MALLOC(3) GENIX Programmer's Manual MALLOC(3)
NAME
malloc, free, realloc, calloc - main 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;
DESCRIPTION
Malloc and free provide a simple 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 allocates the first big enough contiguous reach of free space
found in a circular search from the last block allocated or freed,
coalescing adjacent free blocks as it searches. It calls sbrk (see
break(2)) to get more memory from the system when there is no suit-
able 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 con-
tents will be unchanged up to the lesser of the new and old sizes.
Realloc also works if ptr points to a block freed since the last
call of malloc, realloc or calloc; thus sequences of free, malloc
and realloc can exploit the search strategy of malloc to do storage
compaction.
Calloc allocates space for an array of nelem elements of size
elsize. The space is initialized to zeros.
Each of the allocation routines returns a pointer to space suitably
aligned (after possible pointer coercion) for storage of any type of
object.
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MALLOC(3) GENIX Programmer's Manual MALLOC(3)
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
storing outside the bounds of a block. Malloc may be recompiled to
check the arena very stringently on every transaction; see the
source code.
BUGS
When realloc returns 0, the block pointed to by ptr may be des-
troyed.
The current incarnation of the allocator is unsuitable for direct
use in a large virtual environment where many small blocks are to be
kept, since it keeps all allocated and freed blocks on a single cir-
cular list. Just before more memory is allocated, all allocated and
freed blocks are referenced; this can cause a huge number of page
faults.
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