malloc, calloc, free, memalign, realloc, valloc, alloca - memory allo‐
void *malloc(size_t size);
void *calloc(size_t nelem, size_t elsize);
void free(void *ptr);
void *memalign(size_t alignment, size_t size);
void *realloc(void *ptr, size_t size);
void *valloc(size_t size);
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(), or realloc(). 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 ter‐
mination of the application. If ptr is a null pointer, no action
occurs. If a random number is passed to free(), the results are unde‐
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 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 tem‐
porary space is automatically freed when the caller returns. If the
allocated block is beyond the current stack limit, the resulting behav‐
ior is undefined.
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(), realloc(), memalign(), val‐
loc(), and calloc() 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(), or realloc() returns unsuccessfully, errno will
be set to indicate the error. The free() function does not set errno.
The malloc(), calloc(), and realloc() functions will fail if:
The physical limits of the system are exceeded by size bytes
of memory which cannot be allocated.
There is not enough memory available to allocate size bytes
of memory; but the application could try again later.
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 multi‐
threaded applications but are not scalable. Concurrent accesses by mul‐
tiple threads are single-threaded through the use of a single lock.
Multithreaded applications that make heavy use of dynamic memory allo‐
cation should be linked with allocation libraries designed for concur‐
rent 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 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.
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ See below. │
│MT-Level │ Safe │
The malloc(), calloc(), free(), realloc(), valloc() functions are Stan‐
dard. The memalign() and alloca() functions are Stable.
SEE ALSObrk(2), getrlimit(2), libbsdmalloc(3LIB), libmalloc(3LIB), libmapmal‐
loc(3LIB), libmtmalloc(3LIB), libumem(3LIB), umem_alloc(3MALLOC),
Undefined results will occur if the size requested for a block of mem‐
ory exceeds the maximum size of a process's heap, which can be obtained
The alloca() function is machine-, compiler-, and most of all, system-
dependent. Its use is strongly discouraged.
Mar 21, 2005 MALLOC(3C)