MALLOC(3) | Library Functions Manual | MALLOC(3) |
malloc
, calloc
,
realloc
, free
—
#include <stdlib.h>
void *
malloc
(size_t
size);
void *
calloc
(size_t
number, size_t
size);
void *
realloc
(void
*ptr, size_t
size);
void
free
(void
*ptr);
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.
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 free
() function causes the allocated
memory referenced by ptr to be made available for
future allocations. If ptr is
NULL
, no action occurs.
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
() function returns 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. If size is 0, either
NULL
or a pointer that can be safely passed to
free(3) is returned.
The free
() function returns no value.
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, reallocarr(3) 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"); }
The above test is not sufficient in all cases. For example, multiplying ints requires a different set of checks:
int num, size; ... /* Avoid invalid requests */ if (size < 0 || num < 0) errc(1, EOVERFLOW, "overflow"); /* Check for signed int overflow */ if (size && num > INT_MAX / size) errc(1, EOVERFLOW, "overflow"); if ((p = malloc(size * num)) == NULL) err(1, "malloc");
Assuming the implementation checks for integer overflow as
NetBSD does, it is much easier to use
calloc
() or
reallocarr(3).
The above examples could be simplified to:
ptr = NULL; if ((e = reallocarr(&ptr, num, size))) errx(1, "reallocarr", strerror(e));
or at the cost of initialization: if ((p = calloc(num, size)) == NULL) err(1, "calloc");
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;
For the implementation details, see jemalloc(3).
malloc
(), calloc
(),
realloc
() and free
() functions
conform to ISO/IEC 9899:1990
(“ISO C90”).
free
() internal kernel function and a predecessor to
malloc
(), alloc
(), first
appeared in Version 1 AT&T UNIX. The C
Library functions alloc
() and
free
() appeared in Version 6
AT&T UNIX. The functions malloc
(),
calloc
(), and realloc
() first
appeared in Version 7 AT&T UNIX.
A new implementation by Chris Kingsley was introduced in 4.2BSD, followed by a complete rewrite by Poul-Henning Kamp (“phk's malloc” or “new malloc”) which appeared in FreeBSD 2.2 and was included in NetBSD 1.5 and OpenBSD 2.0. These implementations were all sbrk(2) based.
The jemalloc(3) allocator became the default system allocator first in FreeBSD 7.0 and then in NetBSD 5.0.
June 1, 2016 | NetBSD 9.2 |