MALLOC(3) — System Interface Manual — Subroutines

NAME

malloc, free, realloc, calloc, cfree, 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;

cfree(ptr)
char ∗ptr;

char ∗alloca(size)
int size;

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 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.

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, and can be freed with cfree.

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.

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; 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 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 circular list. Just before more memory is allocated, all allocated and freed blocks are referenced; this can cause a huge number of page faults.

Alloca is machine dependent; it’s use is discouraged.