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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | 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 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
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 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
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);
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 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) (but see "Nonportable behavior" for
portability issues).
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 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 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 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()
C11, POSIX.1-2008.
reallocarray()
None.
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.
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().
#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);
}
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-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. 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
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[email protected]
Linux man-pages 6.9.1 2024-05-02 malloc(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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