malloc(3)

NAME

malloc, free, realloc, calloc, alloca − memory allocator

SYNTAX

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

The malloc and free subroutines provide a simple general-purpose memory allocation package. The malloc subroutine 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.

The malloc subroutine maintains multiple lists of free blocks according to size, allocating space from the appropriate list. It calls sbrk to get more memory from the system when there is no suitable space already free. For further information, see brk(2).

The realloc subroutine 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.

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.

The calloc subroutine allocates space for an array of nelem elements of size elsize. The space is initialized to zeros.

The alloca subroutine 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.

RESTRICTIONS

When realloc returns 0, the block pointed to by ptr may be destroyed.

Currently, the allocator is unsuitable for direct use in a large virtual environment where many small blocks are kept, since it keeps all allocated and freed blocks on a circular list. Just before more memory is allocated, all allocated and freed blocks are referenced.

The alloca subroutine is machine dependent.

DIAGNOSTICS