allocates size bytes of memory in kernel space. If successful, a kernel virtual address is returned; otherwise, NULL is returned.

The free function releases the memory at addr—that was previously allocated by malloc—for reuse. Note that free doesn’t clear this memory, which means that you should explicitly zero any memory whose contents you need to keep private. If addr is NULL, then free does nothing.

The realloc function changes the size of the memory at addr to size bytes. If successful, a kernel virtual address is returned; otherwise, NULL is returned, and the memory is left alone. Note that the returned address may differ from addr, because when the size changes, the memory may be relocated to acquire or provide additional room. Interestingly, this implies that you should not have any pointers into the memory at addr when calling realloc. If addr is NULL, then realloc behaves identically to malloc.

The reallocf function is identical to realloc except that on failure it releases the memory at addr.

The malloc, realloc, and reallocf functions provide a flags argument to further qualify their operational characteristics. Valid values for this argument are shown in Table 2-1.

The flags argument must include either M_NOWAIT or M_WAITOK.