MALLOC(3C) Standard C Library Functions MALLOC(3C)

NAME


malloc, calloc, free, freezero, memalign, realloc, reallocarray,
recallocarray, valloc, alloca - memory allocator

SYNOPSIS


#include <stdlib.h>

void *
malloc(size_t size);

void *
calloc(size_t nelem, size_t elsize);

void
free(void *ptr);

void
freezero(void *ptr, size_t size);

void *
memalign(size_t alignment, size_t size);

void *
realloc(void *ptr, size_t size);

void *
reallocarray(void *ptr, size_t nelem, size_t elsize);

void *
recallocarray(void *ptr, size_t oldnelem, size_t newnelem, size_t elsize);

void *
valloc(size_t size);

#include <alloca.h>

void *
alloca(size_t size);

DESCRIPTION


The malloc() and free() functions provide a simple, general-purpose memory
allocation package. The malloc() function returns a pointer to a block of
at least size bytes suitably aligned for any use. If the space assigned by
malloc() is overrun, the results are undefined.

The argument to free() is a pointer to a block previously allocated by
malloc(), calloc(), realloc(), reallocarray(), or recallocarray(). After
free() is executed, this space is made available for further allocation by
the application, though not returned to the system. Memory is returned to
the system only upon termination of the application. If ptr is a null
pointer, no action occurs. If a random number is passed to free(), the
results are undefined.

The freezero() function is similar to the free() function except it ensures
memory is explicitly discarded. If ptr is NULL, no action occurs. If ptr
is not NULL, the size argument must be equal or smaller than the size of
the earlier allocation that returned ptr. freezero() guarantees the memory
range starting at ptr with length size is discarded while deallocating the
whole object originally allocated.

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

The memalign() function allocates size bytes on a specified alignment
boundary and returns a pointer to the allocated block. The value of the
returned address is guaranteed to be an even multiple of alignment. The
value of alignment must be a power of two and must be greater than or equal
to the size of a word.

The realloc() function 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. If
the new size of the block requires movement of the block, the space for the
previous instantiation of the block is freed. If the new size is larger,
the contents of the newly allocated portion of the block are unspecified.
If ptr is NULL, realloc() behaves like malloc() for the specified size. If
size is 0 and ptr is not a null pointer, the space pointed to is freed.

The reallocarray() function is similar to realloc(), but operates on nelem
elements of size elsize and checks for overflow in nelem*elsize
calculation.

The recallocarray() function is similar to reallocarray() except it ensures
newly allocated memory is cleared similar to calloc(). If ptr is NULL,
oldnelem is ignored and the call is equivalent to calloc(). If ptr is not
NULL, oldnelem must be a value such that oldnelem*elsize is the size of the
earlier allocation that returned ptr, otherwise the behaviour is undefined.

The valloc() function has the same effect as malloc(), except that the
allocated memory will be aligned to a multiple of the value returned by
sysconf(_SC_PAGESIZE).

The alloca() function allocates size bytes of space in the stack frame of
the caller, and returns a pointer to the allocated block. This temporary
space is automatically freed when the caller returns. If the allocated
block is beyond the current stack limit, the resulting behavior is
undefined.

RETURN VALUES


Upon successful completion, each of the allocation functions returns a
pointer to space suitably aligned (after possible pointer coercion) for
storage of any type of object.

If there is no available memory, malloc(), calloc(), realloc(),
reallocarray(), recallocarray(), memalign(), and valloc() return a null
pointer.

When realloc() is called with size > 0 and returns NULL, the block pointed
to by ptr is left intact. If size, nelem, or elsize is 0, either a null
pointer or a unique pointer that can be passed to free() is returned.

If malloc(), calloc(), realloc(), reallocarray(), or recallocarray()
returns unsuccessfully, errno will be set to indicate the error. The
free() and freezero() functions do not set errno.

ERRORS


The malloc(), calloc(), realloc(), and reallocarray() functions will fail
if:

ENOMEM The physical limits of the system are exceeded by size
bytes of memory which cannot be allocated, or there's
integer overflow in reallocarray().

EAGAIN There is not enough memory available to allocate size
bytes of memory; but the application could try again
later.

The recallocarray() function will fail if:

EINVAL ptr is not NULL and multiplying oldnelem and elsize
results in integer overflow.

USAGE


Portable applications should avoid using valloc() but should instead use
malloc() or mmap(2). On systems with a large page size, the number of
successful valloc() operations might be 0.

These default memory allocation routines are safe for use in multithreaded
applications but are not scalable. Concurrent accesses by multiple threads
are single-threaded through the use of a single lock. Multithreaded
applications that make heavy use of dynamic memory allocation should be
linked with allocation libraries designed for concurrent access, such as
libumem(3LIB) or libmtmalloc(3LIB). Applications that want to avoid using
heap allocations (with brk(2)) can do so by using either libumem(3LIB) or
libmapmalloc(3LIB). The allocation libraries libmalloc(3LIB) and
libbsdmalloc(3LIB) are available for special needs.

Comparative features of the various allocation libraries can be found in
the umem_alloc(3MALLOC) manual page.

INTERFACE STABILITY


The malloc(), calloc(), free(), realloc(), valloc() functions are Standard.

The freezero(), reallocarray(), and recallocarray() functions are
Committed.

The memalign() and alloca() functions are Stable.

MT-LEVEL
Safe.

SEE ALSO


brk(2), getrlimit(2), libbsdmalloc(3LIB), libmalloc(3LIB),
libmapmalloc(3LIB), libmtmalloc(3LIB), libumem(3LIB), umem_alloc(3MALLOC),
watchmalloc(3MALLOC), attributes(5)

WARNINGS


Undefined results will occur if the size requested for a block of memory
exceeds the maximum size of a process's heap, which can be obtained with
getrlimit(2).

The alloca() function is machine-, compiler-, and most of all, system-
dependent. Its use is strongly discouraged.

illumos July 28, 2017 illumos