MALLOC(3)MALLOC(3)NAME
malloc, free, realloc, calloc, alloca - memory allocator
SYNOPSIS
char *malloc(size)
unsigned size;
free(ptr)
char *ptr;
char *realloc(ptr, size)
char *ptr;
unsigned size;
char *calloc(nelem, elsize)
unsigned nelem, elsize;
char *alloca(size)
int size;
DESCRIPTION
Malloc and free provide a general-purpose memory allocation package.
Malloc returns a pointer to a block of at least size bytes beginning on
a word boundary.
The argument to free is a pointer to a block previously allocated by
malloc; this space is made available for further allocation, but its
contents are left undisturbed.
Needless to say, grave disorder will result if the space assigned by
malloc is overrun or if some random number is handed to free.
Malloc maintains multiple lists of free blocks according to size, allo‐
cating space from the appropriate list. It calls sbrk (see brk(2)) to
get more memory from the system when there is no suitable space already
free.
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. If ptr is
null, realloc simply returns the value of malloc called with an argu‐
ment of size.
In order to be compatible with older versions, realloc also works if
ptr points to a block freed since the last call of malloc, realloc or
calloc; sequences of free, malloc and realloc were previously used to
attempt storage compaction. This procedure is no longer recommended.
Calloc allocates space for an array of nelem elements of size elsize.
The space is initialized to zeros.
Alloca allocates size bytes of space in the stack frame of the caller.
This temporary space is automatically freed on return.
Each of the allocation routines returns a pointer to space suitably
aligned (after possible pointer coercion) for storage of any type of
object. If the space is of pagesize or larger, the memory returned
will be page-aligned.
SEE ALSObrk(2), pagesize(2)DIAGNOSTICS
Malloc, realloc and calloc return a null pointer (0) if there is no
available memory or if the arena has been detectably corrupted by stor‐
ing outside the bounds of a block. Malloc may be recompiled to check
the arena very stringently on every transaction; those sites with a
source code license may check the source code to see how this can be
done.
BUGS
When realloc returns 0, the block pointed to by ptr may be destroyed.
The current implementation of malloc does not always fail gracefully
when system memory limits are approached. It may fail to allocate mem‐
ory when larger free blocks could be broken up, or when limits are
exceeded because the size is rounded up. It is optimized for sizes
that are powers of two.
Alloca is machine dependent; its use is discouraged.
4th Berkeley Distribution June 9, 1989 MALLOC(3)
