MALLOC(2) MALLOC(2) NAME malloc, mallocz, free, realloc, calloc, msize, setmalloctag, setrealloctag, getmalloctag, getrealloctag, malloctopoolblock - 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) ulong msize(void *ptr) void setmalloctag(void *ptr, ulong tag) ulong getmalloctag(void *ptr, ulong tag) void setrealloctag(void *ptr, ulong tag) ulong getrealloctag(void *ptr, ulong tag) 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. Incompatibly with ANSI C, 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. The call realloc(0, size) means the same as `malloc(size)'. Further, the call realloc(ptr, 0) means the same as `free(ptr)'. MALLOC(2) MALLOC(2) 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 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 malloc and returns the address of the corresponding block allocated by the pool(2) routines. SOURCE /sys/src/libc/port/malloc.c SEE ALSO leak(1), brk(2), debugmalloc(2), getcallerpc(2), pool(2) 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. 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.