MALLOC(3)                                               MALLOC(3)

     NAME
          malloc, mallocz, free, realloc, calloc, setmalloctag,
          setrealloctag, getmalloctag, getrealloctag - memory
          allocator

     SYNOPSIS
          #include <u.h>
          #include <libc.h>

          void* malloc(ulong size)

          void* mallocz(ulong size, int clr)

          void  free(void *ptr)

          void* realloc(void *ptr, ulong size)

          void* calloc(ulong nelem, ulong elsize)

          void  setmalloctag(void *ptr, ulong tag)

          ulong getmalloctag(void *ptr)

          void  setrealloctag(void *ptr, ulong tag)

          ulong getrealloctag(void *ptr)

     DESCRIPTION
          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 fur-
          ther 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.

          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 are zero:

          realloc(0, size)
               means `malloc(size)'; returns a pointer to the newly-

     MALLOC(3)                                               MALLOC(3)

               allocated memory

          realloc(ptr, 0)
               means `free(ptr)'; returns null

          realloc(0, 0)
               no-op; returns null

          Calloc allocates space for an array of nelem elements of
          size elsize. The space is initialized to zeros.  Free frees
          such a block.

          The memory allocator on Plan 9 maintains two word-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(3)
          to setmalloctag) to provide more useful information about
          the source of allocation.

     SOURCE
          /src/lib9/malloc.c
          /src/lib9/malloctag.c

     SEE ALSO
          trump (in acid(1)), getcallerpc(3)

     DIAGNOSTICS
          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.

          The trump library for acid can be used to obtain traces of
          malloc execution; see acid(1).

     BUGS
          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.

          To avoid name conflicts with the system versions of these
          functions, malloc, realloc, calloc, and free are

     MALLOC(3)                                               MALLOC(3)

          preprocessor macros defined as p9malloc, p9realloc,
          p9calloc, and p9free; see intro(3).