MALLOC(9) MALLOC(9)
NAME
malloc, mallocz, smalloc, realloc, calloc, free, msize,
setmalloctag, setrealloctag, getmalloctag, getrealloctag,
malloctopoolblock - kernel memory allocator
SYNOPSIS
void* malloc(ulong size)
void* mallocalign(ulong size, ulong align, long offset,
ulong span)
void* mallocz(ulong size, int clr)
void* smalloc(ulong size)
void* realloc(void *p, ulong size)
void* calloc(ulong n, ulong szelem)
void free(void *ptr)
ulong msize(void *ptr)
void setmalloctag(void *ptr, ulong tag)
ulong getmalloctag(void *ptr)
void setrealloctag(void *ptr, ulong tag)
ulong getrealloctag(void *ptr)
void* malloctopoolblock(void*)
DESCRIPTION
These are kernel versions of the functions in malloc(2).
They allocate memory from the mainmem memory pool, which is
managed by the allocator pool(2), which in turn replenishes
the pool as required by calling xalloc(9). All but smalloc
(which calls sleep(9)) may safely be called by interrupt
handlers.
Malloc returns a pointer to a block of at least size bytes,
initialised to zero. The block is suitably aligned for
storage of any type of object. The call malloc(0) returns a
valid pointer rather than null. Mallocz is similar, but
only clears the memory if clr is non-zero.
Smalloc returns a pointer to a block of size bytes, ini-
tialised to zero. If the memory is not immediately avail-
able, smalloc retries every 100 milliseconds until the
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MALLOC(9) MALLOC(9)
memory is acquired.
Mallocalign allocates a block of at least n bytes of memory
respecting alignment contraints. If align is non-zero, the
returned pointer is aligned to be equal to offset modulo
align. If span is non-zero, the n byte block allocated will
not span a span-byte boundary.
Realloc changes the size of the block pointed to by p to
size bytes, if possible without moving the data, and returns
a pointer to the block. The contents are unchanged up to
the lesser of old and new sizes, and any new space allocated
is initialised to zero. Realloc takes on special meanings
when one or both arguments are zero:
realloc(0, size)
means `malloc(size)'; returns a pointer to the newly-
allocated memory
realloc(ptr, 0)
means `free(ptr)'; returns null
realloc(0, 0)
no-op; returns null
Calloc returns a pointer to a block of memory of at least
n*szelem bytes, initialised to zero. New code should use
mallocz instead.
The argument to free is a pointer to a block of memory allo-
cated by one of the routines above, which is returned to the
allocation pool, or a null pointer, which is ignored.
When a block is allocated, sometimes there is some extra
unused space at the end. Msize grows the block to encompass
this unused space and returns the new number of bytes that
may be used.
The memory allocator maintains two word-sized fields associ-
ated with each block, the ``malloc tag'' and the ``realloc
tag''. By convention, the malloc tag is the PC that allo-
cated the block, and the realloc tag the PC that last real-
located the block. These may be set or examined with
setmalloctag, getmalloctag, setrealloctag, and
getrealloctag. When allocating blocks directly with malloc
and realloc, these tags will be set properly. If a custom
allocator wrapper is used, the allocator wrapper can set the
tags itself (usually by passing the result of getcallerpc(2)
to setmalloctag) to provide more useful information about
the source of allocation.
Malloctopoolblock takes the address of a block returned by
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malloc and returns the address of the corresponding block
allocated by the pool(2) routines.
SOURCE
/sys/src/9/port/alloc.c
DIAGNOSTICS
All functions except smalloc return a null pointer if space
is unavailable. If the allocated blocks have no malloc or
realloc tags, getmalloctag and getrealloctag return ~0.
SEE ALSO
pool(2), xalloc(9)
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