malloc, calloc, free, memalign, realloc, valloc, alloca -
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 mal-
loc() 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 allo-
cation by the application, though not returned to the sys-
tem. 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
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 (possi-
bly moved) block. The contents will be unchanged up to the
lesser of the new and old sizes. 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 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.
The valloc() function has the same effect as malloc(),
except that the allocated memory will be aligned to a multi-
ple 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 unde-
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(), valloc(), 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
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
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.
Comparative features of malloc(3C), bsdmalloc(3MALLOC), and
malloc(3MALLOC) are as follows:
o The bsdmalloc(3MALLOC) routines afford better perfor-
mance, but are space-inefficient.
o The malloc(3MALLOC) routines are space-efficient, but
have slower performance.
o The standard, fully SCD-compliant malloc routines are
a trade-off between performance and space-efficiency.
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | malloc(), calloc(), free(),|
| | realloc(), valloc() are|
| | Standard; memalign() and|
| | alloca() are Stable. |
| MT-Level | Safe |
brk(2), getrlimit(2), bsdmalloc(3MALLOC), malloc(3MALLOC),
mapmalloc(3MALLOC), watchmalloc(3MALLOC), attributes(5)
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.
Man(1) output converted with