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NAME
malloc, free, calloc, realloc - allocate and free dynamic memory
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
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.
RETURN VALUE
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.
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).
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├─────────────────────┼───────────────┼─────────┤
│malloc(), free(), │ Thread safety │ MT-Safe │
│calloc(), realloc() │ │ │
└─────────────────────┴───────────────┴─────────┘
CONFORMING TO
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.
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-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.
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⟩.
COLOPHON
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