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 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.

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 returns a pointer to the newly-allocated memory

realloc(ptr, 0)
    means 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 preprocessor macros defined as p9malloc, p9realloc, p9calloc, and p9free; see intro((3)).