MALLOC(3)

NAME
malloc, calloc, realloc, reallocf, free, freezero, malloc_usable_size - general purpose memory allocation functions

LIBRARY
libc

SYNOPSIS
#include <stdlib.h>
void *
malloc(size_t size);
void *
calloc(size_t number, size_t size);
void *
realloc(void *ptr, size_t size);
void *
reallocf(void *ptr, size_t size);
void
free(void *ptr);
void
freezero(void *ptr, size_t size);
#include <malloc_np.h>
size_t
malloc_usable_size(const void *ptr);

DESCRIPTION
The malloc() function allocates size bytes of uninitialized memory. The allocated space is suitably aligned (after possible pointer coercion) for storage of any type of object. If the space is at least pagesize bytes in length (see getpagesize(3)), the returned memory will be page boundary aligned as well.

The calloc() function allocates space for number objects, each size bytes in length. The result is identical to calling malloc() with an argument of “number * size”, with the exception that the allocated memory is explicitly initialized to zero bytes.

The realloc() function changes the size of the previously allocated memory referenced by ptr to size bytes. The contents of the memory are unchanged up to the lesser of the new and old sizes. If the new size is larger, the value of the newly allocated portion of the memory is undefined. Upon success, the memory referenced by ptr is freed and a pointer to the newly allocated memory is returned. Note that realloc() may move the memory allocation, resulting in a different return value than ptr. If ptr is NULL, the realloc() function behaves identically to malloc() for the specified size.

The reallocf() function call is identical to the realloc function call, except that it will free the passed pointer when the requested memory cannot be allocated. This is a FreeBSD / DragonFly specific API designed to ease the problems with traditional coding styles for realloc causing memory leaks in libraries.

The free() function causes the allocated memory referenced by ptr to be made available for future allocations. If ptr is NULL, no action occurs.

The freezero() function is similar to the free() function. Cached free objects are cleared with explicit_bzero(3). The size argument must be equal to or smaller than the size of the earlier allocation.

The malloc_usable_size() function returns the usable size of the allocation pointed to by ptr. The return value may be larger than the size that was requested during allocation. The malloc_usable_size() function is not a mechanism for in-place realloc(); rather it is provided solely as a tool for introspection purposes. Any discrepancy between the requested allocation size and the size reported by malloc_usable_size() should not be depended on, since such behavior is entirely implementation-dependent.

IMPLEMENTATION NOTES
DragonFly's malloc implementation is based on a port of the DragonFly kernel slab allocator, appropriately modified for a user process environment.

The slab allocator breaks memory allocations up to 8KB into 80 zones. Each zone represents a fixed allocation size in multiples of some core chunking. The chunking is a power-of-2 but the fixed allocation size is not. For example, a 1025-byte request is allocated out of the zone with a chunking of 128, thus in multiples of 1152 bytes. The minimum chunking, used for allocations in the 0-127 byte range, is 8 bytes (16 of the 80 zones). Beyond that the power-of-2 chunking is between 1/8 and 1/16 of the minimum allocation size for any given zone.

As a special case any power-of-2-sized allocation within the zone limit (8K) will be aligned to the same power-of-2 rather than that zone's (smaller) chunking. This is not something you can depend upon for malloc(), but it is used internally to optimize posix_memalign(3).

Each zone reserves memory in 64KB blocks. Actual memory use tends to be significantly less as only the pages actually needed are faulted in. Allocations larger than 8K are managed using mmap(2) and tracked with a hash table.

The zone mechanism results in well-fitted allocations with little waste in a long-running environment which makes a lot of allocations. Short-running environments which do not make many allocations will see a bit of extra bloat due to the large number of zones but it will be almost unnoticeable in the grand scheme of things. To reduce bloat further the normal randomized start offset implemented in the kernel version of the allocator to improve L1 cache fill is disabled in the libc version.

The zone mechanism also has the nice side effect of greatly reducing fragmentation over the original malloc.

calloc() is directly supported by keeping track of newly-allocated zones which will be demand-zero'd by the system. If the allocation is known to be zero'd we do not bother bzero()ing it. If it is a reused allocation we bzero().

POSIX threading is supported by duplicating the primary structure. A thread entering malloc() which is unable to immediately acquire a mutex on the last primary structure it used will switch to a different primary structure. At the moment this is more of a quick hack than a solution, but it works.

RETURN VALUES
The malloc() and calloc() functions return a pointer to the allocated memory if successful; otherwise a NULL pointer is returned and errno is set to ENOMEM.

The realloc() and reallocf() functions return a pointer, possibly identical to ptr, to the allocated memory if successful; otherwise a NULL pointer is returned, and errno is set to ENOMEM if the error was the result of an allocation failure. The realloc() function always leaves the original buffer intact when an error occurs, whereas reallocf() deallocates it in this case.

The free() function returns no value.

If malloc(), calloc(), realloc() or free() detect an error, a message will be printed to file descriptor STDERR_FILENO and the process will dump core.

The malloc_usable_size() function returns the usable area for the specified pointer or 0 if the pointer is NULL.

EXAMPLES
When using malloc(), be careful to avoid the following idiom:

    if ((p = malloc(number * size)) == NULL)
    	err(EXIT_FAILURE, "malloc");

The multiplication may lead to an integer overflow. To avoid this, calloc() is recommended.

If malloc() must be used, be sure to test for overflow:

    if (size && number > SIZE_MAX / size) {
    	errno = EOVERFLOW;
    	err(EXIT_FAILURE, "allocation");
    }

When using realloc(), one must be careful to avoid the following idiom:

    nsize += 50;
    if ((p = realloc(p, nsize)) == NULL)
    	return NULL;

Do not adjust the variable describing how much memory has been allocated until it is known that the allocation has been successful. This can cause aberrant program behavior if the incorrect size value is used. In most cases, the above example will also leak memory. As stated earlier, a return value of NULL indicates that the old object still remains allocated. Better code looks like this:

    newsize = size + 50;
    if ((p2 = realloc(p, newsize)) == NULL) {
    	if (p != NULL)
    		free(p);
    	p = NULL;
    	return NULL;
    }
    p = p2;
    size = newsize;

SEE ALSO
madvise(2), mmap(2), sbrk(2), alloca(3), atexit(3), emalloc(3), getpagesize(3), memory(3), posix_memalign(3), reallocarray(3)

STANDARDS
The malloc(), calloc(), realloc() and free() functions conform to isoC.

HISTORY
The reallocf() function first appeared in FreeBSD 3.0.

The freezero() function appeared in OpenBSD 6.2 and DragonFly 5.5.

DragonFly's malloc implementation is based on the kernel's slab allocator (see posix_memalign(3)'s IMPLEMENTATION NOTES). It first appeared in DragonFly 2.3.

AUTHORS
Matt Dillon