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


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


#include <stdlib.h>

void *
malloc(size_t size);

void *
calloc(size_t nelem, size_t elsize);

free(void *ptr);

freezero(void *ptr, size_t size);

void *
memalign(size_t alignment, size_t size);

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

void *
reallocf(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);


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(), reallocf(), 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 reallocf() function behaves in the same way as realloc() except that
the passed pointer is freed automatically on failure.

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

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

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


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(), reallocf(),
reallocarray(), recallocarray(), memalign(), and valloc() return a null

When realloc() is called with size > 0 and returns NULL, the block pointed
to by ptr is left intact. By contrast, when reallocf() is called with size
> 0 and returns NULL, the block pointed to by ptr will have been freed.

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


The malloc(), calloc(), realloc(), reallocf(), 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

The recallocarray() function will fail if:

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


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.


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

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

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



brk(2), getrlimit(2), libbsdmalloc(3LIB), libmalloc(3LIB),
libmapmalloc(3LIB), libmtmalloc(3LIB), libumem(3LIB), umem_alloc(3MALLOC),
watchmalloc(3MALLOC), attributes(7)


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

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

illumos September 12, 2019 illumos