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DESCRIPTION

The kfree() function releases memory at address addr that was previously allocated by kmalloc() for re-use. The memory is not zeroed. The kernel implementation of kfree() does not allow addr to be NULL.

The krealloc() 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 krealloc() function behaves identically to kmalloc() for the specified size.

kmalloc_raise_limit() is used to increase the internal pool limit to bytes. Under most of the cases the default internal pool limit should be more than enough, so this function is currently rarely used and must be used with care.

Unlike its standard C library counterpart (malloc(3)), the kernel version takes two more arguments. The flags argument further qualifies kmalloc()'s operational characteristics as follows:

M_ZERO

Causes the allocated memory to be set to all zeros.

M_NOWAIT

Causes kmalloc() and krealloc(), 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. Before the internal pool limit is reached, the kmalloc() and krealloc(), functions cannot return NULL if M_WAITOK is specified. If the internal pool limit is reached and M_NULLOK is not specified along with M_WAITOK, the system will panic. If the internal pool limit is reached and M_NULLOK is specified along with M_WAITOK, the kmalloc() and krealloc(), functions return NULL instead of panicing the system.

M_INTWAIT

Indicates kmalloc() to dig into the system's reserved free pages looking for enough room to perform the allocation. This is typically used in interrupts where you cannot afford kmalloc() to fail. Before the internal pool limit is reached, the kmalloc() and krealloc(), functions cannot return NULL if M_INTWAIT is specified. If the internal pool limit is reached and M_NULLOK is not specified along with M_INTWAIT, the system will panic. If the internal pool limit is reached and M_NULLOK is specified along with M_INTWAIT, the kmalloc() and krealloc(), functions return NULL instead of panicing the system.

M_USE_RESERVE

Indicates that the system can dig into its reserve in order to obtain the requested memory.

M_POWEROF2

Rounds up the size to the nearest power of 2.

M_NULLOK

This flag is usually specified along with M_WAITOK or M_INTWAIT, so when the interal pool limit is reached, kmalloc() and krealloc(), functions will not panic the system, instead, NULL will be returned. This flag is usually used on the kernel code path that is triggered by user space programs' requests.

Exactly one of either M_WAITOK, M_INTWAIT 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 the malloc_type_t typedef 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 = kmalloc(sizeof *buf, M_FOOBUF, M_NOWAIT);