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 *_Nullable ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *_Nullable ptr, size_t size);
void *reallocarray(void *_Nullable 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 functions.
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 mem-
ory 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 result
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 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 ptr is not
NULL and the requested size is zero; this is not considered an error.
(See "Nonportable behavior" for portability issues.) Otherwise, the re-
turned 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 fol-
lowing 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.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│ malloc(), free(), calloc(), realloc() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
malloc()
free()
calloc()
realloc()
C11, POSIX.1-2008.
reallocarray()
None.
HISTORY
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.
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 sys-
tem 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/over-
commit_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 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 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 in-
ternally to protect the memory-management data structures employed by
these functions. In a multithreaded application in which threads simul-
taneously allocate and free memory, there could be contention for these
mutexes. To scalably handle memory allocation in multithreaded applica-
tions, glibc creates additional memory allocation arenas if mutex con-
tention is detected. Each arena is a large region of memory that is in-
ternally 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 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 li-
brary 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 corrup-
tion, 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 real-
loc(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().
EXAMPLES
#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 nmemb, 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 nmemb, size_t size)
{
return reallocarray(NULL, nmemb, size);
}
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
]8;;https://sourceware.org/glibc/wiki/MallocInternals\https://sourceware.org/glibc/wiki/MallocInternals]8;;\.
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