realloc - reallocate memory dynamically
malloc, free, calloc, realloc, reallocarray - allocate and free dynamic memory
#include <stdlib.h>
void *realloc(void *ptr, size_t size);
Think of void *
as meaning the address of any type of value in memory. Think of size_t
as a long
.
#include <stdlib.h>
void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *ptr, size_t size);
void *reallocarray(void *ptr, size_t nmemb, size_t size);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
reallocarray():
Since glibc 2.29: _DEFAULT_SOURCE Glibc 2.28 and earlier: _GNU_SOURCE
This function dynamically resizes a block of memory that was returned by malloc
, the address of whose first byte is ptr
, to be size
contiguous bytes instead, moving (and copying) the original bytes in memory as needed.
The malloc() function allocates size
bytes
and returns a pointer to the allocated memory. The memory is not
initialized. If size
is 0, then malloc()
returns either NULL, or a unique pointer value that can later be
successfully passed to free().
The free() function frees the memory space pointed
to by ptr
, which must have been returned by a previous call to
malloc(), calloc(), or
realloc(). Otherwise, or if free(ptr)
has
already been called before, undefined behavior occurs. If ptr
is NULL, no operation is performed.
The calloc() function allocates memory for an array
of nmemb
elements of size
bytes each and returns a
pointer to the allocated memory. The memory is set to zero. If
nmemb
or size
is 0, then calloc()
returns either NULL, or a unique pointer value that can later be
successfully passed to free(). If the multiplication of
nmemb
and size
would result in integer overflow, then
calloc() returns an error. By contrast, an integer
overflow would not be detected in the following call to
malloc(), with the result that an incorrectly sized
block of memory would be allocated:
malloc(nmemb * size);
The realloc() function changes the size of the
memory block pointed to by ptr
to size
bytes. The
contents will be unchanged in the range from the start of the region up
to the minimum of the old and new sizes. If the new size is larger than
the old size, the added memory will not
be initialized. If
ptr
is NULL, then the call is equivalent to
malloc(size)
, for all values of size
; if size
is equal to zero, and ptr
is not NULL, then the call is
equivalent to free(ptr)
. Unless ptr
is NULL, it must
have been returned by an earlier call to malloc(),
calloc(), or realloc(). If the area
pointed to was moved, a free(ptr)
is done.
The reallocarray() function changes the size of the
memory block pointed to by ptr
to be large enough for an array
of nmemb
elements, each of which is size
bytes. It is
equivalent to the call
realloc(ptr, nmemb * size);
However, unlike that realloc() call,
reallocarray() fails safely in the case where the
multiplication would overflow. If such an overflow occurs,
reallocarray() returns NULL, sets errno
to
ENOMEM, and leaves the original block of memory
unchanged.
This function returns the address of the reallocated block’s first byte (which may or may not be the same as ptr
) or NULL
in cases of error (as when insufficient memory is available).
The malloc() and calloc() functions
return a pointer to the allocated memory, which is suitably aligned for
any built-in type. On error, these functions return NULL. NULL may also
be returned by a successful call to malloc() with a
size
of zero, or by a successful call to
calloc() with nmemb
or size
equal to
zero.
The free() function returns no value.
The realloc() function returns a pointer to the
newly allocated memory, which is suitably aligned for any built-in type,
or NULL if the request failed. The returned pointer may be the same as
ptr
if the allocation was not moved (e.g., there was room to
expand the allocation in-place), or different from ptr
if the
allocation was moved to a new address. If size
was equal to 0,
either NULL or a pointer suitable to be passed to
free() is returned. If realloc()
fails, the original block is left untouched; it is not freed or
moved.
On success, the reallocarray() function returns a pointer to the newly allocated memory. On failure, it returns NULL and the original block of memory is left untouched.
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char *s = malloc(3);
if (s == NULL)
{
return 1;
}
s[0] = 'h';
s[1] = 'i';
s[2] = '\0';
printf("%s\n", s);
char *tmp = realloc(s, 4);
if (tmp == NULL)
{
free(s);
return 1;
}
s = tmp;
s[2] = '!';
s[3] = '\0';
printf("%s\n", s);
free(s);
return 0;
}
calloc(), malloc(), realloc(), and reallocarray() can fail with the following error:
Out of memory. Possibly, the application hit the RLIMIT_AS or RLIMIT_DATA limit described in getrlimit(2).
reallocarray() first appeared in glibc in version 2.26.
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
malloc(),
free(), calloc(), realloc() |
Thread safety | MT-Safe |
malloc(), free(), calloc(), realloc(): POSIX.1-2001, POSIX.1-2008, C89, C99.
reallocarray() is a nonstandard extension that first appeared in OpenBSD 5.6 and FreeBSD 11.0.
By default, Linux follows an optimistic memory allocation strategy.
This means that when malloc() returns non-NULL there is
no guarantee that the memory really is available. In case it turns out
that the system is out of memory, one or more processes will be killed
by the OOM killer. For more information, see the description of
/proc/sys/vm/overcommit_memory
and
/proc/sys/vm/oom_adj
in proc(5), and the Linux
kernel source file
Documentation/vm/overcommit-accounting.rst
.
Normally, malloc() allocates memory from the heap, and adjusts the size of the heap as required, using sbrk(2). When allocating blocks of memory larger than MMAP_THRESHOLD bytes, the glibc malloc() implementation allocates the memory as a private anonymous mapping using mmap(2). MMAP_THRESHOLD is 128 kB by default, but is adjustable using mallopt(3). Prior to Linux 4.7 allocations performed using mmap(2) were unaffected by the RLIMIT_DATA resource limit; since Linux 4.7, this limit is also enforced for allocations performed using mmap(2).
To avoid corruption in multithreaded applications, mutexes are used
internally to protect the memory-management data structures employed by
these functions. In a multithreaded application in which threads
simultaneously allocate and free memory, there could be contention for
these mutexes. To scalably handle memory allocation in multithreaded
applications, glibc creates additional memory allocation arenas
if mutex contention is detected. Each arena is a large region of memory
that is internally allocated by the system (using
brk(2) or mmap(2)), and managed with
its own mutexes.
SUSv2 requires malloc(), calloc(),
and realloc() to set errno
to
ENOMEM upon failure. Glibc assumes that this is done
(and the glibc versions of these routines do this); if you use a private
malloc implementation that does not set errno
, then certain
library routines may fail without having a reason in errno
.
Crashes in malloc(), calloc(), realloc(), or free() are almost always related to heap corruption, such as overflowing an allocated chunk or freeing the same pointer twice.
The malloc() implementation is tunable via environment variables; see mallopt(3) for details.
valgrind(1), brk(2), mmap(2), alloca(3), malloc_get_state(3), malloc_info(3), malloc_trim(3), malloc_usable_size(3), mallopt(3), mcheck(3), mtrace(3), posix_memalign(3)
For details of the GNU C library implementation, see https://sourceware.org/glibc/wiki/MallocInternals">.
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.