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)
unsigned 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 maintains multiple lists of free blocks according to size, allocating 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.

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.

When any of the allocation routines increases a program's address space, it also attempts to increase its allowable resident set size (see vm_ctl(2)).

When asked to allocate zero bytes, malloc, realloc, calloc, and alloca allocate a small block (typically eight bytes) and return a pointer to the block.

NOTES
Malloc allocates memory from process private heap. In a parallel program (see Guide to Parallel Programming), the default size of the private heap is less than 64k (see ld(1) for how to change this default size and see shmalloc() for how to allocate memory in the shared heap).

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.

Alloca is machine dependent; its use is discouraged.

SEE ALSO
ld(1), Guide to Parallel Programming