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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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malloc(3) Library Functions Manual malloc(3)
malloc, free, calloc, realloc, reallocarray - allocate and free
dynamic memory
Standard C library (libc, -lc)
#include <stdlib.h>
void *malloc(size_t size);
void free(void *_Nullable p);
void *calloc(size_t n, size_t size);
void *realloc(void *_Nullable p, size_t size);
void *reallocarray(void *_Nullable p, size_t n, 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
malloc()
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 a unique pointer value that can later
be successfully passed to free(). (See "Nonportable behavior" for
portability issues.)
free()
The free() function frees the memory space pointed to by p, which
must have been returned by a previous call to malloc() or related
functions. Otherwise, or if p has already been freed, undefined
behavior occurs. If p is NULL, no operation is performed.
calloc()
The calloc() function allocates memory for an array of n elements
of size bytes each and returns a pointer to the allocated memory.
The memory is set to zero. If n or size is 0, then calloc()
returns a unique pointer value that can later be successfully
passed to free().
If the multiplication of n 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(n * size);
realloc()
The realloc() function changes the size of the memory block
pointed to by p to size bytes. The contents of the memory 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 p is NULL, then the call is equivalent to malloc(size), for all
values of size.
If size is equal to zero, and p is not NULL, then the call is
equivalent to free(p) (but see "Nonportable behavior" for
portability issues).
Unless p is NULL, it must have been returned by an earlier call to
malloc or related functions. If the area pointed to was moved, a
free(p) is done.
reallocarray()
The reallocarray() function changes the size of (and possibly
moves) the memory block pointed to by p to be large enough for an
array of n elements, each of which is size bytes. It is
equivalent to the call
realloc(p, n * size);
However, unlike that realloc() call, reallocarray() fails safely
in the case where the multiplication would overflow. If such an
overflow occurs, reallocarray() returns an error.
The malloc(), calloc(), realloc(), and reallocarray() functions
return a pointer to the allocated memory, which is suitably
aligned for any type that fits into the requested size or less.
On error, these functions return NULL and set errno. Attempting
to allocate more than PTRDIFF_MAX bytes is considered an error, as
an object that large could cause later pointer subtraction to
overflow.
The free() function returns no value, and preserves errno.
The realloc() and reallocarray() functions return NULL if p is not
NULL and the requested size is zero; this is not considered an
error. (See "Nonportable behavior" for portability issues.)
Otherwise, the returned pointer may be the same as p if the
allocation was not moved (e.g., there was room to expand the
allocation in-place), or different from p if the allocation was
moved to a new address. If these functions fail, the original
block is left untouched; it is not freed or moved.
calloc(), malloc(), realloc(), and reallocarray() can fail with
the following error:
ENOMEM Out of memory. Possibly, the application hit the RLIMIT_AS
or RLIMIT_DATA limit described in getrlimit(2). Another
reason could be that the number of mappings created by the
caller process exceeded the limit specified by
/proc/sys/vm/max_map_count.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ malloc(), free(), calloc(), │ Thread safety │ MT-Safe │
│ realloc() │ │ │
└──────────────────────────────────────┴───────────────┴─────────┘
malloc()
free()
calloc()
realloc()
C23, POSIX.1-2024.
reallocarray()
POSIX.1-2024.
realloc(p, 0)
The behavior of realloc(p, 0) in glibc doesn't conform to any of
C99, C11, POSIX.1-2001, POSIX.1-2004, POSIX.1-2008, POSIX.1-2013,
POSIX.1-2017, or POSIX.1-2024. The C17 specification was changed
to make it conforming, but that specification made it impossible
to write code that reliably determines if the input pointer is
freed after realloc(p, 0), and C23 changed it again to make this
undefined behavior, acknowledging that the C17 specification was
broad enough that undefined behavior wasn't worse than that.
reallocarray() suffers the same issues in glibc.
musl libc and the BSDs conform to all versions of ISO C and
POSIX.1.
gnulib provides the realloc-posix module, which provides wrappers
realloc() and reallocarray() that conform to all versions of ISO C
and POSIX.1.
There's a proposal to standardize the BSD behavior:
⟨https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3621.txt⟩.
malloc()
free()
calloc()
realloc()
POSIX.1-2001, C89.
reallocarray()
glibc 2.26. OpenBSD 5.6, FreeBSD 11.0.
malloc() and related functions rejected sizes greater than
PTRDIFF_MAX starting in glibc 2.30.
free() preserved errno starting in glibc 2.33.
realloc(p, 0)
C89 was ambiguous in its specification of realloc(p, 0). C99
partially fixed this.
The original implementation in glibc would have been conforming to
C99. However, and ironically, trying to comply with C99 before
the standard was released, glibc changed its behavior in glibc
2.1.1 into something that ended up not conforming to the final C99
specification (but this is debated, as the wording of the standard
seems self-contradicting).
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.
If your program uses a private memory allocator, it should do so
by replacing malloc(), free(), calloc(), and realloc(). The
replacement functions must implement the documented glibc
behaviors, including errno handling, size-zero allocations, and
overflow checking; otherwise, other library routines may crash or
operate incorrectly. For example, if the replacement free() does
not preserve errno, then seemingly unrelated library routines may
fail without having a valid reason in errno. Private memory
allocators may also need to replace other glibc functions; see
"Replacing malloc" in the glibc manual for details.
Crashes in memory allocators 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.
Nonportable behavior
The behavior of these functions when the requested size is zero is
glibc specific; other implementations may return NULL without
setting errno, and portable POSIX programs should tolerate such
behavior. See realloc(3p).
POSIX requires memory allocators to set errno upon failure.
However, the C standard does not require this, and applications
portable to non-POSIX platforms should not assume this.
Portable programs should not use private memory allocators, as
POSIX and the C standard do not allow replacement of malloc(),
free(), calloc(), and realloc().
Programmers would naturally expect by induction that
realloc(p, size) is consistent with free(p) and malloc(size), as
that is the behavior in the general case. This is not explicitly
required by POSIX.1-2024 or C11, but all conforming
implementations are consistent with that.
The glibc implementation of realloc() is not consistent with that,
and as a consequence, it is dangerous to call realloc(p, 0) in
glibc.
A trivial workaround for glibc is calling it as
realloc(p, size?size:1).
The workaround for reallocarray() in glibc —which shares the same
bug— would be reallocarray(p, n?n:1, size?size:1).
#include <err.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MALLOCARRAY(n, type) ((type *) my_mallocarray(n, sizeof(type)))
#define MALLOC(type) MALLOCARRAY(1, type)
static inline void *my_mallocarray(size_t n, size_t size);
int
main(void)
{
char *p;
p = MALLOCARRAY(32, char);
if (p == NULL)
err(EXIT_FAILURE, "malloc");
strlcpy(p, "foo", 32);
puts(p);
}
static inline void *
my_mallocarray(size_t n, size_t size)
{
return reallocarray(NULL, n, size);
}
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-07-20 malloc(3)
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