malloc(3) Library Functions Manual malloc(3)
NAME
malloc, free, calloc, realloc, reallocarray - allocate and free dynamic
memory
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
DESCRIPTION
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 ptr, which
must have been returned by a previous call to malloc() or related func-
tions. Otherwise, or if ptr has already been freed, undefined behavior
occurs. If ptr is NULL, no operation is performed.
calloc()
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 a
unique pointer value that can later be successfully passed to free().
If the multiplication of nmemb and size would result in integer over-
flow, then calloc() returns an error. By contrast, an integer overflow
would not be detected in the following call to malloc(), with the re-
sult that an incorrectly sized block of memory would be allocated:
malloc(nmemb * size);
realloc()
The realloc() function changes the size of the memory block pointed to
by ptr 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 mem-
ory 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 equiva-
lent to free(ptr) (but see "Nonportable behavior" for portability is-
sues).
Unless ptr 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(ptr) is done.
reallocarray()
The reallocarray() function changes the size of (and possibly moves)
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 oc-
curs, reallocarray() returns an error.
RETURN VALUE
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 func-
tions 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 ptr 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 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 these func-
tions fail, the original block is left untouched; it is not freed or
moved.
ERRORS
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).
VERSIONS
reallocarray() was added in glibc 2.26.
malloc() and related functions rejected sizes greater than PTRDIFF_MAX
starting in glibc 2.30.
free() preserved errno starting in glibc 2.33.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│malloc(), free(), calloc(), realloc() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
malloc(), free(), calloc(), realloc(): POSIX.1-2001, POSIX.1-2008, C99.
reallocarray() is a nonstandard extension that first appeared in Open-
BSD 5.6 and FreeBSD 11.0.
NOTES
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-account-
ing.rst.
Normally, malloc() allocates memory from the heap, and adjusts the size
of the heap as required, using sbrk(2). When allocating blocks of mem-
ory 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 mal-
lopt(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 in-
ternally to protect the memory-management data structures employed by
these functions. In a multithreaded application in which threads si-
multaneously 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 mu-
tex 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 re-
placing malloc(), free(), calloc(), and realloc(). The replacement
functions must implement the documented glibc behaviors, including er-
rno 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 unre-
lated library routines may fail without having a valid reason in errno.
Private memory allocators may also need to replace other glibc func-
tions; see "Replacing malloc" in the glibc manual for details.
Crashes in memory allocators are almost always related to heap corrup-
tion, such as overflowing an allocated chunk or freeing the same point-
er 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(), cal-
loc(), and realloc().
SEE ALSO
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⟩.
Linux man-pages 6.03 2023-02-05 malloc(3)
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