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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(): _GNU_SOURCE
           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 and may be different from ptr, or NULL if the request fails.
       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).

       ┌─────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├─────────────────────┼───────────────┼─────────┤
       │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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