realloc(3)

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);

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().

   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.

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.

ERRORS

   calloc(), malloc(), and realloc() 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

   POSIX.1-2001, POSIX.1-2008, C89, C99.

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.

   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).  Allocations performed using mmap(2)  are  unaffected
   by the RLIMIT_DATA resource limit (see getrlimit(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

   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)

COLOPHON

   This page is part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs, and the
   latest version of this page, can be found at
   https://www.kernel.org/doc/man-pages/.

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