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 *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)):

           Since glibc 2.29:
           glibc 2.28 and earlier:

       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  "Non‐
       portable behavior" for portability issues.)

       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.

       The  calloc() function allocates memory for an array of nmemb elements of size bytes each and returns a pointer to the allo‐
       cated 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);

       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.

       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.   At‐
       tempting  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  con‐
       sidered  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

       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).

       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.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                                                                        │ Attribute     │ Value   │
       │malloc(), free(), calloc(), realloc()                                                            │ Thread safety │ MT-Safe │

       malloc(), free(), calloc(), realloc(): POSIX.1-2001, POSIX.1-2008, C99.

       reallocarray() is a nonstandard extension that first appeared in OpenBSD 5.6 and FreeBSD 11.0.

       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 pro‐
       cesses 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 allo‐
       cating blocks of memory larger than MMAP_THRESHOLD bytes, the glibc malloc() implementation allocates the memory as  a  pri‐
       vate  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 struc‐
       tures 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 cre‐
       ates 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 de‐

       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 mal‐
       loc(), free(), calloc(), and realloc().

       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 ⟨⟩.

Linux man-pages 6.03                                         2023-02-05                                                   malloc(3)