malloc, mallocalign, mallocz, free, realloc, calloc, msize, setmalloctag,
setrealloctag, getmalloctag, getrealloctag, malloctopoolblock
– memory allocator|
void* malloc(uintptr size)
void* mallocalign(uintptr size, uintptr align, vlong offset, uintptr span)
void* mallocz(uintptr size, int clr)
void free(void *ptr)
void* realloc(void *ptr, uintptr size)
void* calloc(uintptr nelem, uintptr elsize)
void setmalloctag(void *ptr, uintptr tag)
void setrealloctag(void *ptr, uintptr tag)
Malloc and free provide a simple memory allocation package. Malloc
returns a pointer to a new block of at least size bytes. The block
is suitably aligned for storage of any type of object. No two
active pointers from malloc will have the same value. The call
malloc(0) returns a valid pointer rather than null. |
The argument to free is a pointer to a block previously allocated by malloc; this space is made available for further allocation. It is legal to free a null pointer; the effect is a no–op. The contents of the space returned by malloc are undefined. Mallocz behaves as malloc, except that if clr is non–zero, the memory returned will be zeroed.
Mallocalign allocates a block of at least size 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 size–byte block allocated will not span a span–byte boundary.
Realloc changes the size of the block pointed to by ptr to size
bytes and returns a pointer to the (possibly moved) block. The
contents will be unchanged up to the lesser of the new and old
sizes. Realloc takes on special meanings when one or both arguments
Calloc allocates space for an array of nelem elements of size elsize. The space is initialized to zeros. Free frees such a block.
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 pointer–sized fields associated with each block, the ``malloc tag'' and the ``realloc tag''. By convention, the malloc tag is the PC that allocated the block, and the realloc tag the PC that last reallocated 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 malloc
and returns the address of the corresponding block allocated by
the pool(2) routines.
leak(1), trump (in acid(1)), brk(2), getcallerpc(2), pool(2)|
Malloc, realloc and calloc return 0 if there is no available memory.
Errstr is likely to be set. If the allocated blocks have no malloc
or realloc tags, getmalloctag and getrealloctag return ~0. |
After including pool.h, the call poolcheck(mainmem) can be used
to scan the storage arena for inconsistencies such as data written
beyond the bounds of allocated blocks. It is often useful to combine
this with with setting
The trump library for acid can be used to obtain traces of malloc
execution; see acid(1).
The different specification of calloc is bizarre. |
User errors can corrupt the storage arena. The most common gaffes
are (1) freeing an already freed block, (2) storing beyond the
bounds of an allocated block, and (3) freeing data that was not
obtained from the allocator. When malloc and free detect such
corruption, they abort.