table of contents
|MALLOC(9)||Kernel Developer's Manual||MALLOC(9)|
NAME¶malloc, free, realloc, reallocf, MALLOC_DEFINE, MALLOC_DECLARE — kernel memory management routines
#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/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,
NULLis 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:
- Causes the allocated memory to be set to all zeros.
- Causes malloc(),
realloc(), and reallocf() to return
NULLif the request cannot be immediately fulfilled due to resource shortage. Note that
M_NOWAITis required when running in an interrupt context.
- 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
- Indicates that the system can dig into its reserve in order
to obtain the requested memory. This option used to be called
M_KERNELbut 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.
M_NOWAITmust 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);
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_WAITOKwith the mbuf(9) flags
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
NULLif the request could not be satisfied (implying that
DIAGNOSTICS¶A kernel compiled with the
INVARIANTSconfiguration 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)
|October 23, 2008||Debian|